Mechanistic Studies of Dichloro(1,4,7-triazacyclononane)copper(II)-Catalyzed Phosphate Diester Hydrolysis

In order to explore the role of labile metal complexes in promoting phosphodiester hydrolysis, detailed kinetic and mechanistic studies with Cu[9]aneN3Cl2 (1) were performed. The hydrolysis of bis(4-nitrophenyl) phosphate (2) by 1 is catalytic, as determined by the observation of both rate enhancement and turnover. The catalyst is selective for phosphodiesters; the rate of hydrolysis for 4-nitrophenyl phosphate (4) is 50 times slower than the hydrolysis rate of 2. A rate enhancement of 2000 is observed relative to hydroxide ion mediated hydrolysis, and the rates reported herein are comparable to the rates reported for phosphodiester hydrolysis by other labile metal complexes. The reaction of 1 with 2 showed a half-order dependence on the catalyst, implicating a monomer−dimer equilibrium with the monomer as the catalyst. The kinetically determined equilibrium constant is 1220 M-1, favoring the dimer. The reaction of 1 with ethyl 4-nitrophenyl phosphate (3) follows Michaelis−Menten kinetics (KM = 62.3 mM and kcat = 1.96 × 10-5 s-1), providing evidence for the formation of a catalyst−substrate complex. The pH vs rate profile indicates that deprotonation of the metal-coordinated water (pK1 = 7.3) occurs to form the active catalyst, and the reaction becomes pH independent above pH 8.5. The activation parameters for 3 hydrolysis by 1 (pH 9.0) are ΔH⧧ = 90 kJ mol-1 and ΔS⧧ = −128 J mol-1 K-1. A mechanism consistent with the kinetic data is presented.