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

The kinetics and mechanism of the oxidation of [Co(dmgBF)(OH)] (where dmgBF = difluoroboryldimethylglyoximato) by sodium hypochlorite (NaOCl) were investigated by stopped-flow spectrophotometry at 450 nm over the temperature range of 10 °C ≤ θ ≤ 25 °C, pH range of 5.0 ≤ pH ≤ 7.8, and at an ionic strength of 0.60 M (NaCl). The pK value for [Co(dmgBF)(HO)] 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, k = ((k[H] + kK)/([H] + K))[OCl], where at 25.3 °C, k was calculated as 3.54 × 10 M s, and k as 2.51 × 10 M cm. At a constant pH value, while varying the concentration of sodium hypochlorite two rate constants were calculated, viz., k' = 7.56 s (which corresponded to a reaction pathway independent of the NaOCl concentration) and k' = 2.26 × 10 M s, which was dependent on the concentration of NaOCl. From the variation in pH, [Formula: see text], and [Formula: see text] were calculated as 58 ± 16 kJ mol, 46 ± 1 kJ mol, 34 ± 55 J mol K, and -6 ± 4 Jmol K, respectively. The self-exchange rate constant, k, for sodium hypochlorite (as ClO) was calculated to be 1.2 × 10 M s, where an outer-sphere electron transfer mechanism was assumed. A green product, [Co(dmgBF)(OH)(OH)]·1.75NaOCl, which can react with DMSO, was isolated from the reaction at pH 8.04 with a yield of 13%.