Mononuclear Co(III)‐complex promoted phosphate diester hydrolysis: dependence of reactivity on the leaving group

The TrpnCo(III)(OH)(OH2)‐promoted hydrolysis of a range of methyl aryl phosphate diesters was investigated at 37°C and I = 0.1 M (NaClO4). The pH–rate profile confirms that the aqua‐hydroxy form of the complex is the only kinetically significant ionic form. At pH 6.9, all the reactions are first order in both diester and Co(III) complex. Plotting the second‐order rate constant for Co(III) complex‐promoted hydrolysis against the pKa of the leaving aryloxy group revealed a bent LFER indicating a change in rate‐limiting step. This is discussed in terms of either a change from rate‐limiting hydrolysis to rate‐limiting binding or the presence of a phosphorane intermediate.