MAGNETIC RESONANCE IN CHEMISTRY, VOL. 33, S94LS97 (1995)

Radical Cations of Precursor Compounds for Organic Conductors Derived from Carbohydrates : an ESR Study

Markus Scholz and Georg Gescheidt* Institut fur Physikalische Chemie, Universitat Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland

Ulrich Schoberl and Jorg Daub* Institut fur Organische Chemie, Universtitat Regensburg, Universitatsstrasse 3 1, D-93040 Regensburg, Germany

The radical cations of two electron donors containing two 1,3-dithiol moieties were characterized by the use of ESR/ENDOR/general TRIPLE spectroscopy. The spin population resides mainly in the central part of the mol- ecules. The g-factors are correlated with the 33S coupling constants, which are determined from satellite lines.

KEY WORDS ESR; ENDOR; organic conductors; radical cation; 1,3-dithiol

INTRODUCTION

Organic conductors and superconductors are of funda- mental interest in material science.' One class of such substances consists of charge-transfer complexes of elec- tron acceptors and electron donors. Efficient acceptors are mostly derived from planar 'II systems carrying electron-withdrawing substituentsZL4 which effect high electron affinities. A planar geometry is also a common feature of the donor molecules. The low ionization potentials are achieved by the integration of 'electron- rich groups' which are able to stabilize the positive charge. Very often utilized molecular fragments of this type are differently connected 1,3-dithiol-2-ylidene or analogous selenium-substituted

For the analysis of the electronic properties of these materials, ESR spectroscopy is an invaluable tool for providing estimates of the spin distribution inside the molecules. 5-' For the sulphur-containing donors, the parameters supplying this information are the 33S hyperfine coupling constant^'^,'^ and the g - f a ~ t o r s . ~ ~ . ~ '

Here we present the ESR investigation of two very efficient novel electron donors 1 and 2, synthesized from fructose via 5-hydroxymethyl-2-furaldehyde, in which two 1,3-dithiol-2-ylidene moieties are separated by furan spacers. Both compounds possess remarkably low oxidation potentials (1, -0.210; 2, +0.010 V us.

r r

1 2

* Authors to whom correspondence should be addressed

ferrocene) and are, therefore, isolated as the hexa- fluorophosphate salts of the radical cations.

EXPERIMENTAL

Synthesis of the radical cation salts

lf 'PF6 ~ : The electrocrystallization was carried out in a divided cell equipped with a Pt-wire as working elec- trode and a tungsten wire as counter electrode. The cathodic space was filled with 6 ml dry CHCL3/0.03 N tetrabutylammonium hexafluorophosphate (TBAHFP) and the anodic compartment with 2 ml of a 0.01 M solu- tion of 1 in dry CHCL,/0.03 N TBAHFP. The electro- crystallization was undertaken under galvanostatic conditions (i = 20 PA) for 12 h. The dark blue solution of the anodic compartment was separated and the remaining smaragdine crystals were isolated and rinsed with petroleum ether. C,8HloS40PF6 (515.51). mp 285 "C. UVjVIS (acetonitrile): Amax(log E ) = 360 (3.8), 630 (4.9), 767 (3.9), 922 (4.0).

2+'PF6- was obtained by the procedure as described above: Dark blue crystals with mp 228 "C. C2,H12S40PF6 (541.6). &,,ax(lOg E ) = 300 (4.3), 359 (4.Q 397 (4.1), 416 (4.1), 514 (3.9), 560 (4.0), 606 (4.2), 677 (4.6), 827 (4.1), 974 (4.3).

The CH,Cl, used as the solvent for the ESR mea- surements was refluxed with molecular sieves for 2 days and then stored under high vacuum over molecular sieves with exclusion of light.

The radical cation salts lt 'PF6 ~ and 2+'PF6 ~ were dissolved in CH,CI, and degassed by 3-4 freeze-and- thaw procedures.

ESR spectra were recorded on a Varian E9 spectro- meter and g-factors were determined using a Marconi Instruments 2440 microwave counter and a Bruker ER 035 M NMR gaussmeter. ENDOR spectra were taken on a Bruker ESP 300 spectrometer system.

