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., Delta H double dagger = 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. (C) 2012 Elsevier B. V. All rights reserved.