Laser photolysis of chromium(III) porphyrins with axial pyridines in dichloromethane and toluene solutions. Novel effects of a hydrogen bond in the ligand exchange reaction

Laser photolysis studies were carried out for (chloro)(pyridine)(5,10,15,20 -tetraphenylporphyrinato)chromium-(III), [Cr(TPP)(Cl)(Py)], in both dichlor omethane and toluene containing water. The five-coordinate [Cr(TPP)-(CI)] p roduced by the photoinduced dissociation of pyridine from [Cr(TPP)(CI)(Py)] initially reacts with H2O to give [Cr(TPP)(Cl)(H2O)], which eventually exc hanges the axial H20 with Py to regenerate [Cr(TPP)(Cl)(Py)]. The rate for the ligand exchange of [Cr(TPP)(Cl)(H2O)] with exogenous Py is found to exh ibit a bell-shaped pyridine-concentration dependence. Kinetic studies revea led that at a high Py concentration, the exogenous Py probably makes a hydr ogen bond with the axi al H2O of [Cr(TPP)(Cl)(H2O)] to yield [Cr(TPP)(Cl)(H O-H...Py)] as a dead-end complex. A similar structure of the Cr-TPP complex having 2-methylpyridine molecules bound to the coordinated H20 ligand by a hydrogen bond was determined by X-ray structure analysis. The exchange rea ction of the axial HO-H...Py in [Cr(TPP)(Cl)(HO-H...Py)] by Py follows the dissociative mechanism: the first step is the dissociation of Py from [Cr(T PP)(Cl)(HO-H...Py)], and the second step is the dissociation of H2O. The fi ve-coordinate [Cr(TPP)(Cl)] thus produced reacts with Py to regenerate [Cr( TPP)(Cl)(Py)]. The direct ligand exchange reaction of the axial HO-H...Py i n [Cr(TPP)(Cl)(HO-H...Py)] with exogenous Py does not occur. Probably, the hydrogen bond, HO-H...Py, increases the basicity of H2O, and thus, the bond energy between Cr and O in [Cr(TPP)(Cl)(HO-H...Py)] becomes much stronger than that in [Cr(TPP)(Cl)(H2O)]. The mechanism of the ligand substitution r eaction of the chromium(III) porphyrins has been examined in detail on the basis of the laser photolysis studies of [Cr(TPP)(Cl)(L)] (L = pyridine, 3- cyanopyridine, and H2O).