Synthesis, structural characterisation and nuclear magetic resonance studies of cobalt complexes of pyridonate ligands

Abstract

A series of heteroleptic cobalt(II) complexes of diimine and pyridonate ligands have been synthesized from cobalt(II) acetate, with the general stoichiometric formula [Co(xhp)₂L][xhp = 6-methyl-(mhp), 6-chloro-(chp), 6-bromo-(bhp) or 6-fluoro-2-pyridonate (fhp); L = 2,2′-bipyridyl (bipy), 4,4′-dimethyl-2,2′-bipyridyl (dmbipy), 1,10-phenanthroline (phen) or di-2-pyridylamine (Hdpa)]. Four have been characterised by X-ray crystallography. The compounds [Co(mhp)₂(phen)], [Co(chp)₂(Hdpa)]·H₂O and [Co(bhp)₂(dmbipy)] crystallise as mononuclear species containing six-co-ordinate cobalt sites. The co-ordination geometry of the metal is distorted due to the small bite angle of the chelating xhp ligands. The fourth complex, [{Co(µ-fhp)-(fhp)(dmbipy)}₂], crystallises as a dinuclear unit with the cobalt atoms bridged by the exocyclic O atom of one fhp ligand. The Co ⋯ Co distance is 3.208(6)Å. A fifth complex, [{Co(µ-bhp)(O₂CMe)(dmbipy)}₂], has been characterised in which two cobalt atoms are bridged by the O atom from a bhp ligand, giving a Co ⋯ Co separation of 3.158(4)Å. The NMR studies of the complexes [Co(xhp)₂(dmbipy)] revealed large paramagnetic shifts, especially for the 6,6′-protons of the dmbipy unit. The magnitude of the shift varies depending on the pyridonate ligand used, with the largest shifts found for fhp and the smallest for mhp. This shift correlates with the basicity of the N-donor atom of the xhp ligands and hence with the co-ordinative ability of the pyridonate ligands. The synthesis of 6-fluoro-2-pyridone (Hfhp) is reported for the first time.