Mechanistic studies on the reactions of cyanide with a water-soluble Fe(III) porphyrin and their effect on the binding of NO

Article abstract of DOI:10.1021/ic1023345

The reaction of the water-soluble Fe^III(TMPS) porphyrin with CN^- in basic solution leads to the stepwise formation of Fe ^III(TMPS)(CN)(H₂O) and Fe^III(TMPS)- (CN)2. The kinetics of the reaction of CN^- with Fe^III(TMPS)- (CN)(H₂O) was studied as a function of temperature and pressure. The positive value of the activation volume for the formation of Fe ^III(TMPS)(CN)2 is consistent with the operation of a dissociatively activated mechanism and confirms the six-coordinate nature of the monocyano complex. A good agreement between the rate constants at pH 8 and 9 for the formation of the dicyano complex implies the presence of water in the axial position trans to coordinated cyanide in the monocyano complex and eliminates the existence of Fe^III(TMPS)(CN)(OH) under the selected reaction conditions. Both Fe^III(TMPS)- (CN)(H₂O) and Fe ^III(TMPS)(CN)2 bind nitric oxide (NO) to form the same nitrosyl complex, namely, FeII(TMPS)(CN)- (NO⁺). Kinetic studies indicate that nitrosylation of Fe^III(TMPS)(CN)2 follows a limiting dissociative mechanism that is supported by the independence of the observed rate constant on [NO] at an appropriately high excess of NO, and the positive values of both the activation parameters ΔS and ΔV found for the reaction under such conditions. The relatively small first-order rate constant for NO binding, namely, (1.54 (0.01) × 10^-2 s^-1, correlates with the rate constant for CN^- release from the Fe^III(TMPS)(CN)2 complex, namely, (1.3 ± 0.2) × 10^-2 s^-1 at 20 °C, and supports the proposed nitrosylation mechanism.