TOWARD FUNCTIONAL MODELS OF THE NICKEL SITES IN [FENI] AND [FENISE] HYDROGENASES - SYNTHESES, STRUCTURES, AND REACTIVITIES OF NICKEL(II) COMPLEXES CONTAINING [NIN3S2] AND [NIN3SE2] CHROMOPHORES

The reaction of [Ni(terpy)Cl-2] with similar to 2 equiv of 2,4,6-(Me)( 3)C6H2Se- in 3:1 acetonitrile/ethanol affords [Ni(terpy)(2,4,6-(Me)3C( 6)H(2)Se)(2)] (7), while [Ni(DAPA)Cl-2] (DAPA = 2,6-bis[1-(phenylimino )ethyl]pyridine) reacts with similar to 2 equiv of PhSe(-) and PhSe(-) in neat ethanol or acetonitrile to yield [Ni(DAPA)(SPh)(2)] (8) and [ Ni(DAPA)-(SePh)(2)] (9), respectively. All three complexes contain the distorted trigonal bipyramidal (TBP) NiN(3)E(2) (E = S, Se) chromopho re. Previous X-ray absorption spectroscopic data have indicated a dist orted TBP NiN3S2 coordination for the nickel site of the hydrogenase ( H(2)ase) from Thiocapsa roseopersicina. Complex 7 crystallizes in the monoclinic space group P2(1)/n with a = 13.170(6) Angstrom, b = 16.091 (5) A, c = 15.111(8) Angstrom, beta = 114.42(2)degrees, V = 2916(2) An gstrom(3), and Z = 4. The structure of 7 was refined to R = 4.78% on t he basis of 2730 reflections (I > 4 sigma(I). Complex 8.CH3-CN crystal lizes in the monoclinic space group P2(1)/c with a = 23.012(7) Angstro m, b = 17.814(5) Angstrom, c = 15.698(4) Angstrom, beta = 108.52(2)deg rees, V = 6099(5) Angstrom(3), and Z = 8. The structure of 8.CH3CN was refined to R = 6.46% on the basis of 6133 reflections (I > 4 sigma(I) ). Complex 9.CH3CN also crystallizes in the monoclinic space group P(2 )1/c with a = 23.209(2) Angstrom, b = 17.960(1) Angstrom, c = 15.749(1 ) Angstrom, beta = 108.482(6)degrees, V = 6225 Angstrom(3) and Z = 8, The structure of 9.CH3CN was refined to 3.90% on the basis of 5808 ref lections (I > 4 sigma(I)). Reduction of the terpy analogue 7 with aque ous dithionite gives rise to the corresponding Ni(I) complex which bin ds CO (reversibly) and H-. The EPR parameters of the CO and hydride ad ducts resemble the Ni-CO and Ni-C signal of the H(2)ases. Much like th e other terpy analogues reported previously by this group, oxidation o f 7 affords unstable Ni(III) products in low yields. The two DAPA anal ogues (8 and 9), on the other hand, are readily oxidized and reduced b y biologically relevant oxidants and reductants, and the transformatio n Ni(III) <-- Ni(II)) --> Ni(I) is reversible. The Ni(III) species (10 and 13) derived from 8 and 9 via oxidation with [Fe(CN)(6)](3-) are c omparatively stable and do not bind CO (or H-). The single electron in both 10 and 13 resides in the d(z2) orbital. Upon reduction with aque ous dithionite, 8 and 9 produce the corresponding Ni(I) species 11 and 14 with the single electron in the d(x2-y2) orbital. These Ni(I) comp lexes are quite stable at low temperatures but slowly lose thiolates/s elenolates at room temperature to give [Ni(DAPA)(solv)(2)](+). Both 11 and 14 bind CO reversibly. The affinity of the Ni(I) (but not the Ni( III)) model complexes toward CO strongly suggests the presence of Ni(I ) in the C form of the H(2)ases since the enzymes bind CO only in the Ni-C form. Reaction of NaBH4 with 8 and 9 results in the hydride adduc ts 19 and 20. These hydride adducts are stable under basic conditions. The absence of any detectable proton hyperfine coupling indicates tha t the H- ligand is located at the basal plane of the Ni(I) center. The EPR parameters of the CO and hydride adducts are quite similar to tho se of the Ni-CO and Ni-C signals of the H(2)ases. Under basic conditio ns, both 8 and 9 react with dihydrogen at ambient temperature and pres sure to afford the hydride adducts 19 and 20 in significant yields. Th is reaction is quite remarkable since the model complexes mimic the re ductive activation step of the biological nickel site in such a reacti on to ultimately produce Ni-C-like signals. Taken together, the presen t results strongly suggest a Ni(I)-H- formalism for the nickel site in the C form of the H(2)ases. In addition, enhancement of the intensiti es of the EPR signals of the hydride adducts in the presence of a base indicates heterolytic cleavage of H (coordinated or not) at the Ni(I) site of the model complexes and probably also at the enzyme active si tes.