Ionic metal-organic frameworks (iMOFs): Design principles and applications

Ionic metal-organic frameworks (iMOFs): Design principles and applications

Accepted Manuscript Title: Ionic metal-organic frameworks (iMOFs): Design principles and applications Author: Avishek Karmakar Aamod V. Desai Sujit K...

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Accepted Manuscript Title: Ionic metal-organic frameworks (iMOFs): Design principles and applications Author: Avishek Karmakar Aamod V. Desai Sujit K. Ghosh PII: DOI: Reference:

S0010-8545(15)00277-5 http://dx.doi.org/doi:10.1016/j.ccr.2015.08.007 CCR 112129

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

4-6-2015 21-8-2015 21-8-2015

Please cite this article as: A. Karmakar, A.V. Desai, S.K. Ghosh, Ionic metal-organic frameworks (iMOFs): Design principles and applications, Coordination Chemistry Reviews (2015), http://dx.doi.org/10.1016/j.ccr.2015.08.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Highlights a) Classification of ionic metal-organic frameworks (iMOFs)

c) Applications of cationic & anionic frameworks

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b) Pre and post-synthetic approaches to design ionic-MOFs

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d) Ion-exchange performance by cationic and anionic MOFs

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e) Futuristic prospects of iMOFs

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Review

Ionic

metal-organic

frameworks

(iMOFs): Design

and

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applications

principles

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Avishek Karmakar, Aamod V. Desai, Sujit K. Ghosh*

Corresponding author. Tel.: +91-20- 25908076

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Dept. of Chemistry, Indian Institute of Science Education and Research (IISER), Pashan, Pune, Maharashtra 411008, India E-mail: [email protected]

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Contents 1. Introduction

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2. Ionic MOFs: Classifications and brief overview 3. Anionic frameworks

4.1.

15-20

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2-3 3 3-6 6-8 8-9 9-10 10-12 12-14 15

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4.

Synthetic strategies of cationic frameworks

4.2.

5.

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Design principles of anionic frameworks Applications 3.2.1 Adsorption 3.2.2 Catalysis 3.2.3 Non-linear optics 3.2.4 Drug delivery 3.2.5 Sensing 3.2.6 Proton conduction Cationic frameworks

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3.1. 3.2.

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Applications 4.2.1 Sensing. 4.2.1. a Luminescence 4.2.1. b Colorimetric sensors 4.2.2 Magnetism 4.2.3 Structural flexibility driven properties 4.2.4 Other applications Ion-Exchange performance in MOFs 5.1.

Anion exchange by cationic MOFs

20 20-21 22-23 24-25 25-26 26-29 29 29-31 31-32

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32

Acknowledgements

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References

33-41

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5.2. Cation exchange by anionic MOFs Conclusion and future prospects

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6.

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Abstract

Metal-organic frameworks (MOFs) have commanded significant attention in recent years on account of the applicability of these materials across several disciplines in material chemistry.

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The liberty of tuning the coordination nanospaces owing to the infinite choice of organic linkers and multivariate oxidation states of the metal nodes bestows a distinguished advantage of

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designable architectures to this class of materials. Majority of the reported MOFs comprise of neutral frameworks as the net positive charge on the metal ions is satisfied by the negative charge of anionic ligands or the coordinated anions of the metal salt used in synthesis. Although

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being non-trivial, the synthesis of ionic MOFs (iMOFs) affords several distinct advantages over the routine neutral frameworks by virtue of the isolated charged species in confined nanospaces.

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The development and potential applications of such cationic or anionic frameworks has been discussed thoroughly in this review. The design principles governing the formation of such charge-polarized MOFs have been outlined through representative examples. The state-of-the-art ion exchange performances of competing materials have been compared and a future perspective of such ionic-MOFs is proposed.

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Keywords: Metal-organic frameworks Ionic MOFs

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Luminescence

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Ion exchange

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Catalysis

MOF

Metal-Organic Framework

PCP

Porous Coordination Polymer

CP

Coordination Polymer

3D

3 Dimensional

DMF

Dimethylformamide

DEF

Diethylformamide

BDC

Benzene-1,4-dicarboxylic acid

Density functional Theory

RTMPyP

5, 10, 15, 20-tetrakis(1-methyl- 4 pyridinio)porphyrin

COD

1,5-cyclooctadiene

Bpy

4-4’-bipyridine

Dppe

1,2-bis(diphenylphosphino)ethane

NLO

Non-linear optics

SHG

Second harmonic generation

DPASM

4-(4-(diphenylamino)styryl)-1methylpyridinium

DPASB

1-butyl-4-(4(diphenylamino)styryl)pyridinium

DPASN

4-(4-(diphenylamino)styryl)-1nonylpyridinium

DPASD

4-(4-(diphenylamino)styryl)-1dodecylpyridinium

TMA

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DFT

TEA

Tetraethylammonium cation

TBA

Tetrabutylammonium cation

PBS

Phosphate-buffered saline

TCPT

2,4,6-tris-(4-carboxyphenoxy)-1,3,5 triazine

TNP

2,4,6-Trinitrophenol

Qst

Isosteric heat of adsorption

TNT

2,4,6-Trinitrotoluene

CCS

Carbon Capture and Sequestration

DMNB

2,3-dimethyl-2,3-dinitrobutane

H2ppz2+

piperazinium cation

2,6-DNT

2,4-Dinitrotoluene

BTC DMA SBU ZMOF BPTC

1,3,5-benzenetricarboxylate

Dimethyl Ammonium System(?)

Secondary Building Unit(?)

Zeolite-like metal-organic frameworks Biphenyltetracarboxylic Acid

Tetramethylammonium cation

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2,4-DNT

2,4-Dinitrotoluene

DMNB

2, 3-dimethyl-2,3-dinitrobutane

NM

Nitromethane

IDC

Imidazole-4,5-dicarboxyic acid

p-XBP4

Bpp

1,3-bis(4-pyridyl)propane

N,N’-p-phenylenedimethylenebis(pyridin4-one)

Btapa

1,3,5-Benzene Tricarboxylic Acid Tris[N(4-pyridyl)amide]

LDH

Layered double hydroxides

EDS

1,2-ethanedisulfonate

ethylenediaminetetrapropionitrile

Btr

4,4’-bis(1,2,4-triazole)

Mtpm

mtpm = tetrakis(m-pyridyloxy

CD

Cicular dichroism

methylene)methane

MTT

1-(9-(1H-1,2,4-triazol-1-yl)anthracen-10yl)-1H-1,2,4-triazole

3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide

Imid

3-bis(4-carboxy-2,6-dimethylphenyl)-1Himidazolium

di-2-pyrazinylmethane

bpe

1,2-bis(4-pyridyl)ethane

bipy

4-4’-bipyridine

POM

polyoxometalates

RH

Relative Humidity

NHC NOTT

N-heterocycliccarbenes

University of Nottingham

Proton Conduction

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PC

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EDTPN

OG

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Introduction

Orange gelb

Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have emerged as one of the

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most important class of materials simply because of their plethoric applications in the fields of gas storage [1-4], separation [5-8], catalysis [9-11], drug delivery [12-14], sensing [15-19], conduction [20-21], ionexchange [22-26] and photonics [27,28]. Build from an organic linker and metal node or clusters, MOFs

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give rise to a wide variety of architectures which range from one-dimensional coordination polymers (CPs) to three dimensional (3D) PCPs [29]. The large repertoire of organic struts available in nature

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allows for tailor-made syntheses of MOFs which could be used for targeted applications making them one of the most sought materials for chemists and material chemists. The pore structure and shape can be tuned in view of a specific property allowing fabrication of rationally designed MOFs [30,31] and thus making these materials as an exclusive class of materials which has opened a new domain in the field of research since its discovery in the late 90’s [32]. Most of the MOFs reported till date are electrically neutral because the positive charge of the metal ion are satisfied by the negatively charged organic ligands mostly carboxylate based linkers [33, 34]. However in some cases MOFs contain some residual charge as non-framework ions either positive or negative making them ionic [35, 36]. Such ionic MOFs (iMOFs), are attracting significant attention because the ions inside the channels of the frameworks can be utilized for specific interactions with

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various incoming guest molecules lading to improved host-gust interactions [37]. These types of ionic MOFs are classified in two types: i) Anionic frameworks in which the framework is anionic thereby resulting in the existence of a counter cation to balance the overall charge of the framework and ii) cationic frameworks where the cationic nature of the backbone results in the necessity of anions (either

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free or weakly coordinated to the metal centre) to neutralize the charge in order to maintain the electrical neutrality. The ions inside such polarized MOFs are often exchanged with other exogenous ions making them a promising candidate to be used in ion exchange resins [38-39]. Moreover presence of charged

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species inside the framework results in specific interactions and these can be effectively used for diverse

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applications.

In this review we have focused in the commonly used strategies for the design and syntheses of these sub class of MOFs i.e. iMOFs. This report gives detailed information of the various ionic MOFs built from

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both positively charged, neutral and negatively charged organic building blocks. We have also discussed the various applications of these ionic MOFs and given an outlook of these MOFs for fabrication and

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improvement needed for these MOFs to be used extensively in chemical industries. 2. Ionic MOFs: Classifications and brief overview

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Ionic MOFs result when the net charge of the framework is mismatched and therefore the need of extra framework ions either positive or negative are present to maintain the overall electrical neutrality. The

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ionic MOFs are mainly divided into two major subclasses a) anionic MOFs: one in which the framework is anionic and extra framework cations balance the overall charge or b) cationic MOFs in which the

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framework is cationic and subsequently requires some negatively charged species to maintain the neutrality. The role of metal ions/clusters and the choice of ligands are very important to develop ionicity in a MOF which will be discussed in the following part of the review. Also, because of charge-induced dipoles created in such a polarized MOF, specific applications which are characteristic of such an ionic framework result, which may be otherwise difficult to obtain in a neutral MOF. 3. Anionic frameworks

3.1. Design principles of anionic frameworks The routes to build up an anionic framework is challenging as it is often accidental. The conventional strategies which lead to the formation of charged frameworks by rational designing of either positively 1

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charged organic linker molecules or charged inorganic clusters are gruesome as they mostly lead to the formation of either cationic frameworks or are difficult to control because of the complexity in their synthetic procedures [40]. In this section we will discuss some of the commonly used strategies to develop anionic MOFs by pre and post-synthetic approaches (Scheme 1).

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(Space for Scheme 1)

It is well known that solvents like DMF and DEF can undergo hydrolysis and subsequent decarbonylation

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in presence of water to form NH2Me2+ or NH2Et2+ cations (Scheme 2) which resulted in the formation of

(Space for Scheme 2)

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anionic frameworks.

