SYNTHESIS, STRUCTURAL CHARACTERIZATION AND NUCLEAR-MAGNETIC-RESONANCESTUDIES OF COBALT COMPLEXES OF PYRIDONATE LIGANDS

A series of heteroleptic cobalt(II) complexes of diimine and pyridonat e ligands have been synthesized from cobalt(II) acetate, with the gene ral stoichiometric formula [Co(xhp)(2)L] [xhp = 6-methyl- (mhp), 6-chl oro- (chp), 6-bromo- (bhp) or 6-fluoro-2-pyridonate (fhp); L = 2,2'-bi pyridyl (bipy), 4,4'-dimethyl-2,2'-bipyridyl (dmbipy), 1,10-phenanthro line (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 spe cies containing six-co-ordinate cobalt sites. The co-ordination geomet ry of the metal is distorted due to the small bite angle of the chelat ing xhp ligands. The fourth complex, [{Co(mu-fhp)(fhp)(dmbipy)}(2)], c rystallises as a dinuclear unit with the cobalt atoms bridged by the e xocyclic O atom of one fhp ligand. The Co ... Co distance is 3.208(6) Angstrom. A fifth complex, [{Co(mu-bhp)(O(2)CMe)(dmbipy)}(2)], has bee n characterised in which two cobalt atoms are bridged by the O atom fr om a bhp ligand, giving a Co ... Co separation of 3.158(4) Angstrom. T he NMR studies of the complexes [Co(xhp)(2)(dmbipy)] revealed large pa ramagnetic 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 li gands. The synthesis of 6-fluoro-2-pyridone (Hfhp) is reported for the first time.