CCC 0749-1581/95jSIOO94-04 0 1995 by John Wiley & Sons, Ltd.

Received 11 M a y 1995 Accepted 17 M a y 1995

ESR OF PRECURSORS FOR ORGANIC CONDUCTORS s95

RESULTS

The solutions of l+'PF, - and 2+'PF6 - in CH,CI, gave well resolved ESR/ENDOR spectra (Figs 1 and 2) which were almost invariant to temperature changes. For both radical cations, the line patterns in the central part of the spectra are due to splittings from even numbers of equivalent protons revealing that both 1" and 2" possess twofold symmetry on the hyperfine time-scale.

The prominent feature in the ESR spectrum of 1" is a triplet pattern spaced by 0.083 mT due to hyperfine coupling of two equivalent protons. Further proton hyperfine coupling constants, a H , of much smaller size give rise to additional lines. ENDOR spectra indicate two aH of 0.028 and 0.014 mT. The simulation of the ESR spectrum establishes that both uH arise from two sets of four equivalent protons. General-TRIPLE reso- nance experiments show that the signs of all these a H are identical (presumably negative as expected for c(-

protons). In the flanks of the intense ESR spectrum, satellite lines of 13C and 33S isotopes can be discerned. Spectral simulations point to a coupling of 0.567 mT for two equivalent 13C nuclei and of 0.293 mT for four apparently equivalent 33S nuclei (Fig. 1, Tables 1 and 2).

From the ESR spectrum of 2+', two uH of 0.215 and 0.118 mT, each due to a pair of equivalent protons, can be directly determined. The higher resolution ability of the ENDOR technique establishes two further uH of 0.029 and 0.013 mT. Based on simulations, each of them is ascribed to four equivalent protons. As in the case of I", the pattern of the satellite lines exhibits a 13C coup- ling constant of 0.280 mT (2C) and a 33S coupling con- stant of 0.248 mT (4s).

'I ,

Figure 1. Top left: Experimental ESR spectrum of 1 "PF,- (solvent CH,CI,, temperature 263 K) and its simulation (below). The insets on the left-hand side show the satellite lines at a 5 x higher scale. Middle right: corresponding ENDOR spectrum.

Figure 2. Top left: Experimental ESR spectrum of 2+'PF6- (solvent CHzCIz, temperature 263 K) and its simulation (below). The insets on the left-hand side show the satellite lines at a 12 x higher scale. Middle right: corresponding ENDOR spectrum.

Table 1. Experimental and calculated 'H and I3C hyperfine coupling constants of 1" and 2"

Hyperfine coupling constant (mT)

'H' ' 'C

Compound f , P b,b 3.3.6.6' 4,4.5.5'

1" (exp.) 0.083 - 0.028 0.014 0.567 I + ' (calc.)" -0.10 __ -0.02 -0.01 +0.58d 2+ ' (exp.) 0.215 0.118 0.029 0.013 0.280 2+ ' (calc.)" -0.21 -0.17 -0.01 -0.01 +0.22d 2+' (calc.)' -0.17 -0.07 0.05 0.05 -

2+' (talc.). -0.34 -0.08 0.0 0.0 ~

a For labelling, see the structures. HMO-McLachlan calculation, Q = -2.5 mT. AUHF-AM1 calculation.zz Calculated with the use of the Karplus-Fraenkel relationshipz3

CEqn ( 1 11. ' PPP calculation: Q = -2.7 mT.z4

Table 2. Comparison between 33S coupling constants, g-factors and oxidation potentials of electron donors with 1,3- dithiol moieties

Compound 33s g-Factor E;:2 (vs SCE) Ref

I + ' 0.293 (4s) 2.0067 0.186 This work This work 2+' 0.248 (4s) 2.0052 0.406

3" 0.427 (4s) 2.0084 0.356 14,17 4+- 0.398 (4s) 2.0081 0.276' 17 5+' - 2.0072 0.71 6' 25

a Value adapted to the SCE-MeCN standard

S96 M. SCHOLZ ET AL.

