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

Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging Fe hydride, [Fe-2(mu-S(CH2)(2)(NPr)-Pr-n(H)(m)(CH2)(2)S)(mu-H)(n)(CO)(4)(PMe3)(2)](2)((2m+2n)+) (1, m = n = 0; [1-2H(N)](2+), m = 1, n = 0; [1 -2H(N)2H(Fe)](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 azaidiethylenethiolate bridges to construct a dimer-of-dimer structure. P-31 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 [1-2H(N)](2+) and [1-2H(N)2H(Fe)](4+) occurs in the presence of triethanolamine (TEOA), which generates 1 and [1-2H(Fe)](2+), respectively. Deprotonation of the Fe hydride is accomplished by addition of bistriphenylphosphineimminium chloride ([PPN]Cl). It is observed that the Fe hydride species, [1-2H(Fe)](2+), is a kinetic product which converts to its thermodynamically stable tautomer, [1-2H(N)](2+), in solution, as evidenced by IR and NMR spectroscopy. The pK(a) values of the aza nitrogen and the diiron sites are estimated to be 8.9-15.9 and <8.9, respectively. [1-2H(N)2H(Fe)](4+) has been observed to evolve H-2 electrocatalytically at a mild potential (-1.42 V vs Fc/Fc(+)) in CH3CN solution. Catalysis of [1-2H(N)2H(Fe)](4+) is found to be as efficient as that of the related diiron azaidithiolate complexes. In the absence of a proton source, [1-2H(N)2H(Fe)](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 [1-2H(N)2H(Fe)](4+), [1-2H(Fe)](2+), [1-2H(N)](2+), and 1, respectively, according to bulk electrolysis and voltammetry in combination of titration experiments with acids.