title: An altered mechanism of hydrolysis for a metal-complexed phosphate diester.
abstract: Isotope effects in the nucleophile and in the leaving group were measured to gain information about the mechanism and transition state of the hydrolysis of methyl p-nitrophenyl phosphate complexed to a dinuclear cobalt complex. The complexed diester undergoes hydrolysis about 1011 times faster than the corresponding uncomplexed diester. The kinetic isotope effects indicate that this rate acceleration is accompanied by a change in mechanism. A large inverse 18O isotope effect in the bridging hydroxide nucleophile (0.937 +/- 0.002) suggests that nucleophilic attack occurs before the rate-determining step. Large isotope effects in the nitrophenyl leaving group (18Olg = 1.029 +/- 0.002, 15N = 1.0026 +/- 0.0002) indicate significant fission of the P-O ester bond in the transition state of the rate-determining step. The data indicate that in contrast to uncomplexed diesters, which undergo hydrolysis by a concerted mechanism, the reaction of the complexed diester likely proceeds via an addition-elimination mechanism. The rate-limiting step is expulsion of the p-nitrophenyl leaving group from the intermediate, which proceeds by a late transition state with extensive bond fission to the leaving group. This represents a substantial change in mechanism from the hydrolysis of uncomplexed aryl phosphate diesters.
authors.affiliation: Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322-0300, USA.
authors.forename: Tim
authors.initials: T
authors.lastname: Humphry
authors.forename: Marcello
authors.initials: M
authors.lastname: Forconi
authors.forename: Nicholas H
authors.initials: NH
authors.lastname: Williams
authors.forename: Alvan C
authors.initials: AC
authors.lastname: Hengge
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D006868:Hydrolysis; D007700:Kinetics; D009587:Nitrogen Isotopes; D009596:Nitrophenols / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D010755:Organophosphates / Q000737:chemistry*; D009943:Organophosphorus Compounds / Q000737:chemistry; D010103:Oxygen Isotopes
chemical_list: D009587:Nitrogen Isotopes; D009596:Nitrophenols; D009942:Organometallic Compounds; D010755:Organophosphates; D009943:Organophosphorus Compounds; D010103:Oxygen Isotopes; C008644:nitrophenylphosphate; D003035:Cobalt