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) comp lex of 5,10,15,20-tetraphenylporphyrin, [Cr(tpp)(Cl)(L)], where L repr esents an imidazole ligand, is presented. The present study is aimed a t elucidating the effect of a steric strain induced by the 2-methyl su bstituent of the imidazole ligand on the molecular structure and the d ynamics of an axial ligand-substitution reaction. Crystals of the 1-me thylimidazole complex [Cr(tpp)(Cl)(1-Meim)] (1) and 1,2-dimethylimidaz ole complex [Cr(tpp)(Cl)(1,2-Me(2)im)] (2) were obtained from a dichlo roethane-toluene mixture containing a slight excess of imidazole ligan d 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 = 9.9 26(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 demo nstrated 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 pyridi ne ligand of [Cr(tpp)(Cl)(py)] by various imidazole ligands was studie d spectrophotometrically in dichloromethane or in toluene. A steric ef fect due to the 2-methyl substituent of the imidazole ligand was obser ved along with an increase in the ligand-dissociation rate and a decre ase in the binding constant of the imidazole ligand to the metalloporp hyrin. These results were examined in terms of steric interactions bet ween the axial ligand and the porphyrin core.