Systematic and non-systematic substituent effects gleaned from studies on CuTCNQFn (n = 0, 1, 2, 4): Electrocrystallisation and characterisation of CuTCNQF

Abstract Exploitation of ligand substituent effects provides a method for systematically tuning the properties of inorganic compounds. Thus, fluorination of TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) followed by reduction to their anionic and dianionic forms allows a series of copper(I) coordination polymers to be prepared that vary systematically with respect to redox and other properties. However, the catalytic properties of CuTCNQFn (n = 0, 1, 2, 4) do not vary systematically with the extent of fluorination as revealed by combining data obtained in this study using TCNQF and TCNQF2 with that published with TCNQ and TCNQF4. Initial studies confirmed that the cyclic voltammetry of 8.0 mM TCNQF0 (TCNQF0 = 2-fluoro-7,7,8,8-tetracyanoquinodimethane) displays two ideal one-electron reversible diffusion controlled TCNQF0/1- and TCNQF1-/2- processes at a glassy carbon electrode in acetonitrile (0.1M Bu4NPF6). Upon introduction of equimolar [Cu(MeCN)4]+, a series of new processes involving oxidation and reduction of surface confined electrocrystallised Cu+-TCNQF1- as well as a Cu+-TCNQF2- derivative were detected. Mechanisms for the electrocrystallisation of kinetically and thermodynamically favoured phases of CuTCNQF are proposed. Variable morphology of elecrocrystallised TCNQF-based material onto ITO electrodes was evident from SEM images. CuTCNQF in the thermodynamically favoured phase was also synthesised chemically by reaction of TCNQF with copper(I) iodide or copper foil or via the reaction between TCNQF1- generated by bulk electrolysis in MeCN (0.10 M Bu4NPF6 and a stoichiometric amount of [Cu(MeCN)4]+. The catalytic activity of CuTCNQF and CuTCNQF2, formed on the surface of a copper foil, with respect to the [Fe(CN)6]3-/S2O32- aqueous redox reaction was investigated by analysis of spectrophotometric and open circuit potential measurements. Comparison with published data with CuTCNQ-Cu foil and CuTCNQF4-Cu foil revealed that the extent of catalysis achieved in these materials were similar and slow. In contrast, reaction rates in the presence of the CuTCNQF2 and CuTCNQF4-Cu foil analogues were much faster, but also similar to each other. This non-systematic effect implies that catalysis requires the TCNQ(Fn)2- (reduced dianion) to be accessible, thus the reversible potential for neither the Cu-TCNQFn1-/2- (n = 0,1) are sufficient to achieve substantial catalysis while the reversible potential for both the Cu-TCNQFn1-/2- (n = 2,4) reactions are well in excess of the minimum value required to achieve catalysis. The conductivity of CuTCNQF was 1.86 × 10-5 S cm-1 which is at the low end of the semiconductor range, and hence orders of magnitude less than for the kinetically favoured phase of CuTCNQ; 0.2 S cm-1. Interestingly, the fluorinated analogues of the readily accessible, highly conducting (kinetically favoured) CuTCNQ phase I are only detected transiently under voltammetric conditions for CuTCNQFn (n=1, 2) and have not been detected for the CuTCNQF4 derivative.