Impacts of coordination modes (chelate versus bridge) of PNP-diphosphine ligands on the redox and electrocatalytic properties of diiron oxadithiolate complexes for proton reduction

Abstract

As diiron subsite models of [FeFe]-hydrogenases with odt bridge, two new series of PNP-chelate or -bridge diiron oxadithiolate complexes Fe2(μ-odt)(CO)4{(Ph2P)2NR} (1–3 and 4–6) were prepared by selective substitutions of diiron hexacarbonyl complex Fe2(μ-odt)(CO)6 (A, odt = SCH2OCH2S) with different PNP ligands (PNP = (Ph2P)2NR, R = (CH2)3Me, (CH2)3NMe2 and (CH2)3Si(OEt)3). All the new model complexes have been fully characterized by elemental analysis, various spectroscopies, and particularly for 1 and 4–6 by X-ray crystallography. In order to investigate the influence of coordination modes (chelate vs. bridge) of PNP ligands on redox and electrocatalytic properties of diiron model complexes for proton reduction, the protonation and electrochemistry of 1 and 4 as a pair of representative counterparts are well studied and compared in the absence and presence of strong acid (CF3CO2H) and weak acid (CH3CO2H) as different proton sources by using in situ IR and NMR spectroscopies as well as cyclic voltammetry (CV). For example, the protonation studies have shown that i) with excess CF3CO2H, protonations of both 1 and 4 are able to form the respective hydride species 1(μH)+ and 4(μH)+, but in which the former is completely protonated whereas the latter is less reactive; and ii) with excess CH3CO2H, complex 1 is unactive yet counterpart 4 is partially protonated to afford species 4(μH)+. The electrochemical investigations have displayed that i) with CF3CO2H as a proton source, complex 1 has a very simliar turnover frequency (TOF) and a much lower overpotential for cataytic proton reduction to H2 in comparison to 4; and ii) with CH3CO2H as a proton source, complex 1 shows a much greater TOF value and a closer overpotential for H2 evolution in contrast to 4.