Chelate Bite Effects for [Pd(triphosphine)(solvent)](BF4)2 Complexes in Electrochemical CO2 Reduction and the Heterolytic Cleavage of Molecular Hydrogen

A series of [Pd(triphosphine)(CH3CN)](BF4)2 complexes has been prepared with different chelate bites. Stoichiometric reactions of these complexes with triethylphosphine, NaBH4, and H2 have been studied, as well as the catalytic electrochemical reduction of CO2. All of these reactions show significant chelate effects. [Pd(ttpE)(CH3CN)](BF4)2 (where ttpE isibs(3-(diethylphosphino)propyl)phenylphosphine) catalyzes the electrochemical reduction of CO2 to CO in acidic dimethylformamide solutions and reacts with NaBH4 or H2 to form [Pd(ttpE)(H)](BF4). The latter complex is the decomposition product formed under catalytic conditions. X-ray diffraction studies of [Pd(ttpE)(CH3CN)](BF4)2 and [Pd(ttpE)(H)](BF4) provide insight into possible steric originsof reactivity differences between the last two complexes and analogous complexes with smaller chelate bites. [Pd(ttpE)(CH3CN)](BF4)2 has a square-planar structure with one methylene group of the ethyl substituents making a close contact with the nitrogen atom of acetonitrile. This steric interaction likely contributes to some of the reactivity differences observed. [Pd(ttpE)(H)](BF4) also has asquare-planar structure with the two terminal phosphorus atoms of the triphosphine ligand distorted slightly toward the hydride ligand. Extended Huckel molecular orbital calculations suggest that small chelate bites shift electron density onto the hydride ligand, making it more hydridic, while larger chelate bites shift electron density away from the hydride ligand, making it more acidic.