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These extra framework cations often play a templating effects during MOF syntheses [41, 42]. Millange and his group used the templating effect of DMA cations to synthesize an anionic framework which contain a regular charge order FeIII/FeII or FeIII 0.5FeII 0.5(OH,F)(O2C–C6H4–CO2) ·0.5DMA [43]. Using

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negatively charged secondary building units (SBUs) in the construction of anionic MOFs represent one of the prime strategies to design anionic frameworks via pre synthetic approach. Various mono, di, tri and

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polynuclear SBUs have been developed till date which resulted in the formation of anionic MOFs [4446]. “Zeolite-like metal-organic frameworks” (ZMOFs) [47] are one of the fascinating examples of anionic frameworks based on such anionic mononuclear SBU i.e. [In(CO2)2N4]-. Using the anionic SBUs

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Eddaoudi and co-workers have synthesized a number of anionic frameworks containing extra cations to balance the charges [48-49]. Bu and co-workers utilized the {In(CO2)4} SBU to design anionic

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frameworks which showed a very high CO2 uptake capacity [50]. A series of isostructural 3D anionic MOFs [Me2NH2]-[M2(bptc)(μ3-OH)(H2O)2] with scu topology based on

tetranuclear butterfly-like

[M4(OH)2] cluster and organic linker molecules were made. Such pre-synthetic approaches for the construction of anionic MOFs is however a challenge as the desired ionicity is not achieved with a little difference in the synthetic conditions [51]. Post-synthetically an anionic MOF synthesis is one of the most cumbersome approaches in the MOF regime. Post-synthetic modifications in MOFs require sufficient amount of porosity in the parent MOF and also inorganic/organic units which contain sites allowing feasible chemical transformation. Long et al. utilized the chemically robust framework of UiO-66 (Zr6O4(OH)4(bdc)6 for post synthetic modification 2

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to form an anionic framework from a neutral framework. In this fascinating piece of work [52] he explained that upon dehydration of the framework coordinatively unsaturated Zr4+ are formed which have also been utilized as Lewis acidic sites. Upon grafting Lithium tert-butoxide (LiOtBu) in the framework the tert-butoxide anion coordinates to the metal centre resulting in the formation of an anionic framework

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and thereby the lithium ions remain trapped in the framework as extra counter anions (Fig. 1). (Space for Fig. 1)

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3.2. Applications

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3.2.1. Adsorption

Rosi et. al. recently demonstrated how post-synthetic modification in an anionic framework of bio-MOF-

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1 (built from Zn(II) and biphenyl dicarboxylic acid and adenine ligands) with molecular formula [Zn8(ad)4(BPDC)6O•2Me2NH2,8DMF,11H2O] {ad= adeninate; BPDC = biphenyldicarboxylate DMF = dimethylformamide} [53] could be used to strategically modify the pore aperture and thereby resulting in

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tuning of the CO2 adsorption of the material. To chemically modify the pore dimensions the authors introduced various organic cations like tetramethylammonium (TMA), tetraethylammonium (TEA), and

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tetrabutylammonium (TBA) via cation exchange to give [email protected] (b), [email protected] (c) and [email protected] (d). To check the effect of these extra framework cations on the porosity N2 adsorption was carried out at 77K. The isotherms though exhibited type-1 nature indicative of its

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microporous nature, the BET surface area were significantly reduced from 1680 m2/g for bio-MOF-1(a) to 830m2/g for [email protected] (d). To check the effect of such pore modifications of the CO2 adsorption

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ability adsorption measurement was performed at 273K with the cation exchanged samples. Interestingly, (b) showed the highest uptake amount of about 4.5 mmol/g at 1 bar, followed by followed by (c) (4.2 mmol/g at 1 bar). The parent framework showed a much less uptake amount of 3.41mmol/g. The uptake amount is a testimonial of the fact that smaller pore apertures may be responsible for higher uptake amount of CO2. At higher temperatures though the CO2 adsorption abilities of the compounds a-d follows the same trend as in 273 K, the sorption ability of d was significantly improved and even comparable to (b) (Fig. 2). (Space for Fig. 2)

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This result illustrated that smaller pore apertures could eventually improve the adsorbate/sorbent interactions leading to higher CO2 adsorption. Framework engineering in the pores of an anionic MOF, [Zn3(TCPT)2(HCOO)][NH2(CH3)2] (SNU-1000) was shown to enhance the isosteric heat, selectivity, and uptake capacity of the CO2 adsorption in the

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MOF [54]. In this interesting piece of work by Suh and co-workers the dimethylammonium included in SNU-100m was post-synthetically exchanged with Li+, Mg2+, Ca2+, Co2+, and Ni2+ ions, which resulted in

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the formation of SNU-100-Li, SNU-100-Mg, SNU-100-Ca, SNU-100-Co, and SNU-100-Ni, respectively. The impregnated metal ions are coordinated with the water molecules inside the porous channels of the temperatures (Fig. 3).

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(Space for Fig. 3)

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anionic framework resulted in higher isosteric heat of adsorption of CO2 and uptake amount at room

The CO2/N2 selectivity was also improved to 40.4–31.0 kJ mol-1 as compared to 29.3 kJ mol-1 and 25.5

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kJ mol-1of the parent MOF. This result for the very first time illustrated both experimentally and theoretically the incorporation of various inorganic metal ions in the pores of the anionic framework could improve the Qst value of CO2 adsorption and could be an important candidate in clean energy applications

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and in carbon capture and sequestration (CCS) technologies. Long et al. was one of the first scientist to explore the field of H2 adsorption modulation by cation

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exchange process in an anionic framework. In an article back in 2006 [55], he first reported the synthesis of a tetrazolate based MOF, Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 (1-Mn2+, H3BTT ) 1,3,5- tris(tetrazol-5-

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yl)benzene). Following up, in 2007 he exploited the ion exchange capabilities of the anionic framework by post-synthetically exchanging the extra Mn2+ with Li+, Cu+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) resulting in isostructural MOFs [56]. The H2 loading in the ion exchanged compounds varied from 2.00 wt.% for 1Cu+ (Cu+ exchanged MOF) to 2.29 wt.% for 1-Ni2+(Ni2exchanged MOF) at 77 and 900 torr. The difference in strength of the adsorbed H2 molecules due framework cations was prominent from the measurements of the isosteric heat of adsorption at zero coverage which reveal a considerable difference. Moreover, 1-Co2+showed a very high initial enthalpy of 10.5 kJ/mol (Fig. 4). (Space for Fig. 4)

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This pioneering work led way to a new domain of research in which postsynthetic modulation of anionic frameworks could improve the hydrogen storage properties in MOFs [57-59]. In 2009, Schrӧder published a fascinating result in which he used two doubly interpenetrated anionic framework [H2ppz][In2(L)2].3.5DMF.5H2O (NOTT-200) and Li1.5[H3O]0.5[In2(L)2]. 11H2O (NOTT-201)

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for H2 adsorption studies [60]. Interestingly (NOTT-201) was obtained by exchange of the H2ppz2+dications by Li+ ions via ion exchange in 1:1 H2O/acetone mixtures (Fig. 5).

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(Space for Fig. 5)

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The crystal structure of NOTT-200 revealed that the mononuclear [In(O2CR)4] nodes are bridged by tetracarboxylate ligand. Three carboxylate groups bind to each In(III) node in a bidentate fashion, whereas one carboxylate group binds in a monodentate fashion leaving one free O atom which is hydrogen bonded

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to the H2ppz2+dication. The crystal structure of NOTT-201 showed a similar a doubly interpenetrated 4,4connected diamond-type framework as of framework NOTT-200 only that the bulky H2ppz2+ were

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replaced by the much smaller Li+ ions thereby modulating the framework porosity. N2 adsorption of both the compounds at 77K revealed much higher uptake of NOTT-201 as compared to NOTT-200 due to

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replacement of the H2ppz2+ by Li+ ions (Fig. 6).

(Space for Fig. 6)

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The N2 adsorption isotherm of the compound NOTT-200 showed a hysteretic behavior whereas full reversibility and no hysteresis was observed in case of NOTT-201. Similar trend was observed in case of

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H2 adsorption too. Such significant difference in sorption behavior for NOTT-200 and NOTT-201 was attributed by the authors to a kinetic trap created by the bulky H2ppz2+dications in NOTT-200 which acts as a reversible gate that controls the access and release of N2 into and from the channel containing the organic cation resulting in hysteretic behavior. The hysteretic nature was tuned by post-synthetic modulation of the host framework. The H2 uptakes at 1.0 bar for NOTT-200 and NOTT-201 are 0.96 wt% and 1.02 wt%, respectively (Fig. 7).

(Space for Fig. 7) Interestingly, the N2 isotherms show that NOTT-201 had an uptake amount twice to that of NOTT-200. The rationale behind such an observation was correlated to the fact that the different kinetic diameters of 5

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H2 (2.89Å) and N2 (2.99 Å) molecules resulted in easy diffusion of the H2 molecules which was not possible in case of N2 molecules. One of the most intriguing feature of this work was that the nondissociative bonding interactions of NOTT-201 with H2 molecules (confirmed by DFT studies) resulted in higher value of isosteric heat of adsorption of 10.1 kJ mol-1 for H2 as compared to 9 kJ mol-1 for NOTT-

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200. In summary these work presented for the very first time how the hysteretic adsorption for H2 gas can be tuned by cation exchange process.

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3.2.2. Catalysis

Though still at the grass root level, MOFs may prove to be the most sought materials for applications in

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catalysis employed in industries [61]. The pre-requisite for optimization of specific catalytic applications is fulfilled by MOFs due to their highly ordered crystalline nature [62], uniform distribution, controllable

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active sites [63] within the framework and no limitation of the pore sizes. Also because of the high percentage of the high metal content of MOFs, catalysis is one field that draws attention of people from all facets, be it organic synthesis or material chemistry. One of the key aspects of designing the MOFs

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which can be potentially useful in the field of catalysis is by post synthetic modification resulting in chemical alterations in the framework [64]. Thus by judiciously choosing an appropriate MOF capable of

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such versatile behavior, a route to design catalytically active MOFs can be achieved (Table 1). Anionic MOFs thus becomes crucial in this respect because of the presence of extra framework cations which can be exchanged post synthetically with catalytically active cationic species or sometimes direct

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activity [65].

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impregnation of active cationic species during the synthetic procedure may result in significant catalytic

In the year 2008, Eddaoudi and co-workers demonstrated the utilization of In based anionic rho-ZMOF [66] with which acts as a host for large molecules like metalloporphyrins which are found to be catalytically active. The free-basic porphyrin moiety was first readily accommodated in the rho-ZMOF which was then readily metallated and post-synthetically modified by various transition metal ions to produce a wide range of encapsulated metalloporphyrins. Hydrocarbon oxidation was performed in the presence of Mn-RTMPyP (5,10,15,20-tetrakis(1-methyl- 4-pyridinio)porphyrin tetra(p-toluenesulfonate encapsulated in rho-ZMOF) to assess catalytic activity for cyclohexane oxidation (Fig. 8). (Space for Fig. 8) 6

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More recently Sanford et al. reported an interesting procedure to encapsulate transition metal catalysts inside the MOFs by simply exchanging the extra framework cations in ZJU-28 [67]. The DMA cations present in the structure were replaced by different transition metal catalysts by treatment of ZJU-28 with N,N-dimethylformamide

(DMF)

solutions

of

[Pd(CH3CN)4][BF4]2,

[FeCp(CO)2(THF)]BF4],

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[Ir(COD)(PCy3)(py)]PF6, [Rh(dppe)(COD)]BF4, and [Ru(Cp*)(CH3CN)3]-OTf to yield ZJU-28-1a−1e respectively (Figure 9) .

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(Space for Figure 9)

Using the catalyst ZJU-28-1d complete hydrogenation of 1-octene to n-octane was achieved within 48 hrs

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(Fig. 10a).

The catalytic performance was even better than the homogeneous counterpart i.e [Rh(dppe)(COD)]BF4

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(1d) . In addition, ZJU-28-1d can be recycled till four times with no significant leaching in the catalysts TON at ambient conditions (Fig. 10b).

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(Space for Fig. 10)

The epoxidation of styrene by [email protected] [68], with a conversion of 72% (styrene epoxide

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selectivity: 65%) was also observed by Ma and co-workers observed after 16 hrs reaction at 60 °C. The corresponding homogeneous counterpart i.e. Co-Pc, in comparison exhibits a much lower conversion of

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38% (styrene epoxide selectivity: 60%) under the same reaction conditions.

