Charge-transfer spectroscopy and photochemistry of alkylamine cobalt(III) complexes

Abstract

Spectroscopic and photochemical studies are reported for Co(NH 2R) 6 3+, Co(NH 2R) 5OH 2 3+, and Co(NH 2R) 5X 2+ (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 Co 2+, 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.