DISCUSSION

Geometry optimization with the use of the AM1 methodz6 indicates that both radical cations prefer planar geometries of C,, symmetry. Moreover, it is evident that at least the five highest occupied molecular orbitals possess n character. Hence it is justified to carry out simple Huckel M O (HMO) calculations for the pre- dictions of the isotropic hyperfine data of 1" and 2". The parameters used to account for the heteroatoms were15 ko = 2.0, k , = 1.0 and h,, = 0.70. The S and 0 atoms each contribute two electrons to the n system, so 1 represents a 23-centre 28-n-electron system and 2 pos- sesses 25 centres and 30 n electrons. The HMO calcu- lated HOMOs mirroring the singly occupied orbitals of 1" and 2" are shown in Fig. 3. It can easily be under- stood from these frontier orbitals that the spin popu- lation is concentrated at the central part of the molecules and the C atoms at the 2-positions of the 1,3- dithiol rings carry the highest amount of spin. The spin population at the C atoms carrying the protons is low, and consequently the uH are small. This accounts for the narrow span of the ESR spectra, the widths being 0.33 (1") and 0.82 mT (2").

The measured and HMO-McConnell-calculated27 (Q = -2.5 mT) uH values are compared in Table 1. Their agreement for 1+' is very good. The largest uH of 0.083 mT can be unambiguously assigned to the protons of the furan ring whereas the small uH of 0.028 and 0.014 mT correspond to the '(almost) equivalent' protons in the 3,3',6,6'- and 4,4',5,5'-positions, respec- tively, of the phenyl groups. The experimentally estab- lished coupling constant of the four 'approximately equivalent' 33S nuclei of 0.293 mT is reflected by the HMO predicted (Q = 3.3 mT28329) value of 0.240 mT. In 1" the C atoms at the 2-positions of the 1,3-dithiol rings bear the highest spin population. The 13C coup- ling constant is calculated to be 0.585 mT, in good agreement with the experimental value of 0.568 mT, based on the well established equationz3

acp = Q p p p n + C Q p P," (1)

where p = locant for a C atom, v = locant for a neigh- bouring atom bound to atom p, Q p , Q,, = proportionality factors and p p a , p V a = spin populations at atoms p and v, respectively.

a

In the case of 2+', the agreement between the theo- retical (HMO) and the experimental values is not as satisfactory. The HMO coefficients at the furan C atoms carrying protons v) and the corresponding values at the methylidene bridge connecting the furan and the 1,3- dithiol moiety (b) are very similar. Thus an unam- biguous assignment of the two (two-proton) uH of 0.215 and 0.118 mT is unlikely by using on the HMO model (Table 1). As mentioned above, AM1 geometry opti- mizations always led to a c,, symmetric geometry of 2" irrespective of the starting geometry. Therefore, we performed a geometry optimization with restricted C,, symmetry (AM 1 ; keywords : precise, symmetry, UHF) of 2". A force calculation revealed that a minimum had been reached. Based on this geometry, the uH were determined.22'30 The uH for the furan protons is predict- ed to be 0.17 mT whereas the uH for the b position is significantly smaller (0.07 mT). These values are similar to the experimental observation that the two major uH have a ratio of cu. 2: 1. Consonant results are also obtained from PPP calculation^^^ (Table 2; of course, all the theoretical methods furnish the same type of orbital possessing A , symmetry, differing only slightly in the coefficient sizes). Thus the uH value of 0.215 mT is assigned to the two equivalent protons of the furan ring and that of 0.118 mT to the b position. The small aH of 0.029 and 0.013 mT mirror the low spin population in the be nzo subs tit uents.

