Kinetic and Mechanistic Characterization of Oxidation of 2-Thiophenecarboxylic Hydrazide by Hexachloroiridate(IV)

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Oxidation of 2-thiophenecarboxylic hydrazide (TPCH) by hexachloroiridate(IV) ([IrCl6]2−) was characterized by rapid-scan spectral, kinetic and 1H NMR spectroscopic techniques. The oxidation reaction was found to strictly follow second-order kinetics; the apparent second-order rate constant versus pH profile was established experimentally. A reaction stoichiometry of Δ[TPCH]/Δ[IrCl ₆^2 - ] = 1/4 was derived and the oxidation product was identified as 2-thiophenecarboxylic acid by 1H NMR spectroscopic approach. In addition, the dissociation constants of TPCH were firstly determined in this work by kinetic and spectrophotometric titration methods, respectively, which are pKa1 = 3.24 ± 0.06 and pKa2 = 11.5 ± 0.1 at 25.0°C and 1.0 M ionic strength. A reaction mechanism was suggested which involves all the three protolytic forms of TPCH attacking on [IrCl6]2‒ in parallel, being as the rate-controlling steps. A rate expression was derived based on the proposed reaction mechanism, and the values of rate constants of the rate-controlling steps were calculated. The reactivity of the TPCH protolytic forms towards reduction of Ir(IV) was revealed to differ hugely and the reactivity versus pH distribution diagram was constructed. Moreover, activation parameters were determined to be Δ H₂ = 33.4 ± 1.0 kJ mol−1 and Δ S₂ = –75 ± 4 J K−1 mol–1 for the rate-determining step between [IrCl6]2‒ and the neutral form of TPCH. Possible mode of electron transfer is discussed.