acid reduction

acid reduction

Teimtudrtm Letters. Vol. 35, No. 32. pp. 5189-5792. 1994 Etsevta Science Ltd Printed in Great Britain oo404039/94 57.awo.00 0040-4039(94)01195-8 S...

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Teimtudrtm

Letters.

Vol. 35, No. 32. pp. 5189-5792. 1994 Etsevta Science Ltd Printed in Great Britain oo404039/94 57.awo.00

0040-4039(94)01195-8

Synthesis of Hydrogenated

Fullerenes by Zinc/Acid Reduction1

Mark S. Meier,* Perry S. Corbin,t Virginia K. Vance, Mark Clayton, and Michael Mollman, Department

of Chemistry,

University

of Kentucky,

Lexington,

KY 405064055

Magdalena Poplawska Faculty of Chemistry, Warsaw Technical University (Politechnika), 00-664 Warszawa, Noakowskiego 3, Poland

Abstract: Reduction of Cm with zinc and acid resulu in the formation of [email protected]

C&Q,

and more highly

hydrogenated fullerenes.

The chemical modification simplest

types of chemical of Q&l36

synthesis

the literature. the formation

of fullerenes

modification

of fullerenes

by the Birch reduction*

Treatment

is hydrogenation

to be a high-energy may contribute

(2) has been proposed.

species due to the interactions to the Bitch reduction

stopping

of so many eclipsed at the c&36

Cm in particular,

is an aggressive reducing agent with a Li+/Li potential of

men&

with suitable reduction

The protonated products, C&I2 to the eventual

metal reduction potentials,

[email protected]

allow reaction

at a potential

and at the Q-J-‘/-~ potentials

synthesis

of the reduced

fullerenes

“fuller&te”

of

(C&L&

is

h,ydrogen atoms,6 and these

formation

has been extensively

investigated.7-‘6

-3.0 V versus Fc+/Fc. very close to the would be easily reduced

of Qr~H36. We are interested

in the products

that

of Qo using less highly Educing

and we chose to investigate

metals. There are a number of tin (t? (Sn2+/8n) = -0.75 V vs Fc+/Fc),

= -1.37 V vs Fc+/Fc).

where CM is not electrochemically

(-0.98 V and -1.37 V vs F&PC. C&L2,

of w36

stage.

being the simplest,

iron (Eo (Pe2+/Pe) = -1.05 V vs Fc+/Pc), and zinc (Ep (Zn2*/Zn) conveniently

in

2

of fullerenes,

reduction potential.

might result from dissolving

The reported

as being the result of the deconjugation

C&-/6-

leading

double bonds. of a fullerene The formation

Fully hydrogenated

Lithium

under these conditions,”

One of the

as a proton source leads to

ranging from Cfjt-$Ltg to C!&LX.~

species has been rationalized

electrochemistry

reactions

and using t-butanol

1 The reductive

of individual

was among the first chemical

of a series of polyhydrofulletenes

double bonds, and one such structure interactions

a highly active area of research.2*3

of Q-J (1) with lithium in ammonia

rather than more highly reduced predicted

is currently

CaH4,

reduced,

respectively).9

C&H6 and others by dissolving

and acid. 5789

These three reagents as well as near the Herein we report the

metal reduction using zinc

5790

Results Heating a heterogeneous

mixture oft.&,

and Discussion

toluene. tin dust, and aqueous HCl for periods of up to six

hours under a nitrogen or argon atmosphere leads to very little reaction as indicated by gel permeation chromatography

The same was found using iron dust instead of tin, suggesting that if any C& radical

(GPC).

anion is produced under these conditions it is fairly unreactive toward acid. However,

treatment of t&o with

sn excess of zinc dust and acid in refluxing toluene under argon or nitrogen results in a change in color from the magenta color of C&o to a dark brown. GPC analysis of the reaction mixture shows three new, evenly spaced bands that elute after C&. Different acids (6 M HCl. 12 M HCI, glacial HOAc, and concentrated (35 psi Hz, 10% Pd/C, 24 hours) lead to the formation

H2SO4) as well as catalytic hydrogenation three new bands.

The relative

concentrations

of reaction over the six hour reaction period.

extents

Control experiments with each acid alone and with zinc alone

. do not, under these conditions, show the formation mixture

of the same

reflecting different

of the new species differ somewhat,

with DDQ results in smooth conversion

Treatment

of any new compounds.

of the mixture back to %,

consistent

of the Z&I-I+ reaction with the assignment

of

these new bands as reduced fullerenesS4

Fig. 1. Gel Permeation

Chromatogram

We have separated

of Zn/HOAc

Reduction

Mixture.

See text for specific

the new bands by preparative GPC16*18 using a bank of four 19 mm diameter

columns operating at ambient temperature with toluene as the mobile phase. rechromatographed solutions

twice.

are evaporated

conditions.

