Role of water in the dynamic disproportionation of Zn-based TCNQ(F 4) coordination polymers (TCNQ = Tetracyanoquinodimethane)

Abstract

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(F 4)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(F 4)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, ZnMeCN and TCNQ(F4)MeCN 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(F 4))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. © 2014 American Chemical Society.