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

Authors

J. Rawlings
Alvan C. Hengge, Utah State UniversityFollow
W. W. Cleland

Document Type

Article

Journal/Book Title

Journal of the American Chemical Society

Publication Date

1997

Volume

119

First Page

542

Last Page

549

Abstract

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 (¹⁸k_nonbridge), in the bridge oxygen atom (¹⁸k_bridge), 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 ¹⁸k_bridge, 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 ¹⁸k_nonbridge 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 ¹⁸k_nonbridge 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.

Recommended Citation

J. Rawlings, A. C. Hengge, and W. W. Cleland. “Heavy-Atom Isotope Effects on Reactions of Co(III)-Bound p-Nitrophenyl Phosphate: Nucleophilic Displacements of p-Nitrophenol and Dissociation of p-Nitrophenyl Phosphate.” J. Am. Chem. Soc.1997, 119, 542-549.