The experimental coupling constant of the 33S nuclei is 0.248 mT. This is in very good accord to the results of the calculations, which predict that the spin population at the S atoms in 2" is 80% relative to that of 1". For 2+', the 13C coupling constant determined by simula- tions is 0.280 mT; however, a straightforward assign- ment to certain C atoms is not possible because the HMO [Eqn (l)] predictions give similar values for the 2(5)-position of the furan ring and the 'bridge' (b) C atoms in 2". We provisionally attribute the 13C coup- ling to the two equivalent furan C atoms because the AM1 s-spin populations and the PPP coefficients at these centres are the major ones.

In Table 2, the 33S coupling constants, the g-factors and the first oxidation potentials of 1 and 2 are com- pared with data for the related donors tetrathiafulvalene (TTF, 3) and the 'elongated' derivatives 4 and 5. Com- pound 4 represents a TTF with an inserted ethene and planar geometry (s-cis and s-trans isomers possible). In

1 (82) Figure 3. HOMOs of 1 and 2 according to H

2 MO calculations

ESR O F PRECURSORS FOR ORGANIC CONDUCTORS

2.008

2.007

2.006

2.005

s97

. .

- -

- -

--

- -

5 the dicyanovinyl and the phenyl substituent induce a significant deviation from a coplanar arrangement of the two 1,3-dithiol-2-ylidene rings (s-trans-like arrangement). 3 2

Regarding the first oxidation potentials, E l , ; , of 1-5 (Table 2), compound 1 is the most efficient electron donor. The improved electron-donating properties of 1 compared with 3-5 can be attributed predominately to the expanded IT system of 1 which allows an enhanced electron delocalization (the same, but less pronounced, tendency holds for the pair +I). Although the IT system of 2 contains even more atoms than that of 1, it is more difficult to oxidize than 1, 3 and 4. This behaviour, however, is in line with the HOMO energies (HMO) of a -- 0.10498 and a - 0.01608 for 1 and 2, respectively. In 5 the two 1,3-dithiol moieties are arranged almost perpendicular and thus the sterically induced lack of delocalization, amplified by the electron-withdrawing effect of the cyano groups, leads to the larger oxidation potential of 0.716 mV (Table 2).

3 4 5

The coupling constants of the 33S nuclei possess their highest values in TTF (3") followed by 4+', 1" and 2'-' (Table 2) , showing that an expansion of the IT

system causes a decrease in the spin population at the sulphur centres. This is additionally paralleled by the sizes of the g-factors as indicated in Fig. 4. Thus it is predominately the amount of spin population at the sulphur atoms (having a spin--orbit coupling constant

g Factor A

2'+

> 0.398 0.427 33s coupl.

const./mT

Figure 4. Correlation between 33S hyperfine coupling constants andg-factors of 1 +'-4+'.

0.248 0.293

2.004

L :

higher by one order of magnitude relative to that of the carbon which directs the g-factor shifts in 1 + '-5 + *,

Studies of the solid-state charge-transfer properties (which are in progress) should give an insight in to how far the efficient donor properties of 1 (2) lead to efficient conducting materials. In view of the sulphur-sulphur interactions in the stacks of charge-transfer complexes being important factors for good conductivity, it will be of interest to see the interplay between the increased donating quality of 1 (us. 3) and lowered spin popu- lation at the sulphur centres of 1" (us . 3+').

Acknowledgement

This work was supported by the Swiss National Science Foundation and the Deutsche Forschungsgemeinschaft.

REFERENCES

1. S. Roth, One-Dimensional Metals. VCH, Weinheim (1994). 2. A. M . Kini, D. 0. Cowan, F. Gerson and R. Mockel, J . Am.

Chem. SOC. 107,556 (1 985). 3. F. Wudl, Acc. Chem. Res. 17, 227 (1 984). 4. A. Aumuller, E. Hadicke, S. Hunig, A. Schatzle and J. U. v.

Schutz, Angew. Chem., lnt . Ed. Engl. 23, 449 (1 984). 5. M. Adam and K. Mullen, Adv. Marer. 6, 439 (1 994). 6. T. Naito, A. Sato, K. Kawano, A. Tateno, H. Kobayashi and A.

