Structure and Redox Chemistry of Analogous Nickel Thiolato and Selenolato Complexes: Implications for the Nickel Sites in Hydrogenases

Abstract

The syntheses, structures, and redox properties of isomorphous Ni(I1) thiolato and selenolato complexes of the tridentate ligands bis(2-(hydrochalcogeno)ethyl)methylamines are reported. Reaction of Ni(0Ac)z with bis(2mercaptoethy1)methylamine leads to the formation of a dimeric complex, bis{ [@-2-mercaptoethyl)(2-mercaptoethyl)methylaminato(2-)]nickel(II)}, [Ni(l)]2. This complex contains planar, diamagnetic Ni(I1) centers ligated by a tertiary amine N-donor atom, a terminal thiolate, and two thiolates that bridge to the second Ni center in the dimer. Crystals of [Ni(l)]2 form in orthorhombic space group h a 2 1 with cell dimensions a = 19.695(2) 8, b = 6.042(2) 8, c = 13.463(3) 8, V = 1602(1) A3, and Z = 4. Reaction of Ni(0Ac)p with bis(2-(hydroseleno)ethy1)methylamine results in the formation of a structurally analogous dimeric complex, bis([@-2-(hydroseleno)ethyl)(2-(hydroseleno)ethyl)methylaminato(2-)]nickel(II)}, [Ni(2)]2, where all of the chalcogenolate donors are selenolates. Crystals of [Ni(2)]2 are isomorphous with those of [Ni(l)]z, with a = 20.040(8) 8, b = 6.265(2) 8, c = 13.590(5) 8, and V = 1706(2) A3, One-electron oxidation of either dimeric complex leads to the formation of radical cations, which exhibit EPR spectra consistent with S = l/2 radicals. For [Ni(l)]2+ the g values observed (gx = 2.20, g, = 2.14, g, = 2.02) are essentially identical to those observed for a reduced and catalytically viable redox state of Fe,Ni hydrogenases (gx = 2.20, g, = 2.14, g, = 2.01). The substitution of Sefor S-donors in [Ni(2)]2 does not alter the observed g values much (gx = 2.23, g, = 2.14, g, = 2.05) but leads to the observation of 77Se hyperfine coupling (A, = 129 G) that indicates that the molecular orbital containing the unpaired spin is largely Se in character (54%). Reaction of either dimeric complex with CNleads to the formation of mononuclear trans-dichalcogenolate complexes, [Ni(l)CN]and [Ni(2)CN]-. Exposure of [Ni(l)CN]to 0 2 leads to the quantitative formation of a monosulfinato complex. In contrast, the selenolato complex does not react with 0 2 under the same conditions. The role of selenocysteinate ligation in Fe,Ni,Se hydrogenases is discussed in view of this chemistry.