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

The TrpnCo(Ill)(OH)(OH2)-promoted hydrolysis of a range of methyl aryl phosphate diesters was investigated at 37 degreesC 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(111) complex -promoted hydrolysis against the pK(a) 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. Copyright (C) 2004 John Wiley Sons, Ltd.