Effect of a Sterically Hindered Imidazole Ligand on the Molecular Structure and Axial Ligand Substitution Reaction of the Chromium(III) Porphyrin Complex

Article abstract of DOI:10.1246/bcsj.71.883

A structural, thermodynamic, and kinetic study of a chromium(III) complex of 5,10,15,20-tetraphenyrporphyrin, [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-Me₂im)] (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) A, β= 120.05(1) A, V = 4512(1) A₃, and in the orthorhombic space group P2_i2_i2_i, Z = 4, a = 9.926(1), b = 17.902(3), c = 25.247(4) , V = 4486(1) A³, respectively. The axial Cr-N_im bond lengths for 1 and 2 are 2.103(4) and 2.139(5) A, respectively. A steric hindrance has been demonstrated by a 0.036 A 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.