Catalytic Generation of Oxalate through a Coupling Reaction of Two CC₂ Molecules Activated on [(Ir(η⁵-C₅Me₅))₂(Ir(η ⁴-C₅Me₅)CH₂CN)(μ ₃-S)₂]

Electrochemical reduction of [(Ir(μ⁵-C₅Me₅))₃(μ ₃-S)₂](BPh₄)₂ ([Ir₃S₂](BPh₄)₂) in CO₂-saturated CH₃CN at - 1.30 V (vs Ag/AgCl) produced C₂O₄^2- and [(Ir(η⁵-C₅Me₅))₂(Ir(η ⁴-C₅Me₅)CH₂CN)(μ ₃-S)₂]⁺ ([Ir₃S₂CH₂CN]⁺). The crystal structure of [Ir₃S₂CH₂CN](BPh₄) by X-ray analysis revealed that a linear CH₂CN group is linked at the exo-position of a C₅Me₅ ligand, and the C₅Me₅CH₂CN ligand coordinates to an Ir atom with an η⁴-mode. The cyclic voltammogram of [Ir₃S₂CH₂CN]⁺ in CH₃CN under CO₂ exhibited a strong catalytic current due to the reduction Of CO₂, while that of [Ir₃S₂]²⁺ did not show an interaction with CO₂ in the same solvent. The reduced form of [Ir₃S₂CH₂CN]⁺ works as the active species in the reduction of CO₂. The IR spectra of [Ir₃S₂CH₂CN]⁺ in CD₃CN showed a reversible adduct formation with CO₂ and also evidenced the oxalate generation through the reduced form of the CO₂ adduct under the controlled potential electrolysis of the solution at -1.55 V. A coupling reaction of two CO₂ molecules bonded on adjacent μ₃-S and Ir in [Ir₃S₂CH₂CN]₀ is proposed for the first catalytic generation of C₂O₄^2- without accompanying CO evolution.