Solvent H-bonding and structural effects on the photo-reduction of Co(III) complexes in water/propan-2-ol and water/1,4-dioxane media

The photo-reduction of a series of cobalt(III) complexes of the type cis-β-[Co(trien)(4-R-Py)Cl]Cl2 where trien=triethylenetetraamine and R = H, Me, Et, t-Bu, COMe, and CN has been studied in water/propan-2-ol and water/1,4-dioxane media of varying compositions. The photo-reduction quantum yield (ΦCo(II)) data were correlated with solvent and structural parameters with an aim to shed some light on the mechanism of the reaction. Correlation of ΦCo(II) with macroscopic solvent parameters viz. relative permittivity indicates that the reactivity is influenced by both specific and non-specific solute-solvent interactions. Kamlet-Taft solvatochromic comparison method was used to separate and quantify these effects. The observed non-linear concave downwards type Hammett plot is explained by assuming shift in the excited state within the same overall reaction pathway.