Effect of the axial halogen ligand on the substitution reactions of chromium(III) porphyrin complex

The thermodynamics and kinetics of the substitution reaction of the chromium(III) complex of 5,10,15,20-tetraphenylporphyrin, [Cr(TPP)(X)(L)] (X = F, Cl, Br), where L represents a non-charged ligand such as H2O, pyridine, or 1-methylimidazole, was investigated. The present study aimed to elucidate the effect of the axial halogen ligand, X on the substitution reaction of the ligand trans to X. The substitution reaction of the axial pyridine ligand of [Cr(TPP)(X)(Py)] by 1-methylimidazole was studied spectrophotometrically in dichloromethane, and it was found that the reaction proceeds via a limiting dissociative mechanism and the activation enthalpy of the dissociation of pyridine is much smaller for the fluorine complex, [Cr(TPP)(F)(Py)], than for the chlorine complex, [Cr(TPP)(Cl)(Py)], i.e., ΔH‡ = 57.5 ± 1.1 and 97.2 ± 1.4 kJ mol−1, respectively. These results are discussed in terms of the trans effect of the halogen ligand on the axial ligand substitution reaction of [Cr(TPP)(X)(L)], and it is concluded that the strength of the bond shows the tendency of Cr–F > Cr–Cl > Cr–Br, leading to the strong trans effect of the F− ligand.

The kinetics of the axial substitution reaction of the Cr(III) tetraphenylporphyrin complex, [Cr(TPP)(X)(L)] (X = F, Cl, Br; L = H2O, pyridine, 1-methylimidazole) was studied. It was found that the reactivity of the substitution reaction strongly depends on the nature of the axial halogen ligand, and the results are discussed in terms of the trans effect of this ligand.Figure optionsDownload as PowerPoint slideHighlights
► Py substitution reaction of [Cr(TPP)(X)(Py)] (X = halogen) was studied.
► ΔH‡ for F complex is much smaller than that of Cl complex.
► Cr–Br bond dissociates easily in the presence of strongly coordinating ligand.
► Trans effect of halogen on the substitution reaction of [Cr(TPP)(X)(L)] was discussed.
► Reactivity of [Cr(TPP)(X)(Py)] strongly depends on the axial halogen ligand.