INFLUENCE OF SULFUR METALATION ON THE ACCESSIBILITY OF THE NI-II I COUPLE IN 2-MERCAPTOETHYL)-1,5-DIAZACYCLOOCTANATO]NICKEL(II) - INSIGHT INTO THE REDOX PROPERTIES OF [NIFE]-HYDROGENASE/

A redox model study of [NiFe] hydrogenase has examined a series of fiv e polymetallics based on the metalation of the dithiolate complex 1,5- bis(mercaptoethyl)-1,5-diazacyclooctane]Ni(II) Ni-1. Crystal structure s of three polymetallics of the series have been reported earlier: [(N i-1)(2)Ni]Cl-2, [(Ni-1)(2)FeCl2](2), and [(Ni-1)(3)(ZnCl)(2)]Cl-2. Two are described here: [(Ni-1)(2)Pd]Cl-2 . 2H(2)O crystallizes in the mo noclinic system, space group P2(1)/c with cell constants a = 12.212(4) Angstrom, b = 7.642(2) Angstrom, c = 16.625(3) Angstrom, beta = 107.6 9(2)degrees, V = 1443.230(0) Angstrom(3), Z = 2, R = 0.051, and R(W) = 0.056. [(Ni-1)(2)CoCl]PF6 crystallizes in the triclinic system, space group <P(1)over bar>, with cell constants a = 8.14(2) Angstrom, b = 1 3.85(2) Angstrom, c = 15.67(2) Angstrom, alpha = 113.59(10)degrees, be ta = 101.84(14)degrees, gamma = 94.0(2)degrees, V = 1561.620(0) Angstr om(3), Z = 2, R = 0.072, and R(W) = 0.077. In all Ni-1 serves as a bid entate metallothiolate ligand with a ''hinge'' angle in the range 105- 118 degrees and Ni-M distances of 2.7-3.7 Angstrom. The most accessibl e redox event is shown by EPR and electrochemistry to reside in the N2 S2Ni unit and is the N-II/I couple, Charge neutralization of the thiol ate sulfurs by metalation can (dependent on the interacting metal) sta bilize the Ni-1 state as efficiently as methylation forming a thioethe r, The implication of these results for the heterometallic active site of [NiFe]hydrogenase as structured from Desulfovibrio gigas (Volbeda, A., et al. Nature, 1995, 373, 580), the generality of the Ni(mu-SR)(2 )M hinge structure, and a possible explanation for the unusual redox p otentials are discussed.