Kinetics of Axial Substitution of the Iron(II) Complex of 2,3,9,10-Tetraphenyl-1,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene in Acetonitrile. Evidence for a Dissociative-Interchange Mechanism

Abstract

Abstract The reaction of the iron(II) complex of 2,3,9,10-tetraphenyl-1,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene ([FeL(CH3CN)2]2+, 1) with imidazole and thiocyanate ion in acetonitrile has been investigated spectrophotometrically at various temperatures and pressures. The axial substitution of 1 with imidazole is first order with respect to the concentration of the iron(II) complex and imidazole and the rate constant and the activation parameters are obtained as k (25.0 °C)=0.69 mol−1 kg s−1, ΔH\eweq=76±4 kJ mol−1, ΔS\eweq=7±13 J mol−1 K−1, ΔV\eweq=13.7±0.9 cm3 mol−1. It has been demonstrated that at higher concentration of imidazole there exists self-associated imidazole which does not react directly with the complex. The axial substitution of 1 with thiocyanate ion proceeds through the ion-association between 1 and SCN− prior to the rate-determining step from the outer-sphere complex to the inner-sphere complex. The activation parameters for the rate-determining step were determined as follows: k0 (25.0 °C and 0.1 MPa)=2.35 s−1, ΔH\eweq=86±2 kJ mol−1, ΔS\eweq=51±6 J mol−1 K−1, ΔV\eweq=10.1±0.2 cm3 mol−1. The positive activation volumes strongly indicate a dissociative character of the activation process.