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catalytic reaction involved.

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Table 1: Catalytic conversion in % and TON (Turnover number) of some known i-MOFs along with the

3.2.3. Non-linear optics (NLO)

Over the past two decades, significant attention have been given to establish second-order NLO materials by rational designing [69]. In view of the plethoric applications of NLO materials in photonics [70], fabrication of new materials which can overcome the disadvantages of the organic chromophore [71, 72] as NLO materials is absolutely crucial. MOFs have been a key candidate in this respect because of the wide variety of organic ligands available and the coordination bonds of inorganic struts with such ligands lead to a wide variety of structures. The tuneable pore size/shape of the MOF structures and the highly directional nature of the coordination bonds can deal with the unfavorable centrosymmetric dipole-dipole interactions. Ionic MOFs score over the conventional neutral MOFs in this aspect because the extra 8

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framework cations can often be exchanged with other inorganic or organic cations to tune the SHG activity. The important pre-requisite for exhibiting NLO activity i.e. being non-centrosymmetric [73-75] can be achieved by carefully choosing an acentric MOF or otherwise by incorporation of chromophoric molecules with acentric orientation in the highly ordered MOF cavities.

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Cui et al. [76] reported an anionic octupolar 3D complex [(H2NMe2)2Cd3(C2O4)4]·MeOH·2H2O (Fig. 11a) crystallizing in the acentric space group I43d. The anionic framework which contains [H2NMe2]+ cation showed very high cation exchange capacity. The parent compound showed a powder SHG intensity of

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(Space for Fig. 11)

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150 versus α -quartz, which is about 15 times higher than that standard KH2PO4 (Fig. 11b).

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The open framework with 3553 and 3454 cages showed very efficient cation exchange abilities with NH4+, Na+, and K+. After exchanging the cations in with NH4+ , K+ , and Na+, the SHG intensity changes to 155, 90, and 110, respectively versus α–quartz. The cations were further re-exchanged with the H2NMe2+

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species which again showed a powder intensity of 147 versus α –quartz and is consistent with the intensity of the parent compound. This proved that the effect of low crystallinity had very little effect on

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the NLO properties of the compound. This work reports the first example of tuning of NLO properties of a MOF by cationic guests and leads a way for the fabrication of a whole domain of opto-electronic devices. an

another

exemplary

work

by

Qian

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In

and

co-workers

the

anionic

framework

of

(Me2NH2)3[In3(BTB)4]·12DMF·22H2O (ZJU-28) [77] was utilized to incorporate various cationic

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chromophores organic dyes to tune the NLO activity. By systematically changing the alkyl chain length of the cationic chromophores i.e.4-(4-(diphenylamino)styryl)-1-methylpyridinium (DPASM), 1-butyl-4(4-(diphenylamino)styryl)pyridinium

(DPASB),

4-(4-(diphenylamino)styryl)-1-nonylpyridinium

(DPASN), and 4-(4-(diphenylamino)styryl)-1-dodecylpyridinium (DPASD) the NLO activity can be tuned by controlling the rotational freedom of the included hemi cyanine guest ions and enhances the interactions with the host framework (Fig. 12). (Space for Fig. 12)

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Therefore ZJU-28DPASD showed the highest SHG activity of 18.3 as compared to ZJU-28DPASM, ZJU-28DPASB, ZJU-28DPASN which showed SHG activity of 0.25, 0.28, 3.5 respectively. This feature even enabled the authors to visualize the distribution of the cationic dyes by optical microscopy (Fig. 13).

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(Space for Fig. 13)

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3.2.4. Drug delivery Drug delivery by MOFs is one of

the most promising applications linked directly to mankind. The

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exceptional surface area, bio-degradable nature and tuneable pore shape/size are the key factors which enable MOFs to be used as host for a wide variety of drug molecules for delivery [78]. Rosi et al. utilized

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the ionic nature of bio-MOF-1 to administer a cationic drug i.e. procainamide HCl which is used for the treatment of cardiac arrhythmias [79]. The dimethylammonium cations which reside inside the porous channels of bio-MOF-1 were post-synthetically exchanged with the drug molecules via cation exchange

M

process without significantly altering the crystallinity of the MOF. It was observed that cations of the buffer solutions that mimicked the exogenous cations of the biological fluids could trigger the release of

ed

the drug molecules from within the pores of bio-MOF-1(Fig. 14a). The drug release profile was checked in 0.1M PBS buffer and was monitored by HPLC. The release profile indicated complete release of the drug molecule after 72h (Fig. 14b). In a control experiment the drug loaded MOF in nano pure water

pt

showed a drug release of only about 20% which was attributed to the drug molecules adhered to the

Ac ce

exterior surface of bio-MOF-1.

(Space for Fig. 14)

This proved that the cations of the biological buffer solutions prompted the release of procainamide molecules. Such a controlled drug release by bio-MOF-1 showed for the very first time that the anionic nature of the MOF could be utilized for loading and administering drug molecules and thus could be an efficient tool in biomedical applications. However, more research should be dedicated for designing sufficiently porous anionic MOFs which can be a perfect host for such large cationic drug molecules. 3.2.5. Sensing

10

Page 17 of 114

MOFs have emerged as an exclusive class of materials for chemical sensing in recent years [80-82]. Due to the wide variety of organic bridging ligands available during syntheses fabrication of such materials for tailor made applications in sensing have attracted immense attraction. The ordered crystalline nature and well defined channel structures in MOFs allows significant host guest interaction with the incoming

ip t

analyte molecule. Moreover, the pre-concentration effect which is an important factor in such sensing application [83] is easily achieved in MOFs due to their highly porous nature and regularity in framework structures. Sensing by MOFs are mostly achieved due to the specific host-guest interactions, favorable

cr

hydrogen bonding interactions of the analyte with the host framework [84] and also by selective encapsulation of specific guests molecules inside the porous channels of the MOFs,

resulting in

us

significant signal transduction. The analytes which are sensed by MOFs are often neutral guest molecules which are a result of molecular recognition as demonstrated by Kitagawa and co-workers [85] and Morris

an

et al. [86]. More recently anionic MOFs have added a new dimension in such sensing application. Because of the advantage of exchangeable counter cations in the framework selective sensing of cationic

M

species like metal cations or other exogenous cationic species is possible in such charged frameworks. In a pioneering work by Rosi et al. he utilized bio-MOF-1 for sensing various visible and NIR-emitting lanthanide cations [87]. By simply exchanging the DMA cations present in the porous channels of the

ed

anionic framework by several lanthanide metal ions resulted in the formation of [email protected], [email protected], [email protected], and [email protected] (Fig. 15).The cation loaded samples

pt

excited in a UV lamp (365 nm), even show a visual chromic emission with their unique colors (Eu3+, red; Tb3+, green; Sm3+, orange-pink). Aqueous phase detection of such lanthanide ions especially NIR emitting

Ac ce

lanthanide like Yb3+ showed for the very first time sensitization of lanthanide metal ions using the antennae effect of the MOFs scaffold. (Space for Fig. 15)

Interestingly, for Yb3+, either a phonon-assisted energy transfer from the triplet state or a metal-to-ligand charge transfer state of the sensitizer or otherwise an internal double-electron transfer mechanism was attributed to the naked eye detection of such NIR lanthanide ions. Ma et al. utilized the same anionic nature of bio-MOF-1 to exchange the DMA cations with Co2+, Ni2+ and Cu2+ to form [email protected], [email protected] and [email protected] respectively. The "de novo" (ship-in-a-bottle) assembly strategy was further utilized for encapsulation of functional guest molecules i.e. 1,2-dicyanobenzene inside the 11

Page 18 of 114

porous channels of bio-MOF-1(Scheme 3) resulting in the formation of [email protected], [email protected] and [email protected] (Space for Scheme 3)

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The complete formation of the metal(II) phthalocyanine complex inside bio-MOF-1 formation was characterized by UV-Vis absorption spectroscopy which revealed strong absorption band: Co-Pc at 386

cr

(B-band), 618 (Q-band), and 681 nm (Q-band). The completion of the self assembly was further characterized by FT-IR, X-ray photoelectron spectroscopy (XPS) and mass spectrometry (MS) studies.

also observed (Fig. 16).

an

(Space for Fig. 16)

us

Moreover a visual color change from pink ([email protected]) to deep green ([email protected]) was

This metal cation directed assembly of functionalized guest molecules inside the framework represents a

M

versatile method of encapsulation of organometallic complexes inside the MOF. The catalytic activity of the [email protected] has been discussed previously in this review article.

ed

Recently our group exploited the hydrolytically stable characteristic of bio-MOF-1 for selective and sensitive detection of 2,4,6-Trinitrophenol (TNP) in aqueous medium [88]. Picric acid which is considered to be one of the most deadliest explosive used in chemical warfare also has a mutagenic

pt

activity which therefore makes it an environmental pollutant too [89]. Therefore selective detection of

Ac ce

TNP is vital and considerable attention has been devoted for its detection methods [90]. On the other hand bio-MOF-1 because of its highly porous nature, water stable nature and highly luminescent core is one of the best candidates to be used as a fluorescent probe for sensing applications. Upon incremental addition of TNP up to 10-3 M (200 μL) almost 93% quenching was observed whereas negligible quenching was observed for other nitro analytes like TNT, RDX, DMNB, NM, 2,4-DNT and 2,6-DNT (Fig. 17). (Space for Fig. 17)

The mechanism of such quenching was attributed to the combined influence of electron transfer and energy transfer processes which are well known for quenching mechanism. Moreover aqueous phase

12

Page 19 of 114

detection using easy to handle fluorometric methods could render the possiblity of fabrication of devices for real time sensing applications in the near future too. In a recent report [91], Maji and co-workers synthesized a Mg-based anionic MOF bearing uncoordinated DMA cations. Owing to the liberty of porous channels, the authors exchanged the free cation with several

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transition metal & lanthanide cations. Among the first row transition metals, the authors observed selective high capture of Cu(II) ions which lead to significant quenching of the luminescence in the parent

cr

compound. In the latter case, although few lanthanide ions were exchanged, sensitization of Eu(III) was observed in a selective manner leading to red emission.

us

3D lanthanide-based luminescent anionic MOFs of {K5[Ln5(IDC)4(ox)4]}n.(20H2O)n [Ln= Gd (1),Tb (2), and Dy (3)] was reported by Lu et al. [92] which contains K+ ions as the extra counter anions. The extra

an

framework cations of 2 were post-synthetically exchanged with various cations like Na+, NH4+, Mg2+, Sr2+, Ba2+,Zn2+, Cd2+, Hg2+, and Pb2+ cations. The luminescent intensity either showed a negligible change or showed minor increase upon addition of 3 equivalent of the corresponding metal cations. However,

M

interestingly for Ca2+ the luminescence intensity of 2 was significantly increased almost upto twice as much as that without the addition of Ca2+ ions. The reason put forward by the authors was that due to ligand (S1)

ligand (T1)

ed

greater binding. In addition to the strong interactions between Ca2+ and ox2-the process energy transfer Ln* energy flow resulted in a more efficient intramolecular energy transition

Ac ce

selective sensing of Ca2+ ions.

pt

from ox2- to Tb(III). This work reported the first example of a luminescent lanthanide based MOF for

(Space for Fig. 18)

