Kinetics and mechanism of the oxidation of a cobaloxime by sodium hypochlorite in aqueous solution: Is it an outer-sphere mechanism?

Abstract

The kinetics and mechanism of the oxidation of [Co(dmgBF2)2(OH2)2] (where dmgBF2=difluoroboryldimethylglyoximato) by sodium hypochlorite (NaOCl) were investigated by stopped-flow spectrophotometry at 450nm over the temperature range of 10°C⩽θ⩽25°C, pH range of 5.0⩽pH⩽7.8, and at an ionic strength of 0.60M (NaCl). The pKa1 value for [Co(dmgBF2)2(H2O)2] was calculated as 5.27±0.14 at I=0.60 (NaCl). The redox process was dependent on pH and oxidant concentration in a complex manner, that is, kobs=((k2[H+]+k1Ka)/([H+]+Ka))[OCl−]T, where at 25.3°C, k1 was calculated as 3.54×104M−1s−1, and k2 as 2.51×104M−1cm−1. At a constant pH value, while varying the concentration of sodium hypochlorite two rate constants were calculated, viz., k′1=7.56s−1 (which corresponded to a reaction pathway independent of the NaOCl concentration) and k′2=2.26×104M−1s−1, which was dependent on the concentration of NaOCl. From the variation in pH, ΔH1≠, ΔH2≠, ΔS1≠, and ΔS2≠ were calculated as 58±16kJmol−1, 46±1kJmol−1, 34±55Jmol−1K−1, and −6±4Jmol−1K−1, respectively. The self-exchange rate constant, k11, for sodium hypochlorite (as ClO−) was calculated to be 1.2×103M−1s−1, where an outer-sphere electron transfer mechanism was assumed. A green product, [Co(dmgBF2)2(OH2)(OH)]·1.75NaOCl, which can react with DMSO, was isolated from the reaction at pH 8.04 with a yield of 13%.