Chelate bite effects for [Pd(triphosphine)(solvent)](BF4)(2) 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 H-2 have been studied, as well as the catalytic electrochemical reduction of CO2. AU of these reactions show significant chelate effects. [Pd(ttpE)(CH3CN)](BF4)(2) (where ttpE is bis(3-(diethylphosphino)propyl)phenylphosphine catalyzes the electrochemical reduction of CO2 to CO in acidic dimethylformamide solutions and reacts with NaBH4 or H-2 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)]CBF4)(2) and [Pd(tpE)(H)](BF4) provide insight into possible steric origins of reactivity differences between the last two complexes and analogous complexes with smaller cheIate bites. [Pd(fttpE)(CH3CN)](BF4)(2) has a square-planar structure with one methylene group of the ethyl subtituents making a close contact with the nitrogen atom of acetonitrile. This steric interaction likely contributes to some of the reactitrity differences observed. [Pd(ttpE)(H)](BF4) also has a square-planar structure with the two terminal phosphol-us atoms of the triphosphine ligand distorted slightly toward the hydride ligand. Extended Hiickel molecular orbital calculations suggest thatsmall ckelate bites shift electrondensity onto the hydride ligand, making it more hydridic, while larger chelate bites shift electron density away from the hydride ligand, making it more acidic.