Synthesis, characterization, and reactivity of iron(II) and cobalt(II) complexes containing diphosphine ligands that feature a pendant amine base

A series of iron(II) and cobalt(II) complexes have been synthesized featuring the diphosphine ligand PNP (bis((diethylphosphino)methyl)methylamine) or the cyclic diphosphine ligand PR 2NR' 2(1,5-diaza-3,7-diphosphacyclooctanes), where R = phenyl or cyclohexyl and R' = phenyl or benzyl, with the ultimate goal of creation and mechanistic understanding of a hydrogen oxidation/proton reduction catalyst. These compounds contain a pendant nitrogen that is believed to act as an intra and intermolecular proton relay. Iron(II) complexes of the formulation [Fe(PNP)2(X)(Y)] n+ and [Fe(PR 2NR' 2)2(CH3CN)(X)]n+, where X is H or CH3CN and Y is CO, CH3CN, or P(OEt)3 were used to further explore the exact role of the nitrogen base, as well as the effects of ligand choice on reactivity. Analysis of pK a values, cyclic voltammetry, protonation reactions, mmff calculations, hydride/proton exchange data, acetonitrile exchange studies, and deuterium incorporation suggest that both electronics of the metal center as well as location and acidity of the pendant nitrogen must be carefully controlled. Proof of concept was provided through use of cobalt(II) complexes of the formulation [Co(PR 2NR' 2)2(CH 3CN)]2+. These complexes act as proton reduction electro catalysts with high turnover frequencies, although at fairly negative potentials. Cyclic voltammetry was used to explore the kinetics of this reaction and an overall rate law of rate = k [H+] [cat]2 was found.