Kobayashi, J. Chem. Soc., Chem, Commun. 351 (1 995). 7. E. Aqad, J. Y. Becker, J. Bernstein, A. Ellern, V. Khodror-

kovsky and L. Shapiro, J . Chem. Soc., Chem. Commun 2775 (1 994).

8. A. S. Batsanov, M . R . Bryce, G. Cooke, A. S. Dhindsa, J. N. Heaton, J. A. K. Howard, A. J. Moore and M. C. Petty, Chem. Mater. 6, 1419 (1994).

9. M . Fourmigue, S. Jarshow and P. Batail, Phosphorus Sulfur Silicon Relat. Hem. 75, 175 (1 993).

10. A. F. Garito and A. J. Heeger, Acc. Chem. Res. 7, 232 (1 974). 11 J. H . Perlstein, Angew. Chem. 89, 534 (1 977). 12, G. Saito and S. Kagoshirna, The Physics and Chemistry of

Organic Superconductors. Springer, Heidelberg (1 990). 13. J. M Williams, M . A. Beno, H. H. Wang, P. C. W. Leung, T. J.

Emge, U. Geiser and K. D. Carlson, Acc. Chem. Res. 18, 261 (1 985).

14. U. Schoberl, J. Salbeck and J. Daub, Adv. Mater. 4, 41 (1 992).

15. L. Cavara, F. Gerson, D. 0. Cowan and K. Lerstrup, Helv. Chirn.Acta 69, 141 (1986).

16. F. Gerson, R . Heckendorn, D. 0. Cowan, A. M. Kini and M . Maxfield, J . Am. Chem. Soc. 105, 701 7 (1 983).

17. A. M Kini, D. 0. Cowan, F. Gerson and M. Mockel, J . Am.

18.

19.

20.

21.

22. 23.

24.

25.

26.

27. 28. 29.

30. 31 32.

33. 34. 35.

Chem. Soc. 107,556 (1 985). A. Terahara, H. Ohya-Nishiguchi, N. Hirota, H. Awaji, T. Kawase, S. Yoneda, T. Sugimoto and Z. Yoshida, Bull. Chem. Soc.Jpn. 57, 1760 (1984). H. B. Scheffler and H. 6. Stegmann, Nektronenspinresonanz. Springer, Berlin (1 974). J. A. Weil, J. E. Wertz and J. R. Bolton, Electron Paramagneric Resonance. W h y , New York (1994). J. Daub, K M. Rapp, J. Salbeck, U. Schoberl, in Carbo- hydrates as Raw Mererials, edited by F. W. Lichtenthaler, p. 323. VCH, Weinheim (1991). S. F. Nelsen, J . Chem. Soc., Perkin Trans. 2 1005 (1 988). M. Karplus and G. K. Fraenkel, J . Chem. Phys. 35, 1312 (1 961 ) . C. A. Liberko, S. F. Rak and L. L. Miller, J . Org. Chem. 57, 1379 (1 992). H . Hopf, M. Kreutzer, C. Mlynek, M . Scholz and G . Gescheidt, Helv. Chim. Acta 77, 1466 (1 994). M. J. S. Dewar, E. G. Zoebisch, E. G. Healy and J J. P. Stewart, J. Am. Chern, Soc. 107, 3902 (1 985). H. M . McConnel1.J. Chem. Phys. 24, 632 (1 956) P. D. Sullivan, J . Am. Chem. Soc. 90. 361 8 (1 968). F. B. Bramwell, R . C. Haddon, F. Wudl, M . L. Kaplan and J. H. Marshall,J.Am. Chem. Soc. 100, 4612 (1978). B. Wiedel, VAMPC. University of Erlangen, Erlangen (1 994). R . Pariser and R. G. Parr, J . Chem. Phys. 21, 466 (1 953). H. Hopf, M . Kreutzer and P. G. Jones,Angew. Chem. lnr. Ed. Engl. 30, 1127 (1991). A. J. Stone, Mol. Phys. 6, 509 (1 963). A. J. Stone, Proc R . Soc. London, Ser. A 271, 309 (1 963). A. J. Stone, Mol. Phys. 7 , 31 1 (1 964).