Order Versus Disorder in 1,2,3- and 1,2,6- Trimethylpyridinium Chlorocuprate(II) Salts

The vicinal trimethypyridinium (TMP) cations with 1,2,3- or 1,2,6- substitution form distinct compounds when crystallized from aqueous solution. (1,2,6-TMP)2CuCl4 forms asdark green crystals, in which the 1,2,6-TMP cation is ordered, that exhibit a thermochromic transition to a yellow phase above 60 degrees C. (1,2,3-TMP)CuCl3(H2O) forms as emerald green crystals, in which the 1,2,3-TMP cations are two-fold disordered by symmetry, that are non-thermochromic. In order to study the effect of disorder on the choice of compound, a successful attempt was made to prepare mixed cation crystals by slow evaporation of a solution containing a 1:1:1 ratio of 1,2,3-tmP:1,2,6-TMP:CuCl2. Dark green crystals with the characteristic color of the CuCl4 dianion compound were the only ones obtained. Structure determination on on of these crystals verified the presence of the CuCl4 dianion, but now with a mix of 1,2,3-TMP and !,2,6-TMP with occupancies of 0.85 and 0.15 respectively. Thus it appears that a small reduction of the positional disorder of the 1,2,3-TMP cation favors formation of the CuCl4 compound over the CuCl3(H2O) compound. Further, the presence of disorder now in the CuCl4 compound appears to stabilize the low temperature phase. Observation of thermal response of several different crystal to increasing temperature found the transition temperature dramatically higher than in pure (1,2,6-TMP)2CuCl4 or completely absent. The variation of thermal response by different crystal suggest the mixed cation crystals form as a solid solution rather than as a compound with a defined composition. These results suggest several different directions for future work.