The known lability of C&2, to dryness,

dictated

that samples

particularly

Each band was isolated

the sensitivity

were handled

and

toward oxidation

when

in solution and not evaporated

unless

necessary. Mass spectrometric

characterization

back to the parent fullerene.17

However,

parent ion, along with the expected under these conditions,

extensive

m/e 720, again accompanied the 720 line expected fragment

of C&I2

fragmentation

of these compounds

isotope

a strong tendency

by the expected

to t& occurs, and the spectrum isotope lines.

or the adduct of a hydrogen

Excess intensity

of t3C. indicates

for fragmentation

of Band I exhibited

lines (67% M+l, 24% M+2), corresponding

fragmentation

due to the natural abundance

of C&2.

indicates

ion FABS MS analysis

negative

m/e 722 as the

to t33I-I~.

shows a prominent

at m/e 721, beyond

a small quantity

Even line at

the 67% of

of C&H, either a

ion t8 from the FABS matrix to the t&o formed

by

5791

Analysis of the C&-I2 band by high-resolutionCl8 HPLC (70% CH$&30% aCetonitrilC) indicated thatthis band contains a single component, suggestingthata single isomer is fotmed. Calculationsby Cahill suggest thatof the 23 all-exe isomers of -2,

t6 one isomer (1.9) is 3.8 kcal/mole more stable than any

other isomer. This isomer, resulting from the placementof hydrogensacross a 6,6 ring fusion, has been preparedby tmatmentof f&u with BH3-THF followed by protonolysis.19and by hydrogenationover rhodium.” When we prepare a sample of C&II2 using BH$I’I-IP. we find that it is chromatographicallyand spectroscopically(UV/Vis, III NMR) identicalto the compound we prepateby zinc/acid reduction. indicating thatthe same isomer is produced.

3 Reversed-phaseHPLC indicated thatbands II and III each contain a numberof components. Analysis of Band II by negative-ion FABS mass spectromeny shows thatit is primarilyQuI&

Analysis of this band

with a “‘Buckyclutcher”HPLC column showed that threecompounds, presumablydifferent isomers, are present. This is consistentwith resultsreportedby Cahill, althoughthe relativeamountsof the isomers producedby this method are quite different from the relativeamountsproducedby BH3*THF. Buckyclutcheranalyses made before GPC separationconfirm that this ratio is not an artifactof prior GPC treatment.

C6&4 bmm z

Fig. 2. HPLC (19 mm preparativeBuckyclutcher)Chromatogramof Band II (C&-J-k+ Toluene/hexane (70/30,5

mIJmin) mobile phase, UV/Vis detection at 330 nm.

Band III, containingthe most highly reduced materialproducedunder these conditions, exhibits mass lines from ml+?720 (C&) to 730 (M+2 line from Q&Is). fragmentationfrom C&I,5 or -8.

Again, the lower mass lines are most likely due to

Reversed phase HPLC analysis clearly shows that this band is

composed of a mixture of a large number of compounds. Given the difficulty in obtainingfragmentation-free mass spectrawe can not be certain if QoHe is the parentand thatother lines result from fragmentation.

Clearly,8 zmmbcxof hydrogenated fullerenes C&-I6 to c6oHzz are produced, and these compounds are not

separated by GPC. To a suspension of 20 mg (0.0278 mmoIos) Cm in 5 mL toluene was added 360 mg (5.54 mmoles) in powder aud 0.5 mL 6 M HCI, The suspension was heated at reflux under Ar for a period of 6 hours. The resulting brown suspension was diluted with toluene and filtered to remove the metal salts. The resulting solution was washod with water, then dried (NazSO4) and concentrated to 2 mL before being miorofiltered and purified by GPC!. Preparative GPC of hydrogenated fullerencs was performed using a bank of four 19 mm x 300 mm GPC columns (#l - Jordigel E-277 (SKIA).#2 and #3 - Waters Ultrastyragei (NOA), #4 Waters Ultrastyragel (loOA)) using toluene as the mobile phase at a flow rate of 5 &mm. Injections of up to 5 mL of saturated solutions of the fullerones in tolucne were made. Evaporation to dryness was avoided due to difEculty in redissolving

the solid products. Typical mass balances were 70%. In oases where the

samples were evaporated to dryness, the yields obtained were QH2 tQ&6+,

(7%), C&I-I4 (3%) and band III

1%).

AcknowIedgmenb: We gratefully acknowledge support for P. L. C., provided by National Science

Fouudaticm REU grant CHE-9000653.

The authors thank the University of Kentucky Major Fquipment fund

and the Vice Charm&or for Research and Graduate Studies for their support of this research. We also thank Dr. Paul A. Cahill of Sandia National Laboratory for making his results available to us prior to publication. REFERENCES AND NOTES t Participant in the NSF-E’tEWprogram in summer, 1992. 1.

2. 3. 4. 5.

: : lb. :;: :: :z 17: :;* 20:

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(Received in USA 17 May 1994; accepted

16 .&me N!W)