Structural, Spectroscopic, and Electrochemical Characterization of Semi-Conducting, Solvated [Pt(NH3)4](TCNQ)2·(DMF)2 and Non-Solvated [Pt(NH3)4](TCNQ)2

Abstract

The demand for catalysts that are highly active and stable for electron-transfer reactions has been boosted by the discovery that [Pt(NH3)4](TCNQF4)2 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is an efficient catalyst. In this work, we prepare and characterize the two related [Pt(NH3)4]2+ complexes, [Pt(NH3)4](TCNQ)2·(DMF)2 (1) and [Pt(NH3)4](TCNQ)2 (2). Reaction of [Pt(NH3)4](NO3)2 with LiTCNQ in a mixed solvent (methanol/dimethylformamide, 4 : 1 v/v) gives [Pt(NH3)4](TCNQ)2·(DMF)2 (1), whereas the same reaction in water affords [Pt(NH3)4](TCNQ)2 (2). 2 has been previously reported. Both 1 and 2 have now been characterized by single-crystal X-ray crystallography, Fourier-transform (FT)IR, Raman and UV-vis spectroscopy, and electrochemistry. Structurally, in 1, the TCNQ1− anions form infinite stacks with a separation between adjacent anions within the stack alternating between 3.12 and 3.42 Å. The solvated structure 1 differs from the non-solvated form 2 in that pairs of TCNQ1− anions are clearly displaced from each other. The conductivities of pressed pellets of 1 and 2 are both in the semi-conducting range at room temperature. 2 can be electrochemically synthesized by reduction of a TCNQ-modified electrode in contact with an aqueous solution of [Pt(NH3)4](NO3)2 via a nucleation growth mechanism. Interestingly, we discovered that 1 and 2 are not catalysts for the ferricyanide and thiosulfate reaction. Li+ and tetraalkylammonium salts of TCNQ1−/2− and TCNQF41−/2− were tested for potential catalytic activity towards ferricyanide and thiosulfate. Only TCNQF41−/2− salts were active, suggesting that the dianion redox level needs to be accessible for efficient catalytic activity and explaining why 1 and 2 are not good catalysts. Importantly, the origin of the catalytic activity of the highly active [Pt(NH3)4](TCNQF4)2 catalyst is now understood, enabling other families of catalysts to be developed for important electron-transfer reactions.

Semiconductive 2D arrays of pancake-bonded oligomers of partially charged TCNQ radicals

“…In many structures, isolated pancake-bonded dimers (which form no stacking interaction with neighbouring molecules) are present (Keller et al, 1981;Harms et al, 1982;Abashev et al, 1987;Miller et al, 1987;O'Hare et al, 1990;Stein et al, 1991;Grossel & Weston, 1992;Chen et al, 2008;Sutton et al, 2016;Park et al, 2018). Sometimes the dimers form 1D stacks similar to those of semiquinones (Hoekstra et al, 1972;Ashwell et al, 1975aAshwell et al, , 1977aBosch & Bodegom, 1977;Endres et al, 1978,b;Diezt et al, 1981;Konno & Saito, 1981;Waclawek et al, 1983;Visser et al, 1990a,b,c,d,e;Magonov et al, 1991;Ballester et al, 1997;Moore et al, 2001;Mukai et al, 2001;Nishimura et al, 2002;Chen et al, 2008;Qu et al, 2011;Martin et al, 2012;Lu et al, 2017;U ¨ngo ¨r et al, 2021a), but also planar 2D arrays have been described (Ashwell et al, 1982(Ashwell et al, , 1983Bryce et al, 1988;Brook & Koch, 1997;Radva ´kova ´et al, 2010;Qu et al, 2011); in some of these structures the formal charge (i.e. charge derived from stoichiometry) of TCNQ is À1/2 (Ashwell et al, 1977a(Ashwell et al, , 1982.…”