Biomimetic Model Featuring the NH Proton and Bridging Hydride Related to a Proposed Intermediate in Enzymatic H₂ Production by Fe-Only Hydrogenase

Abstract

Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging Fe hydride, [Fe2(μ-S(CH2)2NnPr(H)m(CH2)2S)(μ-H)n(CO)4(PMe3)2]2(2m+2n)+ (1, m = n = 0; [1−2HN]2+, m = 1, n = 0; [1−2HN2HFe]4+, m = n = 1), are prepared to mimic the active site of Fe-only hydrogenase. X-ray crystallographic analyses of these three complexes reveal that two diiron subunits are linked by two azadiethylenethiolate bridges to construct a dimer-of-dimer structure. 31P NMR spectroscopy confirms two trimethylphosphine ligands within the diiron moiety are arranged in the apical/basal configuration, which is consistent with the solid-state structural characterization. Deprotonation of the NH proton in [1−2HN]2+ and [1−2HN2HFe]4+ occurs in the presence of triethanolamine (TEOA), which generates 1 and [1−2HFe]2+, respectively. Deprotonation of the Fe hydride is accomplished by addition of bistriphenylphosphineimminium chloride ([PPN]Cl). It is observed that the Fe hydride species, [1−2HFe]2+, is a kinetic product which converts to its thermodynamically stable tautomer, [1−2HN]2+, in solution, as evidenced by IR and NMR spectroscopy. The pKa values of the aza nitrogen and the diiron sites are estimated to be 8.9−15.9 and [1−2HN2HFe]4+ has been observed to evolve H2 electrocatalytically at a mild potential (−1.42 V vs Fc/Fc+) in CH3CN solution. Catalysis of [1−2HN2HFe]4+ is found to be as efficient as that of the related diiron azadithiolate complexes. In the absence of a proton source, [1−2HN2HFe]4+ undergoes four irreversible reduction processes at −1.26, −1.42, −1.82, and −2.43 V, which are attributed to the reduction events from [1−2HN2HFe]4+, [1−2HFe]2+, [1−2HN]2+, and 1, respectively, according to bulk electrolysis and voltammetry in combination of titration experiments with acids