Bivalent and Tervalent Cobalt Complexes of 4-(2-Pyridylazo)resorcinol, 1-(2-Pyridylazo)-2-naphthol, and their Thiazolyl Analogs in Aqueous and Aqueous Dioxane Media. Rapid-scan Spectral and Kinetic Studies

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Abstract
The reactions of Co2+ with 4-(2-pyridylazo)resorcinol (PAR), 1-(2-pyridylazo)-2-naphthol (PAN), and their thiazolyl analogs TAR and TAN in aqueous or aqueous dioxane media are followed by a rapid-scan spectroscopic method combined with stopped-flow technique. The step of the fast formation of cobalt(II) complexes is clearly discriminated from the subsequent slower step of oxidation to the cobalt(III) state. Formation and redissolution of transient insoluble intermediates are observed in the stopped-flow signals for the light scattered by the reaction mixture. The metal-to-ligand ratio in the short-lived intermediate cobalt(II) complex of PAR and PAN is determined to be 1 : 2 by applying the method of continuous variation to the fast change in stopped-flow signals. The rate constants k1 and k2 for the step of formation of 1 : 1 cobalt(II) complex with the species LHH* and LH− of PAR in aqueous solution and with the species LH2+ and LH of PAN in 5%(v/v) dioxane-water are determined to be k1=4.8×104 and k2=9.8×104 dm3 mol−1 s−1, and k13 and k2=4.6×103 dm3 mol−1 s−1, respectively.
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