Syntheses, structures, magnetic, and spectroscopic properties of cobalt(II), nickel(II) and zinc(II) complexes containing 2-(6-methyl)pyridyl-substituted nitronyl and imino nitroxide

Chelate complexation of 4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)imidazoli n-1-oxyl 3-oxide (NITmepy) and 4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)im idazolin-1-oxyl (IMmepy) to cobalt(II) chloride (bromide) or zinc(II) chlor ide afforded tetrahedral four-co-ordinate (T-4) complexes, [MX2(NITmepy)] a nd [MX2(IMmepy)] (MX = CoCl (1 and 2), CoBr (1Br and 2Br) or ZnCl (3 and 4) , respectively) containing either a six- or a five-membered chelate ring as determined by single-crystal X-ray analysis. The formation of such T-4 com plexes arises from the steric requirements of the methyl group at the o-pos ition of the pyridyl-N donor. The UV-vis-NIR spectra suggest that both comp lexes 1 and 2 in dichloromethane retain their solid state structure. In ace tonitrile, however, 1 exists as an equilibrium mixture due to dissociation of the co-ordinated NITmepy, while 2 is stable. The ligand-field spin-allow ed and spin-forbidden d-d transitions of 1 and 2 are not influenced by the coordinated radical ligands. The reaction of nickel(II) chloride and thiocy anate with IMmepy gave a dichloro-bridged dinuclear complex with a square p yramidal five-co-ordinate (SPR-5) geometry and an OC-6 complex, respectivel y, [{NiCl(IMmepy)}(2)(mu -Cl)(2)] (5) and [Ni(NCS)(2)(IMmepy)(2)] (6). The intramolecular magnetic interactions in I and 2 are estimated to be antifer romagnetic, whereas 5 and 6 give ferromagnetic interactions. Complex 3, wit h one-dimensional columns, forms a dimer so that a moderate intermolecular antiferromagnetic coupling is demonstrated, while complex 4 exists as a dis crete molecule in the crystal and shows paramagnetic behaviour.