J . Chem. SOC., Faraday Trans. 1, 1982, 78, 1401-1404 Coloured and Colourless Charge-transfer Complexes of Small and Polymeric Quaternary Pyridinium Bromides BY ERNEST A. BOUCHER* AND CHRISTOPHER C . MOLLETT School of Molecular Sciences, University of Sussex, Brighton BNl 9QJ Received 18th March, 1981 The products of quaternization reactions involving alkyl bromides are, under certain conditions, coloured, e.g. 4-methyl-N-n-propylpyridinium bromide is red and poly(4-vinyl-N-n-butylpyridinium bromide) is green when prepared in benzene, say, at 333 K. Neither are coloured, however, when heated in sulpholane. These and other products have been shown to be coloured due to charge-transfer bands; the possible involvement of impurities or side reactions has been eliminated: important in establishing the reliability of kinetic measurements by the authors. Comparison has been made with several iodides under varying conditions: these are invariably colourless in the solid and pale yellow in ethanol.They lack the low-energy charge-transfer bands (colours), but show charge-transfer bands in the U.V. Coleman and FUOSS~ (who were interested in polymer quaternization) found colour changes from yellowish-green to dark green with 4-methylpyridine + n-butyl bromide in nitrobenzene (they concluded that the presence of the 4-methyl group was necessary for colour production). Kronick and FUOSS~ found that some reactions in propylene carbonate gave orange solutions deepening to brown. On the other hand, Coleman and FUOSS~ also found orange solutions with 4-methylpyridine + n-butyl bromide in nitromethane and in NN’-dimethylformamide (DMF), which they attributed to side reactions (side reactions are known,3 but it is not yet clear whether they contribute to the colour).Quite separately Kosower and c o ~ o r k e r s ~ - ~ have extensively studied quaternary pyridinium iodides, which in solution are slightly yellow. [Ref. (1) and (4) are in the same journal volume, but are not cross-referenced.] Hantzsch,s incidentally, was aware of coloured quaternary pyridinium salts. Ray9 and Ray and Makurjee’O investigated the electronic spectra of N-ethylpyridinium bromide in water and N-alkylpyridinium iodides in chloroform. Fosterll provides a suitable background to charge-transfer complexes, and Griffiths and Pugh12 have critically reviewed Kosower’s work.When studies13-15 of the kinetics and mechanism of the quaternization of poly(4- vinylpyridine) with alkyl and arylalkyl bromides commenced, coloured products were thought to arise for one of three possible reasons: (i) impurities in the reactants; (ii) side reactions with solvent or (iii) the products being charge-transfer complexes. Apart from the inhererit interest in the coloured products, it was important to investigate them because the occurrence of (i) or (ii) would undermine the kinetic studies. To determine the nature of the polymeric products, some small quaternary pyridine halides were investigated in the U.V. (bromides and iodides) and visible (bromides) regions. EXPERIMENTAL The preparation of quaternary pyridinium halides in several solvents, usually in the range 320-350 K, from purified chemicals under an atmosphere of nitrogen has been adequately 46 1401 F A R 11402 PYRIDINIUM CHARGE-TRANSFER COMPLEXES described in connection with kinetic studies.3* 13-15 Solutions and solid samples were handled and kept under water-free nitrogen. Electronic spectra were recorded on a Unicam SP800 spectrophotometer at 298 K.Other techniques mentioned were carried out according to standard procedures. A reflectance spectrum of compressed powdered 4-methyl-N-n-propylpyridinium bromide was run under dried nitrogen in a Beckmann mark IV Acta spectrometer. RESULTS AND DISCUSSION Table 1 gives the main features found for five N-pyridinium bromides. The coloured products were actually obtained at 333 K from benzene.Coloured species could be obtained at 333 K (arbitrarily chosen) by mixing the pure liquid reagents alone or in the solvents benzene (2.28), toluene (2.38), nitromethane (35.9), DMF (36.7), and propylene carbonate (65.6). Values of the relative permittivity at 298 K are given in TABLE 1 .-ULTRAVIOLET AND VISIBLE ABSORPTION PROPERTIES OF PYRIDINIUM BROMIDES Rrnaxlnm Emax compound (colour) ref. solvent (main peak) AmaX/nm cmaX 4-methyl-N-n-propyl- chloroform 256 7900 455 95.5 4-methyl-N-n-butyl- chloroform 256 8050 455 95.4 pyridinium bromide” ethanol 256 7800 446 85.6 (red) water 256 7800 44 1 83.8 pyridinium bromide water 256 8100 440 84.3 (red) 4-me th yl-N- benzyl- chloroform 256 6900 490 153 pyridinium bromide water 256 6900 46 1 136 (‘ma~ve’)~ 4-et hyl-N-n-butyl chloroform 228 6700 620 40.2 pyridinium bromide ethanol 229 6400 616 38.7 (green) poly(4-vinyl-N-n-butyl- ethanol 237 68 60 660 38.3 pyridinium bromide)c (green) a Colourless when prepared in water or benzene at 298 K (and ethanol or sulpholane at 333 K); similar properties to these in table for product prepared in toluene or propylene carbonate at 333 K.Similar properties when prepared in propylene carbonate, but no colour when