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)](TCNQF(4))(2) (TCNQF(4)=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, 4:1v/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 TCNQ(1-) anions form infinite stacks with a separation between adjacent anions within the stack alternating between 3.12 and 3.42 angstrom. The solvated structure 1 differs from the non-solvated form 2 in that pairs of TCNQ(1-) 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 TCNQ(1-/2-) and TCNQF(4)(1-/2-) were tested for potential catalytic activity towards ferricyanide and thiosulfate. Only TCNQF(4)(1-/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)](TCNQF(4))(2) catalyst is now understood, enabling other families of catalysts to be developed for important electron-transfer reactions.