Steric and electronic effects on arylthiolate coordination in the pseudotetrahedral complexes [(TpPh,Me)Ni–SAr] (TpPh,Me = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate)

Abstract

Synthesis and characterization of several new pseudotetrahedral arylthiolate complexes [(Tp^Ph,Me)Ni–SAr] (Tp^Ph,Me = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate; Ar = Ph, 2,4,6-ⁱPr₃C₆H₂, C₆H₄-4-Cl, C₆H₄-4-Me, C₆H₄-4-OMe) are reported, including X-ray crystal structures of the first two complexes. With prior results, two series of complexes are spanned, [(Tp^Ph,Me)Ni–S–2,4,6-R^′′₃C₆H₂] (R′′ = H, Me, ⁱPr) plus the xylyl analogue [(Tp^Ph,Me)Ni–S–2,6-Me₂C₆H₃], as well as [(Tp^Ph,Me)Ni–S–C₆H₄–4-Y] (Y = Cl, H, Me, OMe), intended to elucidate steric and/or electronic effects on arylthiolate coordination. In contrast to [(Tp^Me,Me)Ni–SAr] analogues that adopt a sawhorse conformation, the ortho-disubstituted complexes show enhanced trigonal and Ni–S–Ar bending, reflecting the size of the 3-pyrazole substituents. Moreover, weakened scorpionate ligation is implied by spectroscopic data. Little spectroscopic effect is observed in the series of para-substituted complexes, suggesting the observed effects are primarily steric in origin. The relatively electron-rich and encumbered complex [(Tp^Ph,Me)Ni–S–2,4,6-ⁱPr₃C₆H₂] behaves uniquely when dissolved in CH₃CN, forming a square planar solvent adduct with a bidentate scorpionate ligand, [(κ²-Tp^Ph,Me)Ni(NCMe)(S–2,4,6-ⁱPr₃C₆H₂)]. This adduct was isolated and characterized by X-ray crystallography. Single-point DFT and TD-DFT calculations on a simplified [(κ²-Tp)Ni(NCMe)(SPh)] model were used to clarify the electronic spectrum of the adduct, and to elucidate differences between Ni–SAr bonding and spectroscopy between pseudotetrahedral and square planar geometries.