Recently some MOFs have also shown pH dependent fluorescence changes and have shown potential to be used as an efficient pH sensor [93-94]. On a broader perspective although these MOFs are not ionic inherently, but have specific sites in the organic parts which respond specifically to acidic and basic environments. In one of the report by Bradshaw and co-workers, he utilized UiO-66-NH2 [UiO= University of Oslo] composed of Zr4+ and 2-aminoterephtalic acid for post-synthetic modification to form UiO-66-N=N-ind by simple diazotization reaction with 1-methylindole. The modified MOF shows a higher base stability (till pH=12) as compared to other analogues. 3.2.6. Proton conduction 13

Page 20 of 114

Recent advancements of MOFs as a potential candidate for proton conduction have attracted a significant interest to the people working in the field of alternative energy sources [95]. Development of alternate proton exchange materials for practical applications in fuel cells have been one of the most sought area of research in recent years. Nafion which is the most commercialized proton exchange membranes in fuel

ip t

cells business have considerably high production cost, poor conductivity at low humidity i.e. at anhydrous conditions and poor mechanical stability [96]. Therefore for practical applications in clean energy, developments of materials which can overcome the problems of nafion membranes are needed. In this

cr

context, MOFs have been a key candidates to be the next generation proton conducting materials which can be used in fuel cells. MOFs, owing to their designable architecture, high thermal stability along with

us

the added advantage of their crystalline nature lead way to molecular level insight of such proton conduction pathways .Anionic MOFs generally contain a cationic residue (formed in-situ) which acts as

an

proton source and this in conjunction with either water molecules or by a proton carrier helps in the conduction of protons. In a pioneering work from our group [97], a 3D anionic MOF was synthesized which conducts proton in both hydrous and anhydrous conditions. Following a similar synthetic route as

M

mentioned by Zhang et. al. [98] new Zn based 3D anionic MOF was developed. The MOF consists of an anionic framework based on Zinc and oxalate units {[Zn (ox3)2-]} and a counter cationic net of DMA

ed

cations and disordered sulfate anions which are extensively hydrogen bonded to each other and occupy the 3D channels of the compound to form a supramolecular cationic node. Since H-bonding interactions play an important role in proton conduction pathways and presence of significant h-bonding interactions

pt

between the DMA cations and the sulfate anions were observed by the authors from the crystal structure proton conduction of the compound was measured under both hydrous and anhydrous conditions. The

Ac ce

compound shows a proton conduction of 710-5 Scm-1at ambient temperature. However, with increasing temperatures the value increases to as high as 110-4 Scm-1. Further, a time dependent PC measurement at high temperatures (150˚C) shows a negligible loss in conduction even after 12hrs. This illustrates that owing to the high mechanical stability and durability of the material it can be used as a proton exchange membrane. The low activation energy of 0.129e.v indicated Grothuss mechanism is operational. Under varying humidity the compound shows a conductivity of 4.410-5 Scm-1 at 30% R.H and the value increases drastically to 4.210-2Scm-1at 98% R.H (Fig. 19). (Space for Fig. 19) 14

Page 21 of 114

This high value was attributed by the authors to be due presence of acid-base pairs units. The hydrophilic DMA cations provide exchangeable protons in aqueous medium. At high R.H the H2O molecules can help to transfer protons to the DMA cations either by grothuss mechanism or in the form of vehicle mechanism. Notably this MOF showed for the very first time high proton conductivity at both hydrous

ip t

and anhydrous conditions. Interestingly the water assisted conductivity is one of the highest reported in MOFs and even compared to nafion.

cr

Kitagawa et al. [99] reported another strategy to get high proton conduction in anionic frameworks. The anionic framework of [Zn (ox3)2-] contains counter cations (NH4+) which serves as a proton source.

us

Moreover adipic acid was introduced in the framework strategically to improve the proton conduction value of the framework. The compound (NH4)2(adp)[Zn2(ox)3] ·3H2O showed a very high water assisted proton conduction value of 8 × 10-3 S cm-1 at 25 °C. The mechanism of proton conduction of the

an

compound was expected to be by Grotthus mechanism,i.e by forming hydrogen bonds between H3O+ and H2O molecules. The high Ea value of 0.63 eV (Fig. 20) was attributed to the high carrier concentration of

M

(Space for Fig. 20)

the included NH4+ cations and the acidic acid which was included in the framework. This work was the

ed

first example of MOF showing superprotonic conductivity at ambient temperature. In an another interesting piece of work, Kitagawa and co-workers [100] showed how the proton

pt

conductivity can be tuned in an oxalate bridged layered MOF {NR3(CH2COOH)}[MCr(ox)3] (R = Me, Et, Bu, and M = Mn, Fe) comprised of a cationic component of carboxylic acid groups that acts as the proton

Ac ce

carriers. The hydrophobicity of the cationic ions depends on the bulkiness of the residue, and the order of hydrophilicity is, {NMe3(CH2COOH)}+> {NEt3(CH2COOH)}+> {NBu3 (CH2COOH)}+. The conductivity value at ambient temperature and low humidity follows the trend of hydrophilicity i.e Me-FeCr [0.8 ×10−4 S cm−1 (65% RH)] >Et-MnCr [1 × 10−7 S cm−1 (65%)] >Bu-FeCr [2 × 10−11 S cm−1 (60%)]>Bu-MnCr [0.8 ×10−11 S cm−1(60%)] (Fig. 21). The interlayer hydrophilicity associated with the presence of the (Space for Fig. 21) cationic counterparts influenced the adsorption of water which in turn was responsible for the high value of proton conduction at such low humidity. 15

Page 22 of 114

4. Cationic frameworks 4.1. Design principles and structural features of cationic frameworks A cationic MOF is mainly comprised of positively charged framework with extra counter anions to

ip t

balance the charge of the overall framework [101]. The extra anions often lie free in the porous channels of the framework or often are weakly coordinated to the metal center via weak electrostatic attraction. The

cr

routes for rational design of a cationic framework are summarized in Scheme 4. (Space for scheme 4)

us

The first method involves the usage of a neutral ligand mostly nitrogen donor ligands (N-donor ligands) for the construction of a MOF resulting in the necessity of extra anions to suffice the net charge of the

an

framework [102]. The most commonly used nitrogen based ligand used for syntheses of cationic MOF is undoubtedly 4-4’-bipyridine [103-106]. Pyridyl functionalized ligands therefore represent the most easy and comprehensively used ligand as a part of a strategy to build up a cationic MOF. Since the very early

M

report by Yaghi and his group where he demonstrated the construction a MOF by using bpy ligands to build an interpenetrated 3D MOF in which the free nitrate anions reside in the porous channels of the

ed

framework, several cationic MOFs have been reported which reports the role of the neutral ligands in the construction of a cationic MOF. Groy et al. also synthesized a non-interpenetrated railroad like network consisting of Ni II cations and bpy ligands with molecular formula [Ni(4,4’-bpy)2.5(H2O)2(ClO4)2·1.5(4,4’-

pt

bpy) ·2H2O]n in which the free ClO4- anions reside in the pores of the framework [107]. Kim and his group members prepared a 2D MOF in which he prepared the N-donor ligand by threading CB[6] with

Ac ce

N,N'-bis(3-pyridylmethyl)-1,5-diaminopentane dihydronitrate and thereafter conjunction with Cu(NO3)2 resulted in the formation a cationic framework in which the free nitrate anions were post-synthetically exchanged with other anions to study the anion exchange behavior [108]. Rheingold and his group came up with 1,3-bis(4-pyridyl)propane (bpp) ligand to build up two novel cationic MOF with molecular formula [Ag(bpp)]ClO4 and [Ag(bpp)]PF6 in which the anions are free and was studied for further anion exchange behavior [109]. In 2003, Tang reported a series of cationic frameworks using N-pyridyl ligands like 1,3,5-tris(1-imidazolyl)benzene and 1,3-bis(1-imidazolyl)-5-(imidazol-1-ylmethyl)benzene [110]. He explained that the structures obtained using these tripodal ligands were mainly governed by the nature of organic linkers used in the syntheses of MOFs. He also remarked that the counter anions used in the 16

Page 23 of 114

synthetic scheme are responsible in the formation of the frameworks. Lee and co-workers [111] used tripyridyl tri-amides with C3-symmetry for the construction of 3D hydrogen bonded cationic frameworks. Anion exchange was also studied with these cationic frameworks which exhibited luminescent properties at room temperature. Biradha and co-workers also reported 2D open (4,4) networks in which again the

ip t

pyridyl ligands were used to build cationic frameworks [112]. Kitagawa et al. used 1,3,5-Benzene tricarboxylic acid Tris[N-(4-pyridyl)amide] (4-btapa) ligand to synthesize a novel three-dimensional coordination network [113]. He also explained that when nitrate metal salts were introduced in the

cr

preparatory stages weaker participation of the anions led to a porous cationic framework whereas when chloride salts were used stronger interaction of the anions with the amide moieties in the framework

us

resulted in a non-porous structure. Thus the choice of anions also played a crucial role in governing the overall structure of the framework.

an

Another set of ligands that are widely used in the construction of cationic frameworks involves the use of cationic imidazolium group in the ligand backbone [114]. This strategy has been utilized by several

M

groups to generate positively charged framework. In an one such example Hupp and co-workers utilized 1,3-bis(2,6-dimethyl-3,5-dicarboxylphenyl)imidazolium ligand to synthesize cationic framework with molecular formula [Cu2(C23H17N2O4)-(DMSO)2][Cl] [115]. The structure was made up of paddle wheel

ed

Cu2(CO2)4 clusters and four imidazolium ligands. Wu et al. mixed 1,1’-methylenebis-(3-(4carboxyphenyl)-imidazol-3-ium) with CuCl2 to get a paddle wheel structure which contained a net

pt

positive charge and therefore requiring the presence of free nitrate anions. Some other examples include the utilization of pyrimidine-2-carboxylate acid to build up a novel cationic framework in which free

Ac ce

hydroxide anions were present to balance the net charge. Post synthetically a cationic MOF can be synthesized as explained by Kim et al. A carboxylate based MOF with molecular formula [Zn3(3-O)(1-H)6] ·2H3O·12H2O (d-POST-1) [116] was synthesized in which the free pyridyl nitrogen atom were aligned inside the pores of the framework. When a suspension of POST-1 in N,N’-dimethylformamide (DMF) with stirred in excess iodomethane at room temperature for 2 h results in the the formation of the N-alkylated product resulting in the formation of a cationic framework and thereby the free iodide ions were found as the charge balancing anions. Bu et al. demonstrated a general method to create a cationic MOF from a neutral framework [117]. In this pioneering work they demonstrated that the differential affinity between distinct metal ions such as Al3+ 17

Page 24 of 114

with framework anionic species could strip the anions that are coordinated to the metal center such as the F- ions in Cr-MIL. AlCl3 was used to carry out this post synthetic modification in which the F- were extracted and subsequently the Cl- ions were left free inside the pores of the framework allowing facile exchange with other anions like OH- ions.

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4.2. Applications

cr

4.2.1. Sensing

The wide choice of metal and ligands used in the synthesis of MOFs bestow ample opportunities to

us

modulate the opto-electronic properties of such porous networked structures [118]. Cationic frameworks by design permit the encapsulation of anionic receptors and thereby make it feasible to investigate noncovalent interactions like anion-pi, in confined environments. Large porous cationic frameworks with

an

luminescence accessory can, therefore, be used as probes for detection of non-polarised molecules upon reports of such systems are discussed herewith.

