Phosphatediesterhydrolysis within a highly reactive dinuclear cobalt(iii) complex. Ligand effect on reactivity, transition state and dissociation

Abstract

The hydrolysis of methyl aryl phosphate diesters coordinated to a dinuclear Co(III) complex ([Co₂(tame)₂(OH)₂{O₂P(OAr)(OMe)}]³⁺; tame = 1,1,1-tris(aminomethyl)ethane; 2) has been studied in aqueous solution at 25 °C. Hydrolysis of the phosphate diester is base catalysed and occurs 30 to 60 fold faster than in analogous complexes where tame is replaced by 1,4,7-triazacyclononane (tacn) (1). The second order rate constants for base catalysed hydrolysis of 2 are highly sensitive to the basicity of the aryloxy leaving group with β_lg = −1.29 ± 0.03. This leaving group dependence is similar to that of 1 (β_lg = −1.38 ± 0.01), showing that the ligand affects reactivity without greatly altering the transition state at phosphorus. The slight decrease in β_lg is consistent with previous rationalisations of this high sensitivity. Dimethyl phosphate coordinated to both types of complex (3, tame; 4, tacn) only dissociates from the complex, with no hydrolysis. Base catalysed dissociation is slower with tame (3 20 fold slower than 4) but the pH independent reaction is faster (3 10 fold faster than 4). These data suggest that the reactivity and turnover properties of these dinuclear complexes may be tuned rationally and independently.