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Structural, Spectroscopic and Electrochemical Characterisation of Semi-conducting, Solvated [Pt(NH3)4](TCNQ)2∙(DMF)2 and Non-solvated [Pt(NH3)4](TCNQ)2
The high demand for catalysts which are highly active and stable for electron transfer reactions has been boosted by the discovery that [Pt(NH3)4](TCNQF4)2 (TCNQ = 7,7,8,8-tetracyanoquinodimethane) is a highly-efficient catalyst. In this work, we prepare and characterise the two related [Pt(NH3)4]2+ complexes, [Pt(NH3)4](TCNQ)2∙(DMF)2 (1) and [Pt(NH3)4](TCNQ)2 (2) The synthesis, structural spectroscopic and electrochemical investigation of 1 and 2 does provide some interesting comparative chemistry. Reaction of [Pt(NH3)4](NO3)2 with LiTCNQ in a mixed solvent (methanol/dimethylformamide, CH3OH/DMF, 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. Although 2 has been previously reported, both 1 and 2 have been characterised by single crystal X-ray crystallography, FTIR, Raman and UV-vis spectroscopy and electrochemistry. Structurally, in complex 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 to the non-solvated form 2 in which 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. Complex 2 can be electrochemically synthesised by reduction of a TCNQ-modified electrode in contact with an aqueous solution of [Pt(NH3)4](NO3)2 via a nucleation growth mechanism. Unexpectedly, we discover that 1 and 2 are not catalysts for the ferricyanide and thiosulphate reaction. Li+ and tetraalkylammonium salts of TCNQ1-/2- and TCNQF41-/2- were tested for potential catalytic activity towards ferricyanide and thiosulphate. Only TCNQF41-/2- salts were active suggesting that the dianion redox level needs to be accessible for efficient catalytic activity and explains 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.
CH17245 Accepted 16 June 2017
© CSIRO 2017