M

their interaction with the host framework through physical adsorption or non-covalent interactions. Some

In an interesting report, Wang et al. [119] synthesized a rare example of a cationic framework built on a

ed

tetrapyridyl based ligand bearing carboxylate anions uncoordinated to the metal canters. The authors have reported a novel and effective route for synthesis of C-C bond forming reaction in pursuit of constructing tetrapodal pyridyl fluorophores. The emission features of the discrete organic compounds have been

Ac ce

pt

investigated thoroughly (Fig. 22)

(Space for Fig. 22)

and developed a scheme to incorporate these properties into a nanoporous polymer. The MOF is centred on Zn(II) metal nodes in tetrahedral geometry with phosphate ions bridged between two metal ions. The framework assembles a supramolecular chain between the ligand and free carboxylate anions, which render near-white light emission properties to the compound. This work provides a new route to construct luminescent cationic MOFs with large porous cavities. Zhang and co-workers employed a flexible porous MOF for sensing solvent vapours. The authors synthesized a cationic framework from a multidentate neutral ligand and using Zn(II) salt. The obtained compound had two zinc centres with an octahedral and a tetrahedral geometry. The authors observed the 18

Page 25 of 114

deprotonation of one of the imidazole fragment in the ligand and the presence of free OH- ions in the porous channels to balance the electrical neutrality. Upon desolvation, the compound was found to undergo a transition to a pseudo-amorphous phase which could be recovered upon resolvation. Also the authors found that this structural transition was accompanied with drastic changes in the luminescence

ip t

profiles. Further investigation of this photoluminescence response was carried by exposing the compound to benzene, water, methanol, ethanol and nitrobenzene. Different solvent vapours altered the emission of the compound drastically with benzene and water shifting the λmax by more than 50 nm. The authors

cr

investigated the mechanism comprehensively and attributed the role of the deprotonated ligand in the observed emission spectra. Excitingly, luminescence changes were observed at low pressure CO2 (Fig.

an

(Space for Fig. 23)

us

23) adsorption and could be a significant advancement in the progress of vapour phase sensors.

4.2.1a. Luminescence

M

Among most sensory techniques, photoluminescence has been one of the most sought after owing to lack of sophistication, ease of sampling, high sensitivity and prompt response [120]. In the MOF regime, it has been used kindly owing to ample opportunities of tuning the different metal-ligand based emissions to suit

ed

the desired electronic band-gaps. Additionally the wide choices of building units makes MOFs as appropriate candidates for luminescence based transducers for understanding molecular behavior in

pt

confined nanospaces [121-122]. The features of photoluminescence based techniques have been exploited recently in cationic frameworks as a probe for anion capture/exchange processes. Some of them have been

Ac ce

discussed herewith.

In a early work of exploring luminescence features in a cationic framework, Tzeng et al. [123] synthesized a Zn(II) centred amide based cationic coordination polymer. Upon heating the two coordinated water molecules were lost allowing the anions to interact with the metal directly. Phase transformations were observed from a neutral framework bearing coordinated perchlorate ions to a cationic framework upon exposure to moisture. Control single crystal experiments were performed in presence of dry solvents was performed to prove this transformation. Solid state luminescence experiments were performed to monitor this phase transformation (Figure 24). (Space for Figure 24) 19

Page 26 of 114

As a continuation, Oliver and co-workers [124] prepared 3 distinct Cd(II) MOFs from a mixture of anionic and neutral ligands under hydrothermal conditions. Owing to different molar ratios of the reactants 3 dissimilar structures were achieved, with two of them being cationic in nature. The authors attempted to exchange the free organosulfonate ions with nitrate, permanganate, perchlorate and

ip t

perrhanate anions but found that alongwith the electrostatic interactions, strong hydrogen bond interaction with the cationic framework precluded any exchange. The separate character of the 3

cr

compounds was reflected and studied via solid state luminescence measurements (Figure 25). (Space for Figure 25)

us

Previously, in a comprehensive work we have shown the utility of a luminescent cationic framework as a probe to sense different anions by a detectable fluorescence signal. A dynamic 1D porous coordination

an

polymer constructed from Zn(II) cations and a neutral N-donor ligand had free nitrate anions in its channels and methanol molecules coordinated to the metal centre [125]. Upon air-drying these coordinated solvent molecules were substituted with moisture and correspondingly structural changes

M

were observed. This flexible behavior of the MOF was extended to its anion exchange studies by ClO4-(Scheme 5).

ed

replacing the NO3- anions with guest anions of different coordinating tendencies like N3-, N(CN)2-, SCN-,

(Space for scheme 5)

pt

The compound underwent significant changes in its structure which were attributed to the varied size and geometry of the replacing anion. Competitive ion exchange experiments were performed to understand

Ac ce

the affinity order of the chosen ions. The notable differences in the solid state luminescence of the ligand and the MOF propelled us to examine this as the pathway to recognize the anion exchange process. Thereby, the solid-state luminescence spectra of all the anion exchanged phases were recorded and a remarkable variation was observed. The differences in the observed intensities were correlated to the coordinating attributes of the anions and their interactions with the metal centre affecting the inter-ligand transitions. This report demonstrated anion-responsive luminescence in a MOF for the first occasion and serves as a significant juncture in the development of MOF based ion sensors.

20

Page 27 of 114

In a recent contribution, we have exhibited a Zn (II) based dynamic MOF as an anion responsive luminescent compound [126]. The parent compound shows guest dependent dynamic nature and underwent a dimensionality reduction upon loss of mother liquor (Scheme 6).

ip t

(Space for Scheme 6) This phase underwent size selective anion exchange (ClO4-, BF4-) and the different phases were found to

cr

exhibit notable differences in their emission profiles (Fig. 26). (Space for Fig. 26)

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The varied interaction of the anions with the framework lattice has been attributed as the plausible reason for this observation.

an

4.2.1b. Colorimetric sensors

The process of ion recognition can be significantly simpler if the host-guest information is transduced via

M

visual chroma [127]. Cationic networks bearing coloured metal ions or photochemically active ligands can be sought as appropriate candidates for such applications. Anion exchange has been recently explored

ed

in a few cationic MOFs, but reports of MOFs acting as naked eye sensors have been rare [128-129]. This part covers the literature reviews of cationic MOFs exhibiting visual post-synthetic anion exchanging

pt

changes.

In a pioneering work by Dong et al. [130], the efficacy of cationic MOFs as anion receptors in a selective

Ac ce

and detectable manner was explored. The authors synthesized a 2D MOF centred on Cu (II) nodes and pillared on a neutral fluorene based ligand which encapsulated nitrate anions in its voids. The free anions were substituted with foreign ions like F-, Cl-, Br-, I-, SCN- and N3-, the progress of which was characterized by IR, XPS experiments. The authors observed drastic colour change (from blue for the pristine sample to different shades of green and brown) (Fig. 27) (Space for Fig. 27)

with every exchanged anion without perturbing the structural integrity in all the cases. Selectivity experiments were performed to elucidate the ability of the framework to separate anions from a mixture and the results have been attributed to the geometry and coordination affinity of the chosen anions. 21

Page 28 of 114

Zhang and co-workers [131] explored photochromism in a viologen based coordination polymer. A Cd based 1D helical CP was synthesized bearing free perchlorate anions in its coordination space. The authors successfully replaced ClO4- ions with foreign anions like I-, Cl-, N3- and SCN- in a SC-SC manner in a short time. The initially photochromically inactive compound was found to be photochemically active

ip t

after the exchange process, owing to the structural changes occurring upon anion exchange. The different charge transfer (CT) complexes between the electron accepting viologen moieties and pseudohalide ions

cr

have been attributed to the multivariate colouration to the various exchanged phases (Fig. 28). (Space for Fig. 28)

us

Upon irradiation, the different phases were found to exhibit different hues originating due to the differential radical generating reactions and this process was found to be reversible.

an

On similar lines to the report by Dong et al., Bu and co-workers [132] synthesized a cationic framework bearing free nitrate ions in its channels and utilized the same as a colorimetric probe for several guest

M

anions. A 4-fold novel topological network was synthesized by the in-situ reduction of Cu(II) to Cu(I) under crystallization conditions. The occluded methanol molecules were replaced with water molecules on heating and the resultant phase was found to be air and moisture stable. On exchanging the nitrate ions

ed

with different anions (F-, Cl-, Br-, I-, N3-, SCN-, CO32-) in aqueous phase, remarkable visual changes to the colour of the parent compound were observed (Fig. 29).

pt

(Space for Fig. 29)

Ac ce

Exploiting the luminescence feature bestowed by the metal centre, the authors also recorded varying intensities of the different phases by the solid state luminescence experiments. Recently, we have demonstrated a multi-functional Cu (II) MOF [133] serving as a colorimetric SCNsensor. The MOF synthesized solvothermally, included weakly coordinated nitrate ions in its porous channels. The dark blue coloured pristine phase of the compound turned to green upon substitution with SCN- ions (Fig. 30)

(Space for Fig. 30)

22

Page 29 of 114

and this drastic change is attributed to the Jahn-Teller distortion associated with Cu (II) complexes. The exchange process was associated with structural changes from a porous framework to a non-porous one as monitored from SC-SC experiments.

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4.2.2. Magnetism More recently development of magnetic materials which can show long range magnetic ordering have attained significant attention due to potential applications in the field of sensors and molecular spintronics

cr

[134]. Since the development the molecular compounds for spontaneous magnetization in the late 80’s several organic, inorganic and organic-inorganic hybrid materials have been reported which undergoes

us

interesting magnetic behavior [135]. Though still at its infancy MOFs have promised to represent the next generation smart materials to be used as functional molecular devices. Because of the long range ordering

an

in MOFs and precise structural determination a molecular level insight in the magnetic properties of the MOFs can be well correlated. Moreover, dynamic MOFs score over the other conventional rigid MOFs in this aspect because of the guest induced structural changes are accompanied by the changes in the

M

magnetic properties of these materials. Cationic MOFs which are the most commonly and comprehensively used dynamic systems in the regime of MOFs.

ed

A novel lanthanide based SIM (single ion magnets) of a 3D assembly with molecular formula [Ln(bipyNO)4](TfO)3·x solvent was reported by Espallargas et al [136]. In this pioneering work he

pt

utilized the square-antiprismatic coordination environment around the lanthanum centers suitable for SIM behaviour. Slow magnetic relaxation was observed in magnetic measurements which was typical of SIMs.

