Catalytic Generation of Oxalate through a Coupling Reaction of Two CO₂ Molecules Activated on [(Ir(η⁵-C₅Me₅))₂(Ir(η⁴-C₅Me₅)CH₂CN)(μ₃-S)₂]

Abstract

Electrochemical reduction of [(Ir(eta(5)-C(5)Me(5)))(3)(&mgr;(3)-S)(2)](BPh(4))(2) ([Ir(3)S(2)](BPh(4))(2)) in CO(2)-saturated CH(3)CN at -1.30 V (vs Ag/AgCl) produced C(2)O(4)(2)(-) and [(Ir(eta(5)-C(5)Me(5)))(2)(Ir(eta(4)-C(5)Me(5))CH(2)CN)(&mgr;(3)-S)(2)](+) ([Ir(3)S(2)CH(2)CN](+)). The crystal structure of [Ir(3)S(2)CH(2)CN](BPh(4)) by X-ray analysis revealed that a linear CH(2)CN group is linked at the exo-position of a C(5)Me(5) ligand, and the C(5)Me(5)CH(2)CN ligand coordinates to an Ir atom with an eta(4)-mode. The cyclic voltammogram of [Ir(3)S(2)CH(2)CN](+) in CH(3)CN under CO(2) exhibited a strong catalytic current due to the reduction of CO(2), while that of [Ir(3)S(2)](2+) did not show an interaction with CO(2) in the same solvent. The reduced form of [Ir(3)S(2)CH(2)CN](+) works as the active species in the reduction of CO(2). The IR spectra of [Ir(3)S(2)CH(2)CN](+) in CD(3)CN showed a reversible adduct formation with CO(2) and also evidenced the oxalate generation through the reduced form of the CO(2) adduct under the controlled potential electrolysis of the solution at -1.55 V. A coupling reaction of two CO(2) molecules bonded on adjacent &mgr;(3)-S and Ir in [Ir(3)S(2)CH(2)CN](0) is proposed for the first catalytic generation of C(2)O(4)(2)(-) without accompanying CO evolution.