ELECTROCHEMICAL REDUCTION OF CO2 CATALYZED BY A DINUCLEAR PALLADIUM COMPLEX CONTAINING A BRIDGING HEXAPHOSPHINE LIGAND - EVIDENCE FOR COOPERATIVITY

The complexes [Pd-2(CH3CN)(2)(eHTP)](BF4)(4) and [Pd-2(PEt(3))(2)(eHTP)](BF4)(4) (where eHTP is bis(bis((diethylphosphino)ethyl)phosphino))methane (Et(2)PCH(2)CH(2))(2)PCH2P(CH(2)CH(2)PEt(2))(2)) were prepared and characterized. [Pd-2(CH3CN)(2)(eHTP)](BF4)(4) catalyzes the electrochemical reduction of CO2 to CO in acidic dimethylformamide solutions. The rate of this reaction exhibits a biphasic dependence on acid, with a first-order dependence at low acid concentrations and zero-order dependence at acid concentrations greater than 0.06 M. At high acid concentrations the rate-limiting step is first order in catalyst and first order in CO2. When compared to the kinetic properties of previously studied mononuclear complexes, these data suggest both palladium atoms are involved in CO2 reduction. The closely related complex [Pd-2(PEt(3))(2)(eHTP)](BF4)(4) undergoes two reversible two-electron reductions. The mixed-valence intermediate resulting from the first two-electron reduction is unstable and undergoes rapid decomposition.