Dimeric FeFe-hydrogenase mimics bearing carboxylic acids: Synthesis and electrochemical investigation

Hydrogen is expected to be important for future sustainable energy applications. Recent interest in adsorbing water splitting catalysts on a semiconductor surface for electrocatalytic hydrogen production warrants careful study of electrocatalysts with functional groups capable of binding to a surface. The monomeric complex Fe2(μ-S2(CH2)3)(CO)5PPh2C6H4CO2H (S2(CH2)3 = 1,3-propanedithiolate, pdt) and the dimeric complexes [{Fe2(μ-pdt)(CO)5}2(μ,κ1,κ1-Ph2PCH2N(Ar)CH2PPh2)] (Ar = p-CO2H-Ph, 3,5-CO2HPh, p-CH2CO2HPh, m-CO2H-Ph) bearing carboxylic acid functional groups have been prepared. The first electrochemical study of these dimeric FeFe complexes indicate that the complexes degrade upon reduction to form the species [Fe2(μ-pdt)(CO)4(κ2-Ph2PCH2N(Ar)CH2PPh2)]. A route is proposed for the formation of [Fe2(μ-pdt)(CO)4(κ2-Ph2PCH2N(Ar)CH2PPh2)] from [{Fe2(μ-pdt)(CO)5}2(μ,κ1,κ1-Ph2PCH2N(Ar)CH2PPh2)] and a crystal structure of an intermediate in the proposed electrochemical degradation pathway is presented. The electrochemical study presented herein provides valuable insight into degradation pathways of FeFe hydrogenase mimics bearing carboxylic acids amenable to surface immobilization.