Substituent effects on Ni–S bond dissociation energies and kinetic stability of nickel arylthiolate complexes supported by a bis(phosphinite)-based pincer ligand

Abstract

Pincer complexes of the type [2,6-(R₂PO)₂C₆H₃]NiSC₆H₄Z (R = Ph and i-Pr; Z = p-OCH₃, p-CH₃, H, p-Cl, and p-CF₃) have been synthesized from [2,6-(R₂PO)₂C₆H₃]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni–S bond length (approx. 2.20 Å) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with “[2,6-(Ph₂PO)₂C₆H₃]Ni” rather than “[2,6-(i-Pr₂PO)₂C₆H₃]Ni”, and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH₂Br, CH₃I, CCl₄, and Ph₃CCl have been examined and several mechanistic pathways have been explored.