Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2

The synthesis and spectroscopic characterization of (CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)Cl, [3-Cl] (where C5F4N is a tetrafluoropyridyl substituent and (P2N2Bn)-N-tBu = 1,5-dibenzyl-3,7-di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane), are reported. Complex 3-Cl and [(CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)Cl], 4-Cl, are precursors to intermediates in the catalytic oxidation of H2, including (CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)H (3-H), (CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)H (4-H), [(CpNFe)-N-C5F4((P2N2Bn)-N-tBu)]BAr4F ([3](BAr4F), [(CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)]BAr4F ([4](BArF4), [(CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)(H2)]BAr4F ([3-H2]BAr4F), and [(CpFe)-Fe-C5F4N((P2N2Bn)-N-tBu)H]BAr4F ([4-FeH(NH)]BAr4F). All of these complexes were characterized by spectroscopic and electrochemical studies; 3-Cl, 3-H, and 4-Cl were also characterized by single crystal diffraction studies. 3-H and 4-H are electrocatalysts for H2 (1.0 atm) oxidation in the presence of an excess of the amine bases N-methylpyrrolidine, Et3N or iPr2EtN. Turnover frequencies at 22 degrees C for 3-H and 4-H with N-methylpyrrolidine as the base are 2.5 and 0.5 s1, and overpotentials at Ecat/2 are 235 and 95 mV, respectively. Studies of individual chemical and electrochemical reactions of the various intermediates provide important insights into the factors governing the overall catalytic activity for H2 oxidation.