Kinetic and characterization studies of iron(II) and iron(III) complex formation reactions with hydrazinopyridine

Abstract

Reactions of iron(II) and iron(III) with hydrazinopyridine have been investigated. The metal-to-ligand ratio of the two complexes was determined to be 1 : 3 and the stability constants of [Fe(hypz)3]2+ and [Fe(hypz)3]3+ were evaluated as 2.51 × 102 L3 mol−3 and 1.94 × 103 L3 mol−3, respectively. The complex formation reactions were studied kinetically as a function of [hzpy] at various temperatures (15–37°C) at fixed pH (7.5) using stopped-flow techniques. Kinetic results suggest that the binding of hzpy with Fe(II) or Fe(III) takes place in two consecutive steps: a slow and [hzpy] dependent rate-determining step followed by [hzpy] independent ring closure. The activation parameters for both steps in the two systems have been calculated. The low (57 ± 1, 46 ± 2 kJ mol−1) as well as (21 ± 0.1, 21 ± 0.1 kJ mol−1) for the first and second steps, respectively, and the large negative values of (−81 ± 4, −110 ± 5 J K−1 mol−1) as well as (−197 ± 3, −197 ± 3 J K−1 mol−1) for the first and second steps for both Fe(II) and Fe(III) systems, respectively, indicate associative modes of activation for both ligand substitution processes. [Fe(hypz)3](PF6)2 and [Fe(hypz)3](PF6)3 were isolated as solids and characterized using CHN, IR, magnetic and UV-Visible measurements.