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(dmgBF2 )2 (OH2 )2 ] (where dmgBF2 = 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 pKa1 value for [Co(dmgBF2 )2 (H2 O)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+ ] + k1 Ka )/([H+ ] + Ka ))[OCl- ]T , where at 25.3 °C, k1 was calculated as 3.54 × 104 M-1 s-1 , and k2 as 2.51 × 104 M-1 cm-1 . At a constant pH value, while varying the concentration of sodium hypochlorite two rate constants were calculated, viz., k'1 = 7.56 s-1 (which corresponded to a reaction pathway independent of the NaOCl concentration) and k'2 = 2.26 × 104 M-1 s-1 , 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-1 , 46 ± 1 kJ mol-1 , 34 ± 55 J mol-1 K-1 , and -6 ± 4 Jmol-1 K-1 , respectively. The self-exchange rate constant, k11 , for sodium hypochlorite (as ClO- ) was calculated to be 1.2 × 103 M-1 s-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%.