Redox, acid - base chemistry, electrochemical synthesis and characterization of materials based on TCNQF4 and TCNQ

Abstract

Redox and acid-base chemistry of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), and their mono- and di-anions in acetonitrile in the presence or absence of trifluoroacetic acid (TFA) have been studied systematically. In acetonitrile containing supporting electrolyte, both compounds undergo two electrochemically and chemically reversible one-electron reduction steps. The reversible formal potentials associated with these reductions of TCNQF4 are 0.36 and 0.37 V more positive than those for TCNQ. This result is consistent with the higher electron affinity of TCNQF4 than TCNQ due to the strong electron withdrawing effect of the fluorine atoms. The underlying mechanisms for the electrochemistry and the acid-base chemistry of TCNQ, TCNQF4 and their anions have been proposed and supported by digital simulations. A wealth of thermodynamic and kinetic parameters has been revealed from the experimental and simulated data. A comparison of the redox and acid-base chemistry between TCNQ and TCNQF4 as well as their anions also has been discussed. Studies of chemical and electrochemical synthesis of new materials based on TCNQF4 in combination with Li+, Pr4N+, Ag+, Cu+, Co2+ and Ni2+ have been undertaken. In acetonitrile, I- (as the I- salt) reduces TCNQF4 to TCNQF4•- or TCNQF4(2-) depending on the reaction conditions. At low temperature (0 oC) and using the molar ratio I-:TCNQF4 3:1, the major product is the TCNQF4(2-) dianion. The LiTCNQF4 and Li2TCNQF4 thus generated were used to chemically synthesize Pr4NTCNQF4 and (Pr4N)2TCNQF4. TCNQF4 can also be electrochemically reduced in acetonitrile to afford either TCNQF4•- or TCNQF4(2-) by applying different potentials. This electrosynthetic approach has, therefore, been applied to crystallize a family of charge-transfer, semiconducting materials, including AgTCNQF4, Ag2TCNQF4, CuTCNQF4, Cu2TCNQF4, CoTCNQF4 and NiTCNQF4 onto an electrode surface. The electrodeposition of these compounds has been monitored using cyclic voltammetry, surface plasmon resonance and electrochemical quartz crystal microbalance. The solubility of these compounds has been determined in acetonitrile in the presence or absence of 0.1 M Bu4NPF6 supporting electrolyte and found to increase in the presence of the supporting electrolyte as a result of ion pairing between ions from the electrolyte and the complexes. These metal-organic charge-transfer complexes also have been chemically synthesized from acetonitrile solutions of the corresponding TCNQF4 anion and metal cation. A wide range of microscopic and spectroscopic techniques has been applied to characterize the newly generated compounds. Besides synthesis of TCNQF4-based compounds in acetonitrile, a new complex Ni(TCNQF4)2(H2O)2 has been electrochemically synthesized in aqueous media via the reduction of TCNQF4 solid immobilized onto an electrode surface and placed in contact with a Ni2+(aq) solution. The solid-solid state transformation TCNQF4/Ni(TCNQF4)2(H2O)2 has been explored via cyclic voltammetry and chronoamperometry. The electrosynthesis involves a one-electron reduction of TCNQF4 to TCNQF4•-, accompanied with the ingress of Ni2+(aq) from the bulk solution. This transformation is governed by a nucleation and growth mechanism, and is independent of the electrode material and the identity of Ni2+(aq) counter anion, but strongly dependent on Ni2+(aq) concentration and scan rate. The composition, structure, magnetic and conducting properties of the Ni(TCNQF4)2(H2O)2 complex have been determined. The disproportionation reaction of TCNQF4•- and TCNQ•- (both designated as TCNQ(F4)•-) in the presence of Zn2+ is reported. Acetonitrile solutions containing TCNQ(F4)•- and Zn2+ form dissolved TCNQ(F4) and ZnTCNQ(F4) precipitates. The neutral TCNQ(F4) dissolved in acetonitrile was detected using UV-vis spectroscopy and steady-state voltammetry, whilst the presence of TCNQ(F4)2- in the precipitates was confirmed by infrared spectroscopy, and also by showing that the precipitates reacts with TCNQ(F4) in the presence of water to form TCNQ(F4)•-. Interestingly, the extent of the disproportionation decreases upon addition of water; > 3% (v/v) of water results in no disproportionation. A mechanism for the disproportionation has been proposed and is attributed to the precipitation of ZnTCNQ(F4), which shifts the equilibrium 2TCNQ(F4)•- TCNQ(F4) + TCNQ(F4)2- to the right-hand side. However, the solubility of ZnTCNQ(F4) in water is higher than in acetonitrile, and hence the disproportionation is less favored in the presence of water. Electrochemical and spectroscopic identification of TCNQF4 redox levels are reviewed. Steady-state voltammetry and UV-vis spectroscopy are simple and powerful techniques for the qualitative and quantitative detection of TCNQF4 and its anions (TCNQF4•- and TCNQF4(2-)) since each species has a characteristic fingerprint. Infrared spectroscopy has been applied extensively and successfully in the qualitative determination of the redox levels of TCNQF4 in a range of compounds. Predominately, the frequency of the CN triple bond stretch is diagnostic of the redox level. Raman vibration modes for the CN triple bond and C=C ring stretches also are sensitive to the TCNQF4 redox levels.