prepared in nitromethane. All solutions were of concentration 0.5 mol dm-3. Similar colour when no solvent present during preparation. the brackets. Colours could not be produced in ethanol (24.3), but a coloured solid retained its colour when dissolved in this solvent. Note also that the products precipitate out of benzene as they are formed. Also, in contrast to ethanol, colours are produced in propylene carbonate, whereas the trend seems to be for the production of the coloured species in solvents of low relative permittivity.No colours have been found in sulpholane (44) under any circumstances. Table 1 indicates the need for a substituent in the 4-position (compared to the colourless N-ethylpyridinium bromide), and the absorption frequency in the visible shows some sensitivity to the nature of this substituent. With a 4-methyl substituentE. A. BOUCHER AND C. C. MOLLETT 1403 the product is red or ‘mauve’, and with a 4-ethyl substituent and the polymer species the products have an even lower transition energy, being green; i.e. A,,, (visible) increases in the order 4-methyl c 4-ethyl c poly(4-vinyl) for the N-n-butyl com- pounds.The colour is not so sensitive to the N substituent, but Amax is slightly greater for N-benzyl compared with N-n-butyl. Regarding the solvents in which the spectra were run, table 1 shows some differences between chloroform and water; ethanol was similar to water. The following are representative observations. When heated above its melting point (395.0 K) red 4-methyl-N-n-propylpyridinium bromide did not change colour, and the colourless sample remained colourless above its observed melting point (395.5 K). However, the colourless sample when dissolved in acetone and heated to 333 K for 4 h turned red; the red colour was not affeLced by 5 fractional recrystallizations from acetone.The red salts retained the colour (3-4 years to date). The green colour of 4-ethyl-N-n-butylpyridinium bromide disappeared over several days at room tem- perature, but could be regenerated by re-heating the sample in 4-ethylpyridine (acting as solvent). The green colour of poly(4-vinyl-N-n-butylpyridinium bromide) persisted for several months. The possible involvement of free halide was eliminated when it was found that the HSO; ion did not affect the yellow iodides or the coloured bromides. Proton n.m.r. and i.r. spectra, although of limited sensitivity, showed no distinction between coloured and colourless varieties and did not reveal any impurities. No free radicals could be detected in the solutions by e.s.r. There is no reason to believe that trapped solvent is present in the solid state (excellent agreement of individual and mixed melting points and no change under vacuum).Coloured solids did not lose any colour when heated under vacuum near their melting points. Vapour above warmed coloured solid did not show evidence of solvent in g.1.c. Colourless solids subjected to a nominal pressure of lo4 kg in a KBr press did not produce any colour. Unfortunately crystal structures have not been determined for any of the solids, and the possibility of small changes in position or separation distances cannot be ruled out as distinguishing coloured from colourless solids. As far as can be judged the solutions and solids are not air-sensitive. The A, values in table 1 for 4-methyl-N-n-propylpyridinium bromide range from 441 to 455 nm depending on the solvent.It was not easy to obtain reflectance spectra of the solid, but that for this compound showed a broad maximum with a definite shift in Amax to ca. 490 nm. This is taken as evidence that the absorption is modified by the presence of solvent, but that the same charge-transfer species exists in the solid and in solution. In connection with a related study (unpublished) of the quaternization of poly(4- vinylpyridine) with iodides, the u.v.-visible spectra were examined of the products in ethanol and water from 4-methylpyridine + ethyl iodide and pyridine +ethyl iodide. These salts are colourless in the solid but appear pale yellow in ethanol solution. No colours other than pale yellow could be produced under conditions where bromides would be coloured.Generally the absorption spectra were very similar to those found for charge-transfer iodides by Kosower et al.4-7 This study, apart from establishing that the coloured pyridinium bromides, including the polymer derivative, are genuine charge-transfer complexes, brings together what have been two separate areas of investigation. There is now no need to avoid solvents in which coloured products are produced for quantitative quaternization studies, unless side reactions also occur (e.g. with DMF present). There is no evidence favouring the participation of impurities in the colour formation or of anomalous species derived from the reactants. 46-21404 PYRIDINIUM CHARGE-TRANSFER COMPLEXES C.C.M. is grateful to the States of Guernsey Education Department for a studentship. We are very grateful to a referee for helpful comments. B. D. Coleman and R. M. Fuoss, J . Am. Chem. 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