Electro- and photochemical properties of a (μ-alkoxo)bis(μ-carboxylato)diruthenium complex having two tetraphenylporphinato zinc(ii) moieties

Abstract

The novel (μ-alkoxo)bis(μ-carboxylato)diruthenium complex K[Ru₂(dhpta)(μ-O₂C-p-ZnTPP)₂] 3 was prepared by simple ligand substitution reaction. Strong antiferromagnetic interaction between two Ru^III ions of 3 was observed with a coupling constant of −425 ∼ −404 cm⁻¹. The cyclic voltammogram of 3 can be explained in terms of superposition of those of ZnTPP-p-CO₂H and K[Ru₂(dhpta)(μ-O₂CPh)₂] 2, indicating no significant electrochemical interaction. The large conproportionation constant estimated from the reduction potentials for Ru^IIIRu^III and Ru^IIRu^III indicates great stability of the mixed-valence state. The mixed-valence species [Ru^IIRu^III(dhpta)(μ-O₂C-p-ZnTPP)₂]²⁻4 was prepared by controlled potential electrolysis. The electronic absorption spectrum of 4 was quite similar to that of [Ru^IIRu^III(dhpta)(μ-O₂CCH₃)₂]²⁻ which is a typical Class II complex. The fluorescence from the S₂ state of the ZnTPP unit of 3 was significantly (78%) quenched. The electron transfer from the ZnTPP unit to Ru^III ions in 3 is a plausible mechanism, even though energy transfer could not be ruled out completely. The free energy change for electron transfer, ΔG_CS, was estimated to be ca. −1.1 eV, which is similar to typical values for the reorganization energy λ in polar solvents. Hence, the electron transfer scheme is situated almost at the top of the Marcus parabola, enabling ultrafast electron transfer.