Homogeneous solvation controlled photoreduction of cobalt(III) complexes in aqueous 2-methyl-2-propanol solutions: Linear solvation energy relationship and cyclic voltammetric analyses

Abstract

The effect of solvent participation on the ligand-to-metal charge transfer (LMCT, L → Co III) reduction of the of Co III(en) 2Br(RC 6H 4NH 2) 2+ where R = m-OCH 3, p-F, H, m-CH 3, p-CH 3, p-OC 2H 5 and p-OCH 3 were examined in aqueous 2-methyl-2-propanol (Bu t OH) solutions. The change in the reduction behavior of Co III centre was also examined through cyclic voltammetric studies. The observed reduction in quantum yield due to LMCT excitation can mainly be accounted using linear solvation energy relationship (LSER) comprising model correlation equations. These consist of empirical parameters such as Grunwald–Winstein's solvent ionizing power, Y, Dimroth–Richardt's solvent micro-polarity parameter, E T N , Gutmann's donor number, DN N, along with Kamlet–Taft's solvatochromic parameters (hydrogen bond acceptor acidity/basicity α/ β and solvent dipolarity/polarizability, π*). The origin of solvent effect is found to be due to microscopic interaction between the solvent donor and the nitrogen-bound hydrogen of the ligand. Cyclic voltammograms show an irreversible reduction of Co III in DMF using Glassy Carbon Electrode, GCE, the redox peaks for the aniline complexes appear at −0.20 and 0.525 V. Irradiation of the complexes with UV light ( λ = 254 nm) in binary mixtures produce Co II aq and the concentration of this species are highly dependent on x alc ( x alc = mole fraction of alcohol). The observed quantum yield (log Φ Co(II)) is found to be linearly related to mole fraction of organic co-solvent added in the mixture, therefore, log Φ Co(II) = 26.41 × 10 −2 when x 2 = 0.0094 and 43.75 × 10 −2 when x 2 = 0.076 for a typical complex Co III(en) 2Br( p-OCH 3C 6H 4NH 2) 2+ in aqueous 2-methyl-2-propanol at 300 K. Cyclic voltammetry and LSER analyses illustrate the variation of reduction property of Co(III) by the aryl ligand and homogeneous solvation of the excited state of the complex Co III(en) 2Br(RC 6H 4NH 2) 2+ in H 2O/Bu t OH mixtures.