Ac ce

The triflate anions were also exchanged with the bulky polyoxometalates (POMs) without interfering with the magnetic relaxation behavior (Fig. 31). (Space for Fig. 31)

Hong and his co-workers [137] introduced flexibility inside the cationic framework using a flexible linker molecule i.e. N,N’-p-phenylenedimethylenebis(pyridin-4-one) (p-XBP4). The resulting flexible coordination polymer which was denoted as [Co(N3) (p-XBP4) (H2O)2]·(N3) showed guest dependent color and magnetic changes (Fig. 32) upon desolvation. Even the crystallinity was lost resulting in the (Space for Fig. 32) 23

Page 30 of 114

formation of an amorphous phase which showed a distinct change in the magnetic behaviour as compared to the parent compound. 4.2.3. Structural flexibility driven properties

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Structural flexibility in cationic frameworks marks them as an important class of materials for solid state structural changes in response to any physical or chemical stimuli [138-141]. It is a very well established phenomenon that guest molecules inside the porous cavities of the MOF play a key role in structural

cr

modulation. Cationic MOFs are perhaps the best manifestation of such flexible frameworks as more often than not the extra framework anions are responsible for regulating the structural tuning in the domain of

us

porous frameworks [142]. The free anions inside the channels of the cationic MOFs are weakly hydrogen bonded to the framework lattice and subsequent anion exchange may often induce drastic structural

an

changes in the framework by the virtue of differential interactions of the anions with the framework. Suh et al. showed that ethylenediaminetetrapropionitrile (EDTPN) ligand could be utilized for

M

constructing a series of novel cationic framework based on AgX salts (X= NO3-, CF3SO3-, and ClO4-) [143]. The CF3SO3- anion was quantitatively exchanged with other anions like NO3- and ClO4- anions and this anion exchange reaction led to supramolecular structural transformations in solid state which were

ed

concomitant with the exchange reactions (Scheme 7).

pt

(Space for scheme 7)

Kitagawa et al. prepared a cationic MOF [144] with molecular formula {[Cu(AF6)(4,4’-bpy)2] ·8H2O}n

Ac ce

(A = Si, Ge, and P) which exhibited dynamic structural changes in presence of water to form 2D networks from 3D networks. This 2D interpenetrated network allowed unique anion exchange capability and more interestingly when PF6- ions were used instead of AF6- anions it served as a framework regulator leading to the formation of various types of coordination frameworks with different topologies (Fig. 33). Thus

(Space for Fig. 33)

framework engineering was achieved depending upon the choice of anions and led to supramolecular structural changes in the frameworks. Recently our group synthesized a novel 3D cationic framework based on a neutral nitrogen donor ligand and CdII metal ions [145]. This framework underwent guest 24

Page 31 of 114

driven structural changes at r.t because the low boiling solvents escape and results in the formation of a 2D network (Fig. 34). (Space for Fig. 34)

ip t

The guest induced structural changes were well demonstrated by SC-SC (Single crystal to single crystal) experiments and were well characterized by SC-XRD studies. The 2D framework which is also cationic in nature then underwent anion exchange behavior in presence of strongly coordinating anions like SCN- and

cr

N3- anions. Kitagawa and his group members used a flexible nitrogen donor ligand [146] resulting in the formation of CuII(mtpm)Cl2·20H2O. The framework contains free chloride ions which could be

us

exchanged with organic tosylate anions via SC-SC experiments. The structural flexibility was also observed during vapor adsorption of alcohols and water and this could be utilized to separate both

an

methanol and water from water−methanol−ethanol mixtures (Fig. 35).

M

(Space for Fig. 35)

Chao et al. represented for the very first time nitrite-dominated in situ N-nitrosation of an amine ligand accompanied via SCSC structural transformation via anion-exchange reaction [147]. Also formation of a

ed

0D loop from a 1D helical array was achieved during anion exchange which was monitored by SC-XRD studies (Fig. 36).

pt

(Space for Fig. 36)

Ac ce

In another piece of work by Zhao [148] showed that molecular capsules could be prepared by using a novel 1-(9-(1H-1,2,4-triazol-1-yl)anthracen-10-yl)-1H-1,2,4-triazole (tatrz) ligand and Copper ions. Using these

combinations

he

synthesized

three

novel

{[Cu(tatrz)2(NO3)2]·(CH3OH)·4H2O}n (1), {[Cu- (tatrz)2(H2O)2](BF4 )2}

cationic n

frameworks

(2) , and [Mn(tatrz)2 ·

(SCN)2(CH3OH)]·2H2O (3). The compound 1 showed solvent and anion dependent structural transformations in the solid state in an irreversible fashion and was further utilized to develop a luminescent probe for Mg2+ cations. On a similar line solvent templated structural dynamism in a porous cationic framework was reported by Sumby et al. in 2012 where he demonstrated a breathing behavior in a MOF based on di-2-pyrazinylmethane (dpzm) and Ag salt [149] (Fig. 37) (Space for Fig. 37) 25

Page 32 of 114

Such a phenomenon was a result of a combined effect of the coordinating nature and the sterics of the solvent used for exchange. SC-SC structural changes were observed with the retention of framework crystallinity and can actually pave way for designing new materials which can constitute a new class of stimuli responsive materials.

ip t

Maji et al. achieved a rare structure bearing sixfold interpenetrated 3D diamondoid packing centered on Cu(I) metal nodes. The BF4- anions used in the reaction were left free in the void spaces due to the

cr

tetrahedral geometry of the metal center. Size dependent reversible anion exchange was observed with the framework retaining its integrity [150] .

us

Tuning of porosity in a flexible cationic framework as a result of anion exchange is an important topic of research. Cationic frameworks are often structurally dynamic and therefore application of any external

an

stimuli often results in a selective sorption behavior or also improvement of uptake amount of gases or vapours [151].

M

Kitagawa et al. showed a bimodal porous functionality in a flexible cationic framework in 2007. In this pioneering work a cationic framework of α-polonium-type based on Ni(II) and bpe (bpe = 1,2-bis(4pyridyl)ethane was synthesized with molecular formula {[Ni(bpe)2(N(CN)2)](N(CN)2)(5H2O)}n which

ed

underwent reversible structural transformation to a dehydrated phase upon guest removal which shows a selective sorption behavior [152]. The guest free phase of the parent compound showed a color change

pt

upon dehydration and therefore could potentially act as a moisture sensor. The dehydrated phase showed a selectivity of CO2 over other gases like N2, O2 and Xe. This was achieved mainly because of the dipole-

Ac ce

quadrupolar interaction of the incoming CO2 gases with the π-electron clouds from the bpe ligands inside the pores of the framework. This selective host guest interaction was also observed in vapor sorption of MeOH, EtOH, H2O and acetone at 298K. The hydrogen bonding interactions of the incoming guest molecules with the framework also resulted in gated type sorption behavior which is typical of such dynamic frameworks. The parent framework which contains counter anions { N(CN)2) − } also exhibited anion exchange properties in which the anions were selectively replaced by N3− anions which resulted in expansion of larger pores of the framework and shrinkage of the smaller pores. The anion exchanged compound also showed a higher uptake amount of CO2 by the virtue of the increase of the permanent porosity of the exchanged compound. Such bifunctionality in a single MOF (Fig. 38)

26

Page 33 of 114

(Space for Fig. 38) makes it a smart material for futuristic applications in ion-exchange resins, sensors and gas storage devices.

ip t

In an novel piece of work by Zhang et al. the cationic azolate framework [Zn7(ip)12](OH)2 (MAF-34) [153] was used to selectively uptake CO2 over N2 at both low and room temperatures (Fig. 39).

cr

(Space for Fig. 39)

The high degree of selectivity of CO2 was attributed to the uncoordinated imidazolate nitrogen donors on

us

the pore surface of the desolvated phase of MAF-34. The higher uptake amount at r.t was ascribed to the framework expansion at higher temperature. At low temperature the presence of anions in the micropores

an

of the framework would correspondingly block the diffusion of the gases inside the framework. In another work by Kitagawa and co-workers [154] utilized the cationic framework of

M

[Zn2(tpa)2(cpb)]·2DMF·H2O (1⊃2DMF·H2O) for methanol adsorption. The adsorption profile reveals distinct hysteretic behavior at P/P0 0.03 and 0.6, respectively (Fig. 40).

ed

(Space for Fig. 40)

cationic pyridinium surface.

pt

The isosteric heat of adsorption was calculated to be 50-95 kJ mol-1 indicative strong interaction with the

Dong et al. [155] fabricated a Ag(I) centred MOF bearing free SbF6- ions in the porous nanotubes. The

Ac ce

large channels can encapsulate C6-C8 cyclic organic compounds and possibility of recyclability was also investigated. The authors utilized this MOF for selective uptake of benzene in a vapour mixture of benzene-cyclohexane-cyclohexene under ambient conditions. This report provides an interesting addition in the development of hydrocarbon separation by using relatively simpler approaches.

4.2.4. Other applications Other than the applications as discussed previously in this review article some applications of cationic frameworks in other areas are also known in literature. For e.g. our group recently reported a one 27

Page 34 of 114

dimensional homochiral compound based on a neutral chelating N-donor 4,4’-(ethane-1,2-diyl)bis(N(pyridin-2-ylmethylene)aniline and zinc metal ions with molecular formula [{Zn(L)(OH2)2}(NO3)2·xG]n denoted as 1NO3− which crystallized in chiral hexagonal space group P6522 [156]. Solid-state CD spectral analysis was performed and the results revealed homochiral nature of the compound. We

ip t

observed from the crystal structure that the helicity was induced by the binding of the N,N-chelating donor to each metal in a syn-conformation and eventually giving rise a helical arrangement to the structure. The 1D helical chain running along the b axis to form hydrogen bonding interactions with free

cr

nitrate ions, coordinated water molecules to form a H-bond-based 3D packing structure. It was observed that each left-handed helix is interwoven by six similar helices and the central one is in H-bonded with six

us

similar helices to form a six fold helical structure. Anion exchange experiment was performed NaN3, KSCN, NaN(CN)2, NaClO4, NaBF4, NaPF6, and NaCF3SO3 to replace the free anions in 1NO3− by the

an

corresponding anions. Bulk phase homochirality as confirmed by the positive Cotton effect in CD (circular dichroism) spectra for compound 1NO3− was retained even in the anion exchanged compounds

M

(Fig. 41).

(Space for Fig. 41)

ed

The parent compound also showed a turn on fluorescence response upon replacement by exogenous ClO4− anions which were denoted as 1 ClO4−. This anion responsive tuneable bulk phase homochirality in CPs

pt

may find important applications enatioselective catalysis and separation . Drug delivery by cationic MOFs also was recently illustrated by Yang et al. in 2014. The MOF used as a

Ac ce

drug carrier was MOF-74-Fe(II) (bearing dihydroxy terephthalic acid ligands) {(Fe(II)2(dobdc), dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate)} which under aerobic condition formed MOF-74-Fe(III) superoxide anions (O2−) as counter ions which underwent dismutation reaction in presence of H2O to form OH



ions [157]. The hydroxide anions were then post-synthetically exchanged with the sodium salt of

ibuprofen which is a well-known drug for anti-inflammatory treatment (Fig. 42). (Space for Fig. 42) The drug load capacity was calculated around 0.21 g for 1 g of MOF-74-Fe(III). PC12 cells were chosen as model cells to evaluate the cytotoxicity effect by MTT assay which revealed that the cells retained their morphology after incubation (Fig. 43) 28

Page 35 of 114

(Space for Fig. 43) with the MOFs and thus making this MOF as an important candidate for drug delivery host. The very low cytotoxicity effect and controlled release of the drug also makes it a promising material for medicinal

ip t

applications. Bharadwaj and co-workers [158] demonstrated that using a cationic framework containing imidazolium

cr

based ditopic ligand and an unknown Zn8O cluster (Fig. 44), proton conduction could be achieved in (Space for Fig. 44)

us

humidified condition. A very high value of 2.3 × 10−3 S cm− 1 at ambient temperature and 95% relative humidity (RH) were observed by the authors which was achieved due to the alignment of the charged

an

imidazolium units along the channels of the MOF. Such a high value and low activation energy was achieved by ligand modification of the ligand moiety and presented a new strategy to synthesize proton

M

conducting MOFs.

