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.

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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\neweq=76±4 kJ mol−1, ΔS\neweq=7±13 J mol−1 K−1, ΔV\neweq=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\neweq=86±2 kJ mol−1, ΔS\neweq=51±6 J mol−1 K−1, ΔV\neweq=10.1±0.2 cm3 mol−1. The positive activation volumes strongly indicate a dissociative character of the activation process.
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