Reaction of the mononuclear Ni(II) thiolate complexes [Ni(L)]hydrogenase ͉ iron ͉ nickel ͉ thiolates ͉ density functional theory calculations H ydrogenases catalyze the reversible oxidation of molecular hydrogen and play a vital role in anaerobic metabolisms of a wide variety of microorganisms (1, 2).[NiFe]hydrogenases are particularly interesting because of their heterobinuclear active site and have been studied extensively (3-12). Thiolate-bridged heterobimetallic Ni-Fe complexes, therefore, have also received considerable attention because of their importance as structural, spectroscopic, and functional models for the active site of the enzyme (13-15). The structure of the oxidized inactive form of [NiFe]hydrogenase in Desulfovibrio gigas has been determined by x-ray crystallography (4, 5) and confirms an unusual heterobimetallic Ni-Fe core incorporating two terminal thiolates and two bridging thiolates at the Ni ion, with the bridging thiolates binding to a Fe(CN) 2 (CO) fragment with a Ni-Fe separation of 2.9 Å (Fig. 1). The vast majority of low-molecular-weight mimics have sought to model these structural patterns and this Ni-Fe separation (13-15).Of particular interest to us was the reported x-ray absorption study of [NiFe]hydrogenase from Chromatium vinosum that estimated the Ni-Fe separation as 2.5-2.6 Å in the reduced and active Ni-SI form of the enzyme (16). This finding was further supported by theoretical (17) and mechanistic (18, 19) studies that suggest a shorter Ni-Fe separation (2.5 Å) in the active form of the enzyme compared with that observed in the structure of its oxidized, inactive form (2.9 Å) (4). Furthermore, recent crystal structure determinations of [NiFe]hydrogenases from Desulfovibrio vulgaris Miyazaki F (8) and D. gigas (11) also exhibit shorter Ni-Fe separations (2.55 and 2.53 Å, respectively), while Cramer and coworkers (20) have identified spin-state changes brought about by stereochemical distortion and͞or addition of further ligands at the active site. We therefore were challenged to synthesize low-molecular-weight heterobinuclear Ni-Fe complexes that would mimic not only the short Ni-Fe distance of the Ni-SI, Ni-C, and related reduced centers, but also to model the proposed stereochemical distortions and spin-state changes at the Ni center on binding to Fe fragment(s). Materials and MethodsAll operations were carried out at room temperature under a pure argon atmosphere by using standard Schlenk techniques. [Ni(pdt) (dppe)] (21, 22) and [Fe(CO) 3 (BDA)] (23) (pdt, propanedithiolate; dppe, 1,2-diphenylphosphinoethane; BDA, benzylidene acetone) were prepared by methods described in the literature. Fe 2 (CO) 9 and Fe 3 (CO) 12 were purchased from Aldrich and used as received. All solvents were dried and distilled before use. CH 2 Cl 2 was distilled from CaH 2 and benzene from Na͞benzophenone ketyl under argon. All calculations on complex 3 were carried out with the program ORCA (24). The functional for the geometry optimization was BP86 (25,26), and the basis set was TZVP on lig...

The asymmetric binuclear complex [(dppe)Ni(mu-'S, S')Ni(L)](PF6)2 [L = (N, N'-diethyl-3,7-diazanonane-1,9-dithiolato)2-] shows a reversible one-electron reduction to afford a mixed-valent Ni(II) x Ni(I) species; the reduced complex has been characterised by EPR spectroscopy and mimics the redox active Nip site in the active A-cluster of acetyl coenzyme A synthase.

A novel [NiS4Fe2(CO)6]cluster (1: 'S(4)'=(CH(3)C(6)H(3)S(2))(2)(CH(2))(3)) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at -1.31 V versus Fc(+)/Fc to generate the EPR-active monoanion 1(-). Multifrequency Q-, X- and S-band EPR spectra of (61)Ni-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single (61)Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g(1) and A(1), and g(3) and A(3) to reproduce all of the features in the S- and X-band spectra. This behaviour provides a rare example of the detection and measurement of non-coincidence effects from frozen-solution EPR spectra without the need for single-crystal measurements, and in which the S-band experiment is sensitive to the non-coincidence. An analysis of the EPR spectra of 1(-) reveals a 24 % Ni contribution to the SOMO in 1(-), supporting a delocalisation of the spin-density across the NiFe(2) cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, nu(CO), shift to lower frequency by about 70 cm(-1) when 1 is reduced to 1(-). Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1(-) and suggest that the SOMO is delocalised over the whole cluster, but with little S-centre participation. This electronic structure contrasts with that of the Ni-A, -B, -C and -L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO.

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