Effect of a sterically hindered imidazole ligand on the molecular structure and axial ligand substitution reaction of the chromium(III) porphyrin complex

A structural, thermodynamic, and kinetic study of a chromium(III) complex of 5,10,15,20-tetraphenylporphyrin, [Cr(tpp)(Cl)(L)], where L represents an imidazole ligand, is presented. The present study is aimed at elucidating the effect of a steric strain induced by the 2-methyl substituent of the imidazole ligand on the molecular structure and the dynamics of an axial ligand-substitution reaction. Crystals of the 1-methylimidazole complex [Cr(tpp)(Cl)(1-Meim)] (1) and 1,2-dimethylimidazole complex [Cr(tpp)(Cl)(1,2-Me(2)im)] (2) were obtained from a dichloroethane-toluene mixture containing a slight excess of imidazole ligand in the monoclinic space group Cc, Z = 4, a = 18.306(2), b = 13.311(2), c = 21.391(5) Angstrom, beta=120.05(1)degrees, V = 4512(1) Angstrom(3), and in the orthorhombic space group P2(1)2(1)2(1), Z = 4, a = 9.926(1), b = 17.902(3), c = 25.247(4) Angstrom, V = 4486(1) Angstrom(3), respectively. The axial Cr-N-im bond lengths for 1 and 2 are 2.103(4) and 2.139(5) Angstrom, respectively. A steric hindrance has been demonstrated by a 0.036 Angstrom increase in the Cr-N-im bond length and a tilting and tipping of the imidazole ligand of 2 from the symmetrical position observed in 1. The substitution reaction of the axial pyridine ligand of [Cr(tpp)(Cl)(py)] by various imidazole ligands was studied spectrophotometrically in dichloromethane or in toluene. A steric effect due to the 2-methyl substituent of the imidazole ligand was observed along with an increase in the ligand-dissociation rate and a decrease in the binding constant of the imidazole ligand to the metalloporphyrin. These results were examined in terms of steric interactions between the axial ligand and the porphyrin core.