Influence of Sulfur Metalation on the Accessibility of the Ni(II/I) Couple in [N,N'-Bis(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 five polymetallics based on the metalation of the dithiolate complex 1,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Ni(II), Ni-1. Crystal structures of three polymetallics of the series have been reported earlier: (Ni-1)(2)()Ni]Cl(2)(), (Ni-1)(2)()FeCl(2)()](2)(), and (Ni-1)(3)()(ZnCl)(2)()]Cl(2)(). Two are described here: (Ni-1)(2)()Pd]Cl(2)().2H(2)()Ocrystallizes in the monoclinic system, space group P2(1)/c with cell constants a = 12.212(4) ?, b = 7.642(2) ?, c = 16.625(3) ?, beta = 107.69(2) degrees, V = 1443.230(0) ?(3), Z = 2, R = 0.051, and R(w) = 0.056. (Ni-1)(2)()CoCl]PF(6)() crystallizes in the triclinic system, space group P&onemacr;, with cell constants a = 8.14(2) ?, b = 13.85(2) ?, c = 15.67(2) ?, alpha = 113.59(10) degrees, beta = 101.84(14) degrees, gamma = 94.0(2) degrees, V = 1561.620(0)?(3), Z = 2, R = 0.072, and R(w) = 0.077. In all Ni-1 serves as a bidentate metallothiolate ligand with a "hinge" angle in the range 105-118 degrees and Ni-M distances of 2.7- 3.7 ?. The most accessible redox event is shown by EPR and electrochemistry to reside in the N(2)S(2)Ni unit and is the Ni(II/I) couple. Charge neutralization of the thiolate sulfurs by metalation can (dependent on the interacting metal) stabilize the Ni(I) state as efficiently as methylation forming a thioether. 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(&mgr;-SR)(2)M hinge structure, and a possible explanation for the unusual redox potentials are discussed.