LOW-SPIN OCTAHEDRAL COBALT(II) COMPLEXES OF CON6 AND CON4P2 CHROMOPHORES - SYNTHESIS, SPECTROSCOPIC CHARACTERIZATION AND ELECTRON-TRANSFER PROPERTIES

The reaction of 2-(arylazo)pyridines (NC5H4)N=NC6H4R L-1-L-7 (R=H, o-M e/Cl, m-Me/Cl, p-Me/Cl) with cobalt(II) perchlorate hexahydrate in abs olute ethanol under anaerobic conditions afforded low-spin [(CoL3)-L-I I](2+) complexes, isolated as ClO4- salts. At room temperature the com plexes are one-electron paramagnetic in nature, low-spin Co-II, t(2g)( 6)e(g)(1), S=1/2 and behave as 1:2 electrolytes in acetonitrile solven t. In acetonitrile solvent they show a ligand-to-metal charge-transfer (LMCT) band near 400 nm, an intraligand transition near 300 nm and li gand-field d-d transitions in the range 860-600 nm. The complexes exhi bit quasi-reversible Co-II-Co-III couples near 1 V and six sequential ligand reductions (N=N groups) in the range 0.2 to -1.8 V versus satur ated calomel electrode (SCE). At room temperature in the solid state t hey exhibit isotropic EPR spectra but at 77 K, both in the polycrystal line state and in the dichloromethane solution, display rhombic spectr a. Reaction of [(CoL3)-L-II](2+) with 2,2'-bipyridine (bpy) and 1,10-p henanthroline (phen) resulted in complete ligand-exchanged products wi th concomitant metal oxidation, low-spin [Co-III(bpy)(3)](3+) and low- spin [Co-III(phen)(3)](3+) respectively. The reaction of PPh3 with the [Ca-II(L-7)(3)](2+) [L-7=2-(p-chlorophenylazo)pyridine] yielded a par tial ligand-exchanged product, low-spin [Co-II(L-7)(2)(PPh3)(2)](2+), isolated as its ClO4- salt. The complex is one-electron paramagnet and a 1 :2 electrolyte in acetonitrile solvent. It displays an LMCT band at 401 nm, an intraligand transition at 305 nm and four d-d transition s in the range 870-640 nm. It exhibits irreversible Co-II to Co-III ox idation at 1.33 V (E-pa) and four successive ligand reductions in the range -0.30 to -1.1 V versus SCE. At 77 K the complex displays an axia l EPR spectrum.