CHARGE-TRANSFER SPECTROSCOPY AND PHOTOCHEMISTRY OF ALKYLAMINE COBALT(III) COMPLEXES

Spectroscopic and photochemical studies are reported for Co(NH2R)63+, Co(NH2R)5OH23+, and Co(NH2R)5X2+ (where R is methyl, ethyl, or n-propyl, and X is Cl- or Br-). These complexes absorb strongly in the ultraviolet region owing to the presence of one or more ligand-to-metal charge transfer (LMCT) band(s). Irradiation into this charge transfer region results in redox decomposition of the complex with the release of Co2+, alkylamine, and a radical. Photoredox quantum yields measured in fluid solution are generally high and dependent upon excitation wavelength, oxygen concentration, temperature, and solution viscosity. In contrast to their well-studied ammine analogues, the alkylamine complexes (1) possess a N --> Co charge transfer band at wavelengths well above 200 nm, (2) undergo redox decomposition from the X --> Co and N --> Co charge transfer excited states, and (3) avoid an intermolecular decomposition pathway involving photooxidation of the solvent. Reasons for these differences between the two families of complexes are discussed.