Studies on the rate of isotopic oxygen exchange between [Re(py)4O2]+ and solvent water

Abstract

AbstractThe rate of oxygen exchange between trans‐[Re(py)4O2]+ and solvent water in pypyH+ buffer solution follows simple first‐order kinetics and both oxygens are equivalent. The half‐life for isotopic oxygen exchange is about 12 h at a pH of 5.0, 25°C, and [py] = 0.10 M. The observed rate constant for exchange increases with acidity, in the pH range 4 to 6, decreases with [py], and is nearly independent of ionic strength. A small but significant increase of kobs occurs with increasing complex concentration. The rate of exchange follows the rate equation kobs/2 = k0 + k1/[py] with k0 = 1.4 × 10−5(2) s−1 and k1 = 4.7 × 10−7(1) M, s−1 at 25°C. The activation parameters for the reaction at pH = 7.15 (predominately the k0 term) are: ΔH* = +137.(1) kJ/M and ΔS* = +126.(1) J/MK. The pH effect and complex concentration effect are discussed in mechanistic terms. These results are compared to those found for [Re(en)2O2]+ and [Re(CN)4O2]3−.