Orientation and Stereodynamic Paths of Planar Monodentate Ligands in Square Planar Nickel N2S Complexes

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N2SNi moieties. Square planar [N2SNiL] n+ complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N2SNi]2 complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N2SNiL square plane. Variable temperature 1H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with density functional theory (DFT)-guided interpretation of Ni−L rotational activation barriers. Full DFT characterization finds Ni−L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C−Hδ+- - Sδ− interaction). Computational studies find this donor−acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N2ONimidNi complexes.