Kinetic Studies of the Complex Formation of Iron(III) with 4-Isopropyltropolone by a High-Pressure Stopped-Flow Technique. Mechanistic Difference between the Hexaaquairon(III) Ion and the Pentaaquahydroxoiron(III) Ion in Their Complexation

The formation rate of the 1:1 iron(III) complex with 4-isopropyltropolone (Hipt) has been studied spectrophotometrically in aqueous solution of low pH at various temperatures and pressures by a stopped-flow technique. The rate law is expressed as d[Fe(ipt)²⁺]/dt = (K₁ + K₂K_FeOH[H⁺]^-1)(1 + K_H[H⁺])^-1[Fe³⁺][Hipt], with k₁ = 21.7 ± 1.1 M^-1 s^-1 and k₂ = (6.30 ± 0.31) × 10³ M^-1 s^-1 at 25 °C and I = 1.00 M at atmospheric pressure. The activation parameters for the reaction of Fe(OH₂)6³⁺ (k₁ path) and the reaction of Fe(OH₂)₅OH²⁺ (k₂ path) are as follows: ΔH‡₁ = 59.5 ± 3.0 kJ mol^-1, ΔS‡₁ = -19.8 ± 3.0 J mol^-1 K^-1, ΔV‡₁ = -8.7 ± 0.8 cm³ mol^-1; ΔH‡₂ = 29.2 ± 3.6 kJ mol^-1, ΔS‡₂ = -72.3 ± 5.0 J mol^-1 K^-1, ΔV‡₂ = 4.1 ± 0.6 cm³ mol^-1. Associative- and dissociative-interchange mechanisms are most probably operative for the k₁ and k₂ paths, respectively.