Biomimetic model featuring the NH proton and bridging hydride related to a proposed intermediate in enzymatic H(2) 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(CH(2))(2)N(n)Pr(H)(m)(CH(2))(2)S)(mu-H)(n)(CO)(4)(PMe(3))(2)((2m+2n)+) (1, m = n = 0; 1-2H(N), m = 1, n = 0; 1-2H(N)2H(Fe), 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. (31)P 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) and 1-2H(N)2H(Fe) occurs in the presence of triethanolamine (TEOA), which generates 1 and 1-2H(Fe), 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), is a kinetic product which converts to its thermodynamically stable tautomer, 1-2H(N), 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) has been observed to evolve H(2) electrocatalytically at a mild potential (-1.42 V vs Fc/Fc(+)) in CH(3)CN solution. Catalysis of 1-2H(N)2H(Fe) is found to be as efficient as that of the related diiron azadithiolate complexes. In the absence of a proton source, 1-2H(N)2H(Fe) 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), 1-2H(Fe), 1-2H(N), and 1, respectively, according to bulk electrolysis and voltammetry in combination of titration experiments with acids.