Phosphate diester hydrolysis within a highly reactive dinuclear cobalt(III) complex. Ligand effect on reactivity, transition state and dissociation

The hydrolysis or methyl aryl phosphate diesters coordinated to a dinuclear Co(in) complex ([CO2(tame)(2)(OH)(2)- {O2P(OAr)(OMe)}](3+); tame = 1,1,1-t ris(aminomethyl)ethane. 2) has been studied in aqueous solution at 25 degre esC. 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 cat alysed hydrolysis of 2 are highly sensitive to the basicity of the aryloxy leaving group with beta (Ig) = -1.29 +/- 0.03. This leaving group dependenc e is similar to that of 1 (beta (Ig) = -1.38 +/- 0.01), showing that the li gand affects reactivity without greatly altering the transition state at ph osphorus. The slight decrease in Ag is consistent with previous rationalisa tions of this high sensitivity. Dimethyl phosphate coordinated to both type s of complex (3, tame; 4, tacn) only dissociates from the complex, with no hydrolysis. Base catalysed dissociation is slower with tame (3 20 fold slow er 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 thes e dinuclear complexes may be tuned rationally and independently.