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