Heavy-Atom Isotope Effects on Reactions ofCo(III)-Bound p-Nitrophenyl Phosphate: Nucleophilic Displacements of p-Nitrophenol andDissociation of p-Nitrophenyl Phosphate

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Journal of the American Chemical Society

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Heavy-atom isotope effects have been measured for the reactions of p-nitrophenyl phosphate (pNPP) in the stable Co complexes cis-[Co(en)2(OH)pNPP] (1), Co(NH3)5pNPP (2), and a Co(cyclen) complex of pNPP (4) which forms reversibly in solution. The isotope effects in the nonbridge oxygen atoms (18knonbridge), in the bridge oxygen atom (18kbridge), and in the nitrogen atom of pNPP (15k) were measured. Complexes 1 and 2 undergo dissociation of pNPP in competition with nucleophilic attack to liberate p-nitrophenolate. The kinetic isotope effects on both processes were measured, and the theory and equations for the determination of isotope effects in parallel reactions are presented. With complex 4 the kinetic isotope effects for the nucleophilic reaction were measured, as well as the equilibrium isotope effects on formation of the complex between Co(cyclen) and pNPP. The isotope effects sensitive to bond cleavage, 15k and 18kbridge, for the nucleophilic reactions were respectively 1.0021 ± 0.0002 and 1.0213 ± 0.0012 for 1, 1.0012 ± 0.0001 and 1.0098 ± 0.0010 for 2, and 1.0016 ± 0.0002 and 1.0207 ± 0.0006 for 4. All are indicative of substantial transition state bond cleavage in the rate-limiting step, and these results are most consistent with a concerted mechanism. The 18knonbridge isotope effects of 1.0006 ± 0.0002, 1.0045 ± 0.0003, and 1.0057 ± 0.0009 with 1, 2, and 4 indicate that the transition state has a slight associative character. The kinetic 18knonbridge isotope effects for dissociation of pNPP from 1 and 2 of 1.0135 ± 0.0004 and 1.0167 ± 0.0003 and the equilibrium one for 4 of 1.0081 ± 0.0006 are consistent with covalent character of the Co−O bond.

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