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. 2017 Jan 1:454:254-265.
doi: 10.1016/j.ica.2016.07.013.

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

Michael J Celestine  1 , Lorne S Joseph  2 , Alvin A Holder  1
Affiliations

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

Michael J Celestine et al. Inorganica Chim Acta. .

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 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(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 × 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%.

Keywords: 59Co NMR spectroscopy; Cobaloximes; Kinetics; Self-exchange rate constants; Sodium hypochlorite.

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Figures

Fig. 1
Fig. 1
Binuclear mixed-metal complexes used for the production of hydrogen in acidic media [10,22,25].
Fig. 2
Fig. 2
Two FTIR spectra of [Co(dmgBF2)2(OH2)(OH)] ·1.75NaOCl.
Fig. 3
Fig. 3
A plot of the molar extinction coefficient versus wavelength of [Co(dmgBF2)2(OH2)2] (black) and [Co(dmgBF2)2(OH2)(OH)] ·1.75NaOCl (red) in DMSO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4
Fig. 4
1H NMR spectra of [Co(dmgBF2)2(OH2)(OH)] ·1.75NaOCl (A), [Co(dmgBF2)2 (OH2)2] (B), and dmg (C) in DMSO-d6.
Fig. 5
Fig. 5
59Co NMR spectrum of [Co(dmgBF2)2(OH2)(OH)] ·1.75NaOCl in DMSO-d6.
Fig. 6
Fig. 6
A plot of absorbance versus wavelength comparing 0.1 mM [Co(dmgBF2)2(- OH2)2] in water to the respective 1.0 mM Co(III) complex (A), and a plot of absorbance versus wavelength for the reaction between 0.10 mM [Co(dmgBF2)2(- OH2)2] and 0.20 mM NaOCl (I = 0.60 M (NaCl), buffer = phosphate buffer, and pH = 7.33 ± 0.02) (B).
Fig. 7
Fig. 7
A plot of absorbance versus pH at room temperature. [Complex] = 0.10 mM, I = 0.60 M (NaCl), λ = 265 nm, acetate buffer (3.50 ≤ pH ≤ 5.50), and phosphate buffer (5.25 ≤ pH ≤ 8.00).
Fig. 8
Fig. 8
A plot of absorbance versus the ratio of [NaOCl]/[Complex] for the oxidation of [Co(dmgBF2)2(OH2)2] by NaOCl. Inset = a plot of Absorbance versus wavelength for the oxidation of [Co(dmgBF2)2(OH2)2] by NaOCl. [Complex] = 0.10 mM, λ = 450 nm, I = 0.60 M (NaCl), buffer = phosphate buffer, and pH = 7.33 ± 0.02.
Fig. 9
Fig. 9
A plot of kobs versus the concentration of NaOCl. [Complex] = 0.10 mM, λ = 450 nm, I = 0.60 M (NaCl), θ = 10.1 °C, buffer = phosphate buffer, and pH = 7.82 ± 0.08.
Fig. 10
Fig. 10
A plot of kobs versus pH. [Complex] = 0.05 mM, [NaOCl] = 3.00 mM, λ = 450 nm, I = 0.60 M (NaCl), θ = 25.3 °C, and buffer = phosphate buffer.
Fig. 11
Fig. 11
A plot of kobs(Ka + [H+])/[OCl]T versus [H+] for the oxidation of [Co (dmgBF2)2(OH2)2] by 3 mM NaOCl. [Complex] = 0.05 mM, λ = 450 nm, I = 0.60 M (NaCl), θ = 10.3 °C, and buffer = phosphate buffer.
Fig. 12
Fig. 12
A cyclic voltammogram of [Co(dmgBF2)2(OH2)2] in water. Working electrode = glassy carbon electrode, auxiliary electrode = Pt wire, reference electrode = Ag/AgCl, supporting electrolyte = 0.6 M (NaCl), [complex] = 1.0 mM, and scan rate = 100 mV s−1.
Fig. 13
Fig. 13
A plot of kex versus ionic strength for [Co(dmgBF2)2(OH2)2] in water.
Scheme 1
Scheme 1
Synthesis of [Co(dmgBF2)2(OH2)(OH)] ·1.75NaOCl.
Scheme 2
Scheme 2
Mechanism for the oxidation of [Co(dmgBF2)2(OH2)2] by sodium hypochlorite.
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