Kinetic evidence for intramolecular proton transfer between nickel and coordinated thiolate

The complexes [Ni(YR)(triphos)]BPh(4) (Y = S, R = Ph or Et or Y = Se, R = Ph; triphos = (Ph(2)PCH(2)CH(2))(2)PPh) have been prepared and characterized, and the X-ray crystal structure of [Ni(SPh)(triphos)]BPh(4) has been solved. In MeCN, [Ni(YR)(triphos)](+) are protonated by [lutH](+) (lut = 2,6-dimethylpyridine) to give [Ni(YHR)(triphos)](2+). Studies on the kinetics of these equilibrium reactions reveal an unexpected difference in the reactivities of [Ni(SPh)(triphos)](+) and [Ni(SEt)(triphos)](+). In both cases, the reactions exhibit a first-order dependence on the concentration of complex. When R = Ph, the dependence on the concentrations of [lutH(+)] and lut is given by k(obs) = k(1)(Ph)[lutH(+)] + k(-1)(Ph)[lut], which is typical of an equilibrium reaction where k(1)(Ph) and k(-1)(Ph) correspond to the forward and back reactions, respectively. Analogous behavior is observed for [Ni(SePh)(triphos)](+). However, for [Ni(SEt)(triphos)](+), the kinetics are more complicated, and k(obs) = (k(1)k(2)[lutH(+)] + (k(-2) + k(2)))/(k(1)[lutH(+)] + k(-1)[lut]), which is indicative of a mechanism involving two coupled equilibria in which the initial protonation of the thiolate is followed by a unimolecular equilibrium reaction that is assumed to involve the formation of an eta(2)-EtS-H ligand. The difference in reactivity between the complexes with alkyl and aryl thiolate ligands is a consequence of the (Ni(triphos))(2+) site "leveling" the basicities of these ligands. The pK(a)'s of the PhSH and EtSH constituents coordinated to the (Ni(triphos))(2+) are 16.0 and 14.6, respectively, whereas the difference in pK(a)'s of free PhSH and EtSH differ by ca. 4 units. The pK(a) of [Ni(SeHPh)(triphos)](+) is 14.4. The more strongly sigma-donating EtS ligand makes the (Ni(triphos))(2+) core sufficiently electron-rich that the basicities of the sulfur and nickel in [Ni(SEt)(triphos)](+) are very similar; therefore, the proton serves as a bridge between the two sites. The relevance of these observations to the proposed mechanisms of nickel-based hydrogenases is discussed.