Intramolecular apical C-H⋯M interactions in square-planar nickel(II) complexes with dianionic tridentate ligands and 2-phenylimidazole

Abstract

Two square-planar ternary nickel(II) complexes [Ni(bhac)-(phim)] (1) and [Ni(ahac)(phim)] (2) with O,N,O-donor Schiff bases [acetylacetone benzoylhydrazone (H2bhac) and acetylacetone acetylhydrazone (H 2ahac)] and sp2 N-donor 2-phenylimidazole (phim) as the ancillary ligand have been synthesised and characterised by analytical, spectroscopic, magnetic and electrochemical methods. The solid-state structures of both complexes have been determined by X-ray crystallography. The asymmetric unit of 1 contains a single complex molecule while that of 2 contains four complex molecules with different conformations. The molecular structures of both complexes reveal that one of the two ortho C-H groups of the pendant phenyl ring of the phim ligand is very close to the metal centre at the apical site indicating the presence of an intramolecular C-H⋯Ni interaction. In the structures of 1 and 2, the shape of the C-H⋯Ni interaction is determined by the twisting of the phim ligand and the extent of orthogonality between the planes containing the chelate rings formed by the tridentate ligand and the imidazole ring. A survey of the reported X-ray structures of similar d 8 metal ion complexes containing intramolecular C-H⋯M interactions has been performed for comparison with the structural features of 1 and 2. The optimised molecular structures of 1 and 2 generated by calculations based on the density functional method are compared with the experimental structures. Calculations also reveal that the four molecules present in the asymmetric unit of 2 are very close in energy. Theoretical studies and the down-field shift of the proton resonance on cooling in the NMR spectra suggest three-centre four-electron hydrogen-bond character of the observed apical C-H⋯Ni interaction in 1 and 2. © Wiley-VCH Verlag GmbH & Co. KGaA, 2006.