Solvent and Structural Effects on the Photoreduction of Cobalt(III)-Aryl Amine Complexes in Water-Organic Solvent Media

摘要

Photoreduction of [Co(En)_2Cl(RC_6H_4NH_2)]~(2+) ions (where R=p-OMe, p-OEt, p-Me, m-Me, H, p-F, and m-OMe) in varying compositions of water-methanol and water-1,4-dioxane mixtures containing 15-40% (vol.) of organic co-solvent is carried out. Ultraviolet excitation of the above complexes in air-equilibrated solutions causes bleaching of its intense LMCT excited states with concurrent production of Co~(2+)ion. As seen from the quantum yield data, PHI_Co(II) increases smoothly iwht increasing content of organic co-solvent in the binary mixtures. The observed values indicate that the metal center is reduced by both ligands and solvent. The quantum yield PHI_Co(II) is considerably aff4ected by the substituent R of the amine, RC_6H_4NH_2 ligand. The effects of solvent and substituted ligands on the PHI_Co(II) are estimated quantitatively using linear regression and multiple correlation methods. The former analysis was carried out using Grunwald-Winstein (Y) Gutmann donor number (DN~N) and Krygowski-Fawcett E_T~N (solvent empirical parameters). In addition, Kamlet-Taft's alpha,beta, and pi~* solvatochromic parameters were also used to study the effect of solute-solvent interaction. The effect of substituent constant sigma. Extensive tabulations of percentage contributions of these parameters, calculated using methods reported earlier, provide suitable values which are presumed to explain the quantitative effects of solvent and structural changes in the aromatic ligand on photoreduction of the cobalt(III) complexes.