X-RAY ABSORPTION-SPECTRA OF NICKEL-COMPLEXES WITH N3S2 CHROMOPHORES AND SPECTROSCOPIC STUDIES ON H- AND CO BINDING AT THESE NICKEL CENTERS - RELEVANCE TO THE REACTIVITY OF THE NICKEL SITE(S) IN [FENI] HYDROGENASES

The reaction of [Ni(terpy)Cl2] with approximately 2.0 equiv of 2,6-(Me)2C6H3S- in acetonitrile affords the distorted trigonal bipyramidal (tbp) complex [Ni(terpy)(2,6-(Me)2C6H3S)2] (4). Complex 4 crystallizes in the orthorhombic space group Pbcn with a = 21.881 (7) angstrom, b = 11.461 (3) angstrom, c = 10.557 (3) angstrom, V = 2647.4 (13) angstrom3, and Z = 4. The structure of 4 was refined to R = 4.31% on the basis of 1673 (F > 4-sigma(F)) data. Comparisons of the results of X-ray absorption spectroscopy (XAS) for complex 4 as well as [Ni(terpy)(2,4,6-(i-Pr)3C6H2S)2] (3) and [Ni(terpy)(C6F5S)2] (A) (desolvated [Ni(terpy)(C6F5S)2(CH3CN)] (2)) with those for the [FeNi] hydrogenase from Thiocapsa roseopersicina indicate that the nickel center in the enzyme is in a distorted tbp geometry with a mixed N/O- and S-donor environment. In this regard, complexes 3, 4, and 2 (without the acetonitrile molecule) are good structural models for the biological nickel site. In solvents like acetonitrile and DMSO, complex 2 generates hexacoordinated solvent adducts whose XAS data are quite different from those of the parent complex A as well as 3 and 4. X-ray absorption near-edge spectroscopy (XANES) shows a weaker 1s --> 3d transition for these hexacoordinated adducts compared to the parent complex A. EXAFS features of the solvated hexacoordinated adducts (maximum at k = 4 angstrom-1) are also different from the EXAFS data for the pentacoordinated complexes (maximum at k = 6 angstrom-1). The model complexes 2-4 are readily reduced to the corresponding Ni(I) species with reductants like Na2S2O4 in DMF. The EPR spectra of the reduced complexes as well as the substrate (CO, H-, and R-)-bound Ni(I) species have been discussed. Close similarities between the EPR spectra of the hydride adducts of reduced 3 and 4 and the Ni-C signal of the hydrogenase from T. roseopersicina support the formalism Ni(I)-H- for the nickel site in form C of the enzyme.