Role of Water in the Dynamic Disproportionation of Zn-Based TCNQ(F4) Coordination Polymers (TCNQ = Tetracyanoquinodimethane)

Intriguingly, coordination polymers containing TCNQ2– and TCNQF42– (TCNQ = 7,7,8,8-tetracyanoquinodimethane, TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, both designated as TCNQ(F4)2–) may be generated from reaction of metal ions with TCNQ(F4)•–. An explanation is now provided in terms of a solvent-dependent dynamic disproportionation reaction. A systematic study of reactions associated with TCNQ(F4) and electrochemically generated TCNQ(F4)MeCN•– and TCNQ(F4)MeCN2– revealed that disproportionation of TCNQ(F4)MeCN•– radical anions in acetonitrile containing a low concentration of water is facilitated by the presence of ZnMeCN2+. Thus, while the disproportionation reaction 2TCNQ(F4)MeCN•– ⇌ TCNQ(F4)MeCN + TCNQ(F4)MeCN2– is thermodynamically very unfavorable in this medium (Keq ≈ 9 × 10–10; TCNQF4), the preferential precipitation of ZnTCNQ(F4)(s) drives the reaction: ZnMeCN2+ + 2 TCNQ(F4)MeCN•– ⇌ ZnTCNQ(F4)(s) + TCNQ(F4)MeCN. The concomitant formation of soluble TCNQ(F4)MeCN and insoluble ZnTCNQ(F4)(s) and the loss of TCNQ(F4)MeCN•– 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 ZnTCNQ(F4)(s). Thus, in this "wet" environment, ZnMeCN2+ and TCNQ(F4)MeCN•– are produced from a mixture of ZnTCNQ(F4)(s) and TCNQ(F4)MeCN and with the addition of water provides a medium for synthesis of [Zn(TCNQ(F4))2(H2O)2]. An important conclusion from this work is that the redox level of TCNQ(F4)-based materials, synthesized from a mixture of metal cations and TCNQ(F4)•–, 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.