Abstract
Intriguingly, coordination polymers containing TCNQ(2-) and TCNQF(4)(2-) (TCNQ= 7,7,8,8-tetracyanoquinodimethane, TCNQF(4) = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, both designated as TCNQ(F-4)(2-)) may be generated from reaction of metal ions with TCNQ(F4)(center dot-). An explanation is now provided in terms of a solvent-dependent dynamic clisproportionation reaction. A systematic study of reactions associated with TCNQ(F4) and electrochemically generated TCNQ(F-4)(MeCN)(center dot-) and TCNQ(F-4)(MeCN)(2-) revealed that disproportionation of TCNQ(F-4)(MeCN)(center dot-) radical anions in acetonitrile containing a low concentration of water is facilitated by the presence of Zn-MeCN(2+). Thus, while the disproportionation reaction 2TCNQ(F-4)(MeCN)(center dot-) + TCNQ(F-4)(MeCN)(2-) is thermodynamically very unfavorable in this medium (K-eq approximate to 9 X 10(-1); TCNQF(4)), the preferential precipitation of ZnTCNQ(F-4)((s)) drives the reaction: Zn-MeCN(2+) + 2 TCNQ(F-4)(MeCN)(center dot-) TCNQ(F-4)(MeCN) + TCNQ(F-4)(MeCN). The concomitant formation of soluble TCNQ(F-4)mecN and insoluble ZnTCNQ(F-4)(s) and the loss of TCNQ(F-4)(MeCN)(center dot-) were verified by UV visible and infrared spectroscopy and steady-state voltammetry. Importantly, the reverse reaction of comproportionation rather than disproportionation becomes the favored process in the presence of >= 3% (v/v) water, due to the increased solubility of solid ZnTCNCI(F-4)(s). Thus, in this "wet" environment, ZriecN and TCNQ(F-4)(MecN)(center dot-) are produced from a mixture of ZnTCNQ(F-4)((s)) and TCNQ(F4)mecN and with the addition of water provides a medium for synthesis of [Zn(TCNQ(F4))2(H20)2]. An important conclusion from this work is that the redox level of TCNQ(F-4)-based materials, synthesized from a mixture of metal cations and TCNQ(F-4)(center dot-), is controlled by a solvent-dependent disproportionation/comproportionation reaction that may be tuned to favor formation of solids containing the monoanion radical, the dianion, or even a mixture of both.