Kinetics of imidazole addition to the axial site of the iron(II) complex of 2, 3, 9,10-tetraphenyl-1,4, 8,11-tetraaza-1,3, 8,10-cyclotetradecatetraene in dimethyl sulfoxide. Evidence for a dissociative-interchange mechanism

The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL²⁺, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k₁ (25.0°C) = (6.8 ±0.2)×10⁵ mol^?1 kg s^?1, ΔH³₁ = 47.5 ± 4.9 kJ mol^?1, ΔS³₁ = 26±16 J K^?1 mol^?1, and ΔV³₁ (30.0°C) = 27.2±1.5 cm³ mol^?1; for the second step with k₂ (25.0°C) = 26.8±0.8 mol^?1 kg s^?1, ΔH³₂ = 91.6± 0.8 kJ mol^?1, ΔS³₂ = 90±3 J K^?1 mol^?1, and ΔV³₂ (35.0°C) = 21.8±0.9 cm³ mol^?1. The large positive activation volumes strongly indicate a dissociative character of the activation process.