Characterization of the protolysis equilibria of 1-naphthoylhydrazide and of its oxidation mechanism by hexachloroiridate(IV) in aqueous solution

Abstract

Oxidation of 1-naphthoylhydrazide (1-NH) by hexachloroiridate(IV) ([IrCl6]2−) was characterized in detail by various spectroscopic techniques. The stoichiometry was determined to be [1-NH]:[Ir(IV)] = 1:4 and 1-NH was oxidized to 1-naphthoic acid which was confirmed by RP-HPLC and mass spectral analyses. The oxidation reaction strictly followed second-order kinetics; subsequently the rate versus pH profile was built up. A convincing reaction mechanism was proposed to account for all the experimental observations; rate constants of the rate-determining steps were calculated. Moreover, Activation parameters were determined to be ΔH2‡ = 36.5 ± 2.4 kJ∙mol−1 and ΔS2‡ = −75 ± 8 J∙K−1∙mol−1 for the reaction between [IrCl6]2− and the neutral form of 1-NH. Possible modes of electron transfer are discussed. This work also reports the protolysis constants of 1-NH for the first time, which are pKa1 = 3.03 ± 0.06 and pKa2 = 12.0 ± 0.1 at 25.0 °C and 1.0 M ionic strength.