Energy & Fuels 1994,8, 1524-1525
1624
Unusual Electronic Properties of Complexes between Coals and the Oxidants TCNQ and TCNE Robert A. Flowers, II,t Layce Gebhard,' John W. Larsen,*vtJYuzo Sanada,B Masahide Sasaki,l and Bernard Silbernagel' Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, Pennsylvania 18015, Metals Research Institute, Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan, Exxon Research and Engineering Company, Route 22 East, Clinton Township, Annandale, New Jersey 08801, and Resources and Energy Division, Hokkaido National Industrial Research Institute, 2-1 7-2-1 Tsukisam u-Higashi, Toyohi ra-k u, Sapporo 062, Japan Received January 7, 1994. Revised Manuscript Received July 15, 1994 When strong electron acceptors like tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE) are added to Illinois No. 6 coal, there is strong spectroscopic evidence that the coals transfer significant electron density to these molecules. However, we do not observe the creation of localized ionized donor molecules in amounts that are commensurate with the extent of electron transfer. Nor are the aromatic groups in Illinois No. 6 coal individually capable of significant electron transfer to these acceptors. We therefore conclude that the electron transfer is a bulk property of the solid, arising from electrons in delocalized states, and not due t o electron transfer from individual independent aromatic structures in the coal. TCNQ and TCNE are good one-electron oxidants which have been used to prepare electrically conducting organic comp1exes.l The complex formed between the electron donor, tetrathiofulualene, and the acceptor TCNQ is one of the most studied organic conductors.l We here present evidence that coals transfer significant electron density to TCNQ and TCNE and that this transfer is a bulk property of the solid (a cooperative property) and is not due to electron transfer from individual independent aromatic structures in the coal. The extent of electron transfer t o TCNQ is easily measured by IR spectroscopy.2 Electron density transferred from a donor t o TCNQ enters the TCNQ lowest unocuppied molecular orbital (LUMO), an antibonding orbital. This alters bond strengths throughout the molecule, including the C s N bonds. Transferring density to TCNQ causes the C=N stretching frequency to shift to lower wavenumbers, a shift which is linear in the amount of electron density transferred.2 With TCNQ, transfer of one electron causes a shift of 44 cm-l. When deposited in Illinois No. 6 coal at all concentrations up to one TCNQ per aromatic structure, the full 44 cm-l shift is ~ b s e r v e d . The ~ IR spectrum obtained (see Figure 2) is that of the TCNQ radical anion. Furthermor, all of the TCNQ is fully shifted; no un-
' Lehigh University. i Exxon
Research and Engineering Co. Hokkaido University. Resources and Energy Division, Hokkaido National Industrial Research Institute. (1) Soos, Z. G.; Klein, D. J. InMoZecuZar Association; Foster, R., Ed.; Academic Press: New York, 1981; Vol. 1. (2) Chappel, J. S.;Bloch, A. B.; Bryden, W. A,; Maxfield, M.; Poehler, T. 0.;Cowan, D. 0. J . Am. Chem. SOC.1981,103,2442-2443. (3)The populations of aromatic systems were calculated from information in: Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. 4
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Figure 1. Overlayed diffuse reflectance IR spectra of Illinois No. 6 coal and TCNQ.
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Figure 2. Diffuse reflectance IR spectrum of Illinois No. 6 coal containing 20 mg of TCNQ/g of coal.
shifted TCNQ is observed. With TCNE, the situation is identical except that the shift in the C=N stretching frequency is 36 cm-l. The TCNQ and TCNE are deposited in coals by soaking the coals for 1 week with a pyridine or acetonitrile solution of the electron acceptor and evaporating the solvent at room temperature under v a ~ u u m .Details ~ are provided in the supplementary material. The electron density transferred to TCNQ does not quantitatively produce independent TCNQ radical an(4) Other solvents can be used and some give larger IR shifts than those reported here. A description of the experimental procedures has been deposited as supplementary material.
0 1994 American Chemical Society
Communications
Energy &Fuels, Vol. 8, No. 6, 1994 1525
Table 1. Change in the CN Stretching Frequency (cm-’) of TCNQ on Formation of Complexes with Aromatics in Chloroform Solution compound A(CN stretch) (cm-’) 2.4 3.2 3.6 3.5
to1u en e biphenyl pyrazole phenanthrene 1-naphthol 3,5-dimethoxyphenyl fluoranthene
3.5 3.6 3.2
ions, although the blue color of the coal-TCNQ complex suggests the presence of TCNQ radical anion dimers. ESR spectra do not show the spectrum of the TCNQ radical anion even a t liquid nitrogen temperatures. Furthermore, when one TCNQ per aromatic structure is deposited in the coal, the radical population is lo2 less than that required for radical anion formation. The best explanation for the ESR observations is the formation of extended electronic systems involving the TCNQ. The individual aromatic structures present in coal are not capable of transferring significant electron density to TCNQ. This can be shown by the absence of large changes in the TCNQ C z N stretching frequency upon complexation with individual aromatic molecules. The changes in C=N stretching frequency for TCNQ complexed to a selected set of aromatics are given in Table 1. The largest shift is less than 10% of that which is observed for TCNQ in Illinois No. 6 coal. This behavior is as e ~ p e c t e d . ~ TCNQ does not undergo chemical reactions with this coal. Both TCNQ and TCNE undergo ready addition reactions to phenols and other nucleophiles.6 The initial addition products involve formation of nonconjugated ~~~
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(5) Foster, R. Molecular Association; Academic Press: New York, 1981;VOl. 1.
CN groups whose IR stretching frequencies will be found a t 2240-2260 There is no IR evidence for the formation of such addition products. Even if the coal TCNE system is warmed to 180 “C, no CN shifts consistent with the formation of addition products are observed.8 Significant electron density is transferred from Illinois No. 6 coal to TCNQ (and TCNE), yet the individual aromatic structures in coal are not capable of such electron transfer. This transfer therefore must be a bulk property of the material and not due to the behavior of independent individually acting aromatic systems. The fact that no “unreacted”TCNQ remains even when one TCNQ per aromatic structure has been deposited in the coal confirms this. Surely not every aromatic group in the coal will transfer a full electron to TCNQ. The observed transfer, not being the result of individual structures, must be the result of some bulk cooperative behavior. Full details on both TCNQ and TCNE interactions with a new series of coals will be reported in a full paper.
Acknowledgment. We are grateful to the U.S. Department of Energy for supporting the work a t Lehigh University. Supplementary Material Available: A description of the experimental procedures used (2 pages). Ordering information is given on a n y current masthead page. (6) Smith, B.; Persmark, U.Acta Chem. Scand. 1963,17,651-656. Smith, B.; Persmark, U.; Edman, E. Acta. Chem. Scand. 1963,17,709722. Farcasiu, M.;Russell, C. S. J. Org. Chem. 1976,41, 571-572. (7)Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1964; pp 200-203. (8) Larsen, J. W.; Rothenberg, S. Unpublished data.
