Electrophilic Alkylation of Pseudotetrahedral Nickel(II) Arylthiolate Complexes

A kinetic study is reported for reactions of pseudotetrahedral nickel­(II) arylthiolate complexes [(Tp^R,Me)­Ni–SAr] (Tp^R,Me = hydrotris­{3-R-5-methyl-1-pyrazolyl}­borate, R = Me, Ph, and Ar = C₆H₅, C₆H₄-4-Cl, C₆H₄-4-Me, C₆H₄-4-OMe, 2,4,6-Me₃C₆H₂, 2,4,6-^<i>i</i>Pr₃C₆H₂) with organic electrophiles R′X (i.e., MeI, EtI, BzBr) in low-polarity organic solvents (toluene, THF, chloroform, dichloromethane, or 1,2-dichloroethane), yielding a pseudotetrahedral halide complex [(Tp^R,Me)­Ni–X] (X = Cl, Br, I) and the corresponding organosulfide R′SAr. Competitive reactions with halogenated solvents and adventitious air were also examined. Akin to reactions of analogous and biomimetic zinc complexes, a pertinent mechanistic question is the nature of the reactive nucleophile, either an intact thiolate complex or a free arylthiolate resulting from a dissociative pre-equilibrium. The observed kinetics conformed to a second-order rate law, first order with respect to the complex and electrophile, and no intermediate complexes were observed. In the absence of a mechanistically diagnostic rate law, a variety of mechanistic probes were examined, including kinetic effects of varying the metal, solvent, electrophile, and temperature, as well as the 3-pyrazolyl and arylthiolate substituents. Compared to zinc analogues, the effect of Ni–SAr covalency is also of interest herein. The results are broadly interpreted with respect to the disparate mechanistic pathways