Dye encapsulation by cationic frameworks was achieved by Kaskel [159] et al. in which [Cu(Imid)(H2O)] (Imid: Imid: 3-bis(4-carboxy-2,6-dimethylphenyl)-1H-imidazolium was utilized to incorporate Nile blue

ed

within the framework. Such an adsorption process is reversible whereas non reversible incorporation of dyes like Methyl red and fluorescein was also achieved because of the deprotonation of the carboxyl

pt

group of the dyes resulting in the permanent incorporation of these dyes within the cationic framework. The coulombic interaction between the negatively charged dyes and the cationic framework of ITCs was

Ac ce

utilized by Feng et al. [160] to selectively incorporate anionic dyes like (Orange gelb dye) OG2- whereas neutral and cationic dyes failed to enter inside the MOFs. The authors concluded that the exchange dynamics were principally governed by 1) Magnitude of charges on the anionic dyes i.e. -2 and -3 charged species were exchanged at a much faster rate than the uni-negavitely charged dyes and 2) size exclusion i.e. dyes with large size could not be exchanged. Such selective encapsulation of dyes within the cationic framework could lead to the application of such materials in photonics industries. Heterogeneous catalysis by a cationic framework was demonstrated by Wu et al. [161] in which he utilized an imidazolium based MOF in Pd catalysis. Post-synthetic modification in the cationic backbone of imidazolium based ligand of the framework resulted in the coordination of the NHCs (N-heterocyclic 29

Page 36 of 114

carbenes) which binds to the Pd(II) ion when Pd(OAc)2 was reacted with it. The resulting Pd incorporated framework showed an excellent catalytic activity for the Suzuki-Miyaura coupling reactions[162]. Similar modifications in such frameworks also showed catalytic activities towards hydrogenation of alkenes and reduction of nitrobenzene which was carried out at ambient temperature.

ketalization of 2-butanone to form 2-ethyl-2-methyl-[1,3]-dioxolane (Fig. 45)

cr

(Space for Fig. 45)

ip t

Oliver and his group members [163] reported the catalytic activity by SLUG-21 and SLUG-22 in

us

which is a very important synthetic strategy for the protection of keto groups. Both the catalysts could be reused several times with very little change in the catalytic activity and shows no evidence of leaching

an

too.

Zhou and his group members utilized zirconium-porphyrin frameworks, PCN-225 comprised of Zr4+ and tetrakis(4-carboxyphenyl)porphyrin (H2TCPP) ligand [164] which shows a wide range of pH stability

M

from pH=2 to pH=11 and also shows pH dependent fluorometric changes which could lead to MOFs being used as an efficient pH sensor.

ed

5. Ion-Exchange performance in MOFs

Naturally occurring ion exchangers which include both inorganic and organic based ion exchange resins

pt

are limited for widespread use mainly because of their non-controllable pore size, thermal stability, low ion exchange capacities and selectivity. The synthetic inorganic ion exchangers like the zeolites are

Ac ce

considered to be a much improved version of modern ion exchangers because of their porous nature and thermal stability. However low ion specificity and high production costs are the hurdles that need to be addressed for them to be the most comprehensively used materials as ion-exchange membranes or resins. Ionic MOFs are one of the next generation materials that have fulfilled all the pre-requisites for being served as an ion exchange matrix as they have high tuneable pore shape/size, high chemical and mechanical stability and often low production costs. The strong coulombic attractive forces and highly polarizable nature allows facile exchange of ions to a great extent and could render a possibility of them to be used in industries. 5.1. Anion exchange by cationic MOFs 30

Page 37 of 114

Heavy oxo-anions like ReO4-, CrO42- ,TcO4−, Cr2O72- , MnO4−, AsO43− , SeO32− etc. are known to cause serious threats to human health and environment [165]. The U.S. Environmental Protection Agency has listed these metal based oxo-anions in the list of top priority pollutants occurring in nature [166]. The commonly used techniques for separation of such pollutants from waste water involve ion exchange,

ip t

photocatalytic oxidation and adsorption . However, most of these methods are either cost effective or have accessibility issues. The commonly used ion exchangers are mainly divided into i) organic based polymer resins or ii) inorganic based zeolites or layered double hydroxides (LDHs). But the usage for widespread

cr

applications is limited because the poor thermal and chemical stability of the organic polymers and the lack of recyclability and low selectivity for the anions restricts the use of the inorganic based materials.

us

Cationic MOFs play a crucial role in this respect because the readily exchangeable anions which remain free inside the pore can be replaced by these heavy pollutants [167].

an

Oilver et al. reported the application of [Ag2(4,4'-bipy)2(O3SCH2CH2SO3) ·4H2O] (SLUG-21) for the treatment of radioactive waste like permanganate and perrhenate. The cationic unit of Ag-bipy chains, charge balanced by interstitial 1,2-ethanedisulfonate (EDS) anions. Anion exchange performed with

M

aqueous solutions of MnO4− showed a trapping of MnO4− and optical images taken at different time adsorption

ed

intervals (Fig. 46) as high as 94% after 48 hrs as confirmed by UV-vis spectroscopy (Fig. 47) with an (Space for Fig. 46)

pt

(Space for Fig. 47)

capacity of 283 mg/g which is much higher as compared to the uptake capacity of LDHs (10/150 mg/g).

Ac ce

The high performance of SLUG-21 in trapping MnO4− was attributed by the authors to the stability of these anion inside the cationic framework upon structural transformation. The extra stability gained upon replacement of the EDS anions by these oxo-anions is the governing factor for such thermodynamic stability. The exchange capacity of SLUG-21 for ReO4- was also studied by the authors which revealed an adsorption capacity of high 602 mg/g based on the molecular weight of ReO4-. The uncalcined and calcined LDHs are only 37 and 125 mg/g which is much lower than SLUG-21. Thus the authors overcame the problem of low uptake capacity by the hydrotalcites for this two oxo-anions. The exchange rate and capacity of SLUG-21 for other problematic oxo-anions as calculated by the authors follows the order MnO4− > ReO4-> ClO4− > CrO42- > NO3−> CO32-. 31

Page 38 of 114

In another report by Wang

and co-workers [168]

a new [Ag2-(btr)2]·2ClO4·3H2O (ABT·2ClO4 ;

(btr=4,4’-bis(1,2,4-triazole)) which shows capture and separation of Cr2O72- in water. This unique MOF composed of distorted octahedral and tetrahedral cages showed contains ClO4− anions in the porous cavities which were readily exchanged by dichromate anions (Fig. 48).

ip t

(Space for Fig. 48) The adsorption capacity of ABT·2ClO4 for Cr2O72- was calculated to be 0.73 mol/mol after 48 h which precedes the only known lead fluoride material for exchanging dichromate anions. The effect of anion

cr

exchange on luminescence was observed as almost complete luminescence quenching was observed after anion exchange detectable even by naked eye. The reason given by the authors was that due to because

us

the electron-transfer transitions of Cr2O72- which in turn reduces the energy transfer from the btr ligand to Ag+ ions by ligand-to-metal charge transfer.

an

A more recent report by Zhang et al. two cationic MOFs , [Zn2(Tipa)2(OH)]·3NO3·12H2O] (FIR-53) and [Zn(Tipa)]·2NO3·DMF·4H2O] (FIR-54) were synthesized and the exogenous anions i.e. OH



and NO3−

were post synthetically exchanged with Cr2O72-[169] (Fig. 49) .

M

(Space for Fig. 49)

Because of the ease of diffusion of the dichromate anions in FIR-54 showed a high adsorption capacity of

ed

100 mg g−1. Both these two compounds trapped the incoming anion by SC-SC transformation (Fig. 50). (Space for Fig. 50)

pt

In particular, FIR-53 showed unprecedented reversibility behavior without losing its uptake capacity. Zhao et al. synthesized an indium trimer-based cationic framework which could be used as an ion

Ac ce

exchange based platform for separation purposes. Various long chain anionic organic dyes like OG2which are otherwise difficult to separate by size exclusion effect were separated by the cation MOF. Moreover molecules within the range ~100Da
5.2. Cation exchange by anionic MOFs 32

Page 39 of 114

The ion exchange polymer~MOF composite utilized the porous nature of the MOF and the ion exchange capability of the impregnated polymer to perform in even gas phase in which the well-known ion exchange resins fail to perform because of highly shielded active sites. Jiang and his group members [171] used a molecular simulation study to investigate the exchange dynamics of Pb2+ cations in rhoprocesses. Martens and co-workers encapsulated [172]

ip t

ZMOF and thus provided a new opportunity of such anionic MOFs to be used in water purification encapsulated [PW12O40]3-, a Keggin-type

polyoxometalates in HKUST-1 and the charge balancing Cu2+ cations were then exchanged with Eu3+ and

cr

Na+ cations and thus could be an alternative approach to use MOFs as cation exchangers. The molecular level insight of the MOF structures and designable open architectures are the essential features that enable

us

them to perform as ion exchange matrices and may be one of the key candidates to be used in ion

an

separation, radioactive waste detection and purification industries. 6. Conclusion and future prospects

In conclusion we have reviewed the general design principles of the iMOFs i.e. both cationic and anionic

M

MOFs and even explained the applications of these charged frameworks in various fields which would enthrall the people working in the field of material chemistry. The inherent polarizable ions present in

ed

such MOFs gives an added advantage in incorporation of ionic species by ion exchange processes which would thereby help to fabricate specific functionality which would otherwise is not possible in neutral MOFs. However, one of the major hurdles that iMOFs have to overcome is the prior design strategies

pt

and thermochemical stability in order to be used as one of the competent materials in the field of gas storage, photonics, fuel cells, catalysis etc. Also the framework ions present in them often holds a

Ac ce

limitation because in multiple cases the porosity is compromised. Once rationally synthesized, these charged frameworks holds the advantage over the conventional neutral MOFs due to free ions present in them which enables them to be used in multifarious areas of science. Also because of the ease of syntheses procedures and low production costs of iMOFs we feel this important sub-class of MOFs hold a promising future as possible commercial usage in ion-exchange resins. We believe judicious designing of iMOFs would enable them to be the next generation MOFs and therefore has a delightful prospect ahead.

Acknowledgements 33

Page 40 of 114

We are grateful to IISER Pune for research facilities. DST (Project No.GAP/DST/CHE-12-0083) is

Ac ce

pt

ed

M

an

us

cr

ip t

acknowledged for the financial support. A.K. and A.V.D. thank IISER Pune for research fellowship.

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Page 41 of 114

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Figure captions:

Scheme1. Design strategies for construction of anionic MOF via pre and post-synthetic approach. Scheme 2. Equation of hydrolysis reaction of DMF/DEF in presence of water resulting in the formation of respective cations.

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Fig.1. Showing (left) Zr6O4(OH)4(O2CR)12 clusters in UiO-66 and (right) the two-step modification process depicting the grafting of lithium tert-butoxide. Reproduced with permission from Ref. [52]. Fig. 2. (a) N2 adsorption isotherm for compounds a-d at 77K and (b) CO2 adsorption of the same

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compounds at 273K. Reproduced with permission from Ref. [53] Fig. 3. CO2 adsorption isotherm for different SNO-100 compounds at r.t and the corresponding isosteric

cr

heat of adsorption (Qst). Reproduced with permission from Ref. [54].

Fig. 4. a) Hydrogen adsorption isotherm for compounds 1-M at 77K and b) enthalpy of adsorption curves

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for the uptake of H2 in compounds 1-M. Reproduced with permission from Ref. [56].

Fig. 5. Cation exchange in 1-ppz-solv (left) to form 1-Li-solv (right) showing that the H2ppz2. Colour

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codes are shown in the right. Reproduced with permission from Ref. [60].

Fig. 6 a) N2 adsorption isotherm of 1-ppz (blue) and 1-Li (red) at 78K and b) graph showing H2 uptake of

M

1-ppz at 78K. Reproduced with permission from Ref. [60].

