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

The synthesis and spectroscopic characterization of Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)­Cl, <b>[3-Cl]</b> (where C₅F₄N is a tetrafluoropyridyl substituent and P^<i>t</i>Bu₂N^Bn₂ = 1,5-dibenzyl-3,7-di­(<i>tert</i>-butyl)-1,5-diaza-3,7-diphosphacyclooctane), are reported. Complex <b>3-Cl</b> and [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)­Cl], <b>4-Cl</b>, are precursors to intermediates in the catalytic oxidation of H₂, including Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)H <b>(3-H)</b>, Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)H (<b>4-H)</b>, [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)]­BAr^F₄ (<b>[3]­(BAr</b>^<b>F</b>_<b>4</b>), [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)]­BAr^F₄ (<b>[4]­(BAr</b>^<b>F</b>_<b>4</b>), [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)­(H₂)]­BAr^F₄ (<b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>), and [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂<i>H</i>)<i>H</i>]­BAr^F₄ (<b>[4-Fe</b><i><b>H</b></i><b>(N</b><i><b>H</b></i><b>)]­BAr</b>^<b>F</b>_<b>4</b>). All of these complexes were characterized by spectroscopic and electrochemical studies; <b>3-Cl</b>, <b>3-H</b>, and <b>4-Cl</b> were also characterized by single crystal diffraction studies. <b>3-H</b> and <b>4-H</b> are electrocatalysts for H₂ (1.0 atm) oxidation in the presence of an excess of the amine bases <i>N</i>-methylpyrrolidine, Et₃N or ^<i>i</i>Pr₂EtN. Turnover frequencies at 22 °C for <b>3-H</b> and <b>4-H</b> with <i>N</i>-methylpyrrolidine as the base are 2.5 and 0.5 s^–1, and overpotentials at <i>E</i>_cat/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