Fig. 7. H2 adsorption isotherm for 1-Li at 78K. Reproduced with permission from Ref. [60].

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Fig. 8. (a) Single crystal structure of rho-ZMOF and (b) Cyclohexane catalytic oxidation using MnRTMPyP as a catalyst at 65 °C. Reproduced with permission from Ref. [66].

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Fig. 9. Schematic showing cation exchange in ZJU-28. Reproduced with permission from Ref. [67].

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Fig. 10. a) Reaction showing hydrogenation of 1-octene to n-octane in neat 1-octene and comparison of the catalysis reaction by ZJU-28-1d (blue) and homogeneous 1d (red) and b) Bar diagram showing recycling of ZJU-28-1d as catalyst in the reduction of octane. Reproduced with permission from Ref. [67]. Fig. 11.a) [Cd6(C2O4)8]4- octupolar unit in [(H2NMe2)2Cd3(C2O4)4]·MeOH·2H2O and b) Comparative bar diagram representing SHG intensities of KH2PO4, [(H2NMe2)2Cd3(C2O4)4]·MeOH·2H2O, NH4+- Na+- and K+ exchanged samples of [(H2NMe2)2Cd3(C2O4)4]·MeOH·2H2O, and the known 2D octupolar MOFs. Reproduced with permission from Ref. [76]. Fig.12. a) Perspective view of cation exchange process leading to pyridinium hemicyanine chromophores incorporated into ZJU-28 b) Fluorescent microscope images of ZJU-28 and ZJU-28DPASD under UV 45

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light of 365 nm UV and c) PXRD patterns of simulated ZJU-28, experimental ZJU-28 and experimental ZJU-28DPASD. Reproduced with permission from Ref. [77]. Fig. 13. a) SHG spectra of ZJU-28DPASD where the inset represent the SHG intensity of ZJU28DPASD as a function of the energy of the fundamental frequency laser, b) Confocal image of a

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triangular prism crystal of ZJU-28DPASD and c) bar diagram showing comparison of the SHG intensities of a quartz, ZJU-28, and the chromophore-including ZJU-28. Reproduced with permission

cr

from Ref. [77].

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Fig. 14. a) Extracellular cation triggered procainamide release from bio-MOF-1 b) corresponding drug release profile and c) zinc adinate units in bio-MOF-1. Reproduced with permission from Ref. [79].

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Fig. 15. a) Cation exchange by different lanthanides in bio-MOF-1 b) emission spectra of such cation exchanged samples under UV light and (c) PXRD patterns of lanthanide inclusion complexes of bio-

M

MOF-1. Reproduced with permission from Ref. [87].

Scheme 3. Schematic representing Encapsulation of a functionalized Guest Molecule into a MOF via the

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Metal-Cation-Directed de Novo Assembly Strategy. Reproduced with permission from Ref. [68] Fig. 16. a) Representation of de novo assembly of [email protected] b) Optical images of bio-MOF-1 and [email protected] (b) UV-vis absorption spectra of bio-MOF-1 (black), [email protected] bio-MOF-1

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(red), and Co-Pc (blue) and c) photos of bio-MOF-1, [email protected], and [email protected]: (a)

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bio-MOF-1, (b) [email protected], (c) [email protected] bio-MOF-1, (d) [email protected], (e) [email protected], (f) [email protected] bio-MOF-1, and (g) [email protected] Reproduced with permission from Ref. [68].

Fig. 17. (a) Porous view of bio-MOF-1 along c axis, zoomed view of the Zn-adenate unit showing free amine group and hydrogen bonding interaction of the free amine group of bio-MOF-1 with TNP. (b) Luminescence quenching upon incremental addition of TNP in bio-MOF-1. (c) Comparison of quenching efficiency by TNP with other nitro analytes (inset showing confocal image of bio-MOF-1 crystal upon exposure to TNP). Reproduced with permission from Ref. [88].

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Fig. 18. (a) Tb6L4 vertices linked to each other forming 1D chains in octahedra in {K5[Tb5(IDC)4(ox)4]}n and porous view of {K5[Tb5(IDC)4(ox)4]}n along c axis showing free potassium ions in CPK model (blue). (b) Bar diagram showing changes in luminescence intensities upon addition of various metal cations in {K5[Tb5(IDC)4(ox)4]}n (Excitation wavelength is 545nm). Reproduced with permission from Ref. [91].

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Fig. 19. Two-in-one proton conduction in {[(Me2NH2)3(SO4)]2[M2(ox)3]}n. Reproduced with permission from Ref. [94].

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is shown as a dotted line. Reproduced with permission from Ref. [96].

cr

Fig. 20. a) Arrhenius plots of the proton conductivity of 1 under 98% RH conditions. Least-squares fitting

Fig. 21. a) Parallel stacking of the cationic and anionic units (b) Perspective view along the b-axis. Guest molecules are omitted. (c Humidity dependence of the proton conductivity at 298 K, colors correspond to

an

the proton conductivity of Me-FeCr(blue), Et-MnCr(red), Bu-FeCr(green), Bu-MnCr(orange,), and NBu4(purple), respectively. Reproduced with permission from Ref. [97].

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Scheme 4. Schematic representation of routes to build a cationic MOF via pre and post-synthetic approach.

permission from Ref. [120].

ed

Scheme 5. Overall Schematic showing anion exchange processes in a SC-SC fashion. Reproduced with

pt

Fig.22. Schematic showing framework engineering accompanied by anion exchange processes.

Ac ce

Reproduced with permission from Ref. [121].

Fig. 23. a) Showing the 3D cationic framework of NTHU-12 and b) emission spectra of NTHU-12. Reproduced with permission from Ref. [133]. Fig. 23. a) Luminescence response upon various guest inclusion b) thin film of compound frown on Zn plate c) emission spectra of cationic MOF in presence of various guests and d) photo-luminescence response in presence of CO2 at different pressures. Reproduced with permission from Ref. [129] Fig. 24. a) Luminescence response of the compound under UV light showing differential response in presence/absence of water (left) and (right) solid state luminescence spectra of the of the parent and desolvated compound at r.t and at low temperature. Reproduced with permission from Ref. [135]. 47

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Fig. 25. Photoluminescence spectra of SLUG-23 (black), SLUG-24 (red), SLUG-25 (blue) and free 4,4’bipy ligands (green). Reproduced with permission from Ref. [136]. Scheme 6. Schematic showing anion dependent tuneable luminescence as a result of anion exchange in

ip t

[{Zn(L)(H2O)2}(NO3)2·2H2O]n. Reproduced with permission from Ref. [137]. Scheme 7. Cartoon representation showing guest solvent molecules and anion induced tuneable

cr

luminescence behavior. Reproduced with permission from Ref. [138].

Fig. 26. Emission spectra of the parent compound and ClO4- , BF4- exchanged compounds and of the free

us

ligand showing variable luminescence properties. Reproduced with permission from Ref. [138]. Fig. 27. a) Ball and stick model representation of compound [CuL2(H2O)0.5](NO3)2 and b) colour changes

an

in presence of various anions. Reproduced with permission from Ref. [141]

Reproduced with permission from Ref. [142].

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Fig. 28. Visual photochromic response as a result of anion exchange processes in upon photo irradiation.

Fig. 29. The colour changes upon anion exchange reactions in the parent compound (1’). Reproduced

ed

with permission from Ref. [143]

Fig. 30. a) Porous view of compound [{CuL2(NO3)2}.o-Xylene.DMF] n along a axis and b) colorimetric from Ref. [144].

pt

anion exchange based structural transformation in presence of SCN- ions. Reproduced with permission

Ac ce

Fig. 31. Magnetic relaxation behavior as represented by fitting the experimental data of various compounds as represented by [Ln(bipyNO)4](TfO)3·x solvent (Ln= Dy3+, Ho3+, Tb3+. Er3+). Reproduced with permission from Ref. [147].

Fig. 32. Guest dependent magnetic changes shown by the difference in magnetic susceptibility graphs in compound [Co(N3) (p-XBP4) (H2O)2]·(N3). Reproduced with permission from Ref. [148] Fig. 33. Schematic showing framework engineering accompanied by anion exchange processes. Reproduced with permission from Ref. [121].

48

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Fig. 34. Guest induced structural changes from 3D to 2D upon air drying. Reproduced with permission from Ref. [122]. Fig. 35. Perspective view of compound CuII(mtpm)Cl2·20H2O along c axis (left) and adsorption isotherms of water (black), methanol (blue) and ethanol (red) of the same compound. Reproduced with permission

ip t

from Ref. [123].

Fig. 36. Figure showing structural transformation from 1D helix to a molecular loop. Reproduced with

cr

permission from Ref. [124].

us

Fig. 37. Solvent dependent structural transformations resulting in breathing and contraction. Reproduced with permission from Ref. [126].

an

Fig. 38. Figure showing (a) multiple functionalities in compound {[Ni(bpe)2(N(CN)2)](N(CN)2)(5H2O)}n, (b) methanol adsorption in the same compound and (c) CO2 adsorption of the assynthesized framework

M

and the azide anion exchanged framework at 195K. Reproduced with permission from Ref. [128]. Fig. 39. CO2 and N2 separation by compound [Zn7(ip)12](OH)2 at a) low temperatures and b) at 298K.

ed

Reproduced with permission from Ref. [129].

Fig. 40. a) Methanol adsorption isotherm for [Zn2(tpa)2(cpb)] ·2DMF·H2O T 298 K and b) isosteric heat

pt

of adsorption for methanol for the same compound. Reproduced with permission from Ref. [130].

Ac ce

Fig. 41. a) Perspective view of helical chain in [{Zn(L)(OH2)2}(NO3)2·xG]n b) one single helical strand in space fill model and c) CD spectra of the parent and different anion exchanged compounds. Reproduced with permission from Ref. [149] Fig. 42. Figure showing drug uptake and release by MOF-74-Fe(II). Reproduced with permission from Ref. [150]

Fig. 43. Live cell images upon incubation with MOF-74-Fe(II) after 12hrs. Reproduced with permission from Ref. [150]

49

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Fig. 44. a) Metallomacrocycles connected to the Zn8O cluster shown in stick model and b) space filling model showing imidazolium cation aligned within the pores. Reproduced with permission from Ref. [151] Fig. 45. Figure showing catalytic activity by SLUG-21 and SLUG-22 in ketalization of 2-butanone to

ip t

form 2-ethyl-2-methyl-[1,3]-dioxolane. Reproduced with permission from Ref. [156]. Fig. 46. Optical images of SLUG-21 in KMnO4 solutions monitored at different time intervals.

cr

Reproduced with permission from Ref. [156]

Fig. 47. UV-VIS spectra showing gradual uptake of permanganate ions in exchange for EDS anions at

us

different time intervals. Reproduced with permission from Ref. [156]

Fig. 48. Figure showing (right) octahedral and tetrahedral cages in [Ag2-(btr)2]·2ClO4·3H2O and b) UV-

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VIS spectra of UV/Vis spectra of aqueous K2Cr2O7 solution during exchange with Ag2(btr)2]·2ClO4·3H2O. Reproduced with permission from Ref. [160]

Reproduced with permission from Ref. [161] 50.

SC-SC

structural

changes

occurring

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Fig.

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Fig. 49. Image showing Reversible trapping and release of Cr2O72- by [Zn2(Tipa)2(OH)]·3NO3·12H2O].

upon

anion

exchange

with

Cr2O72-

in

Ac ce

pt

[Zn2(Tipa)2(OH)]·3NO3·12H2O]. Reproduced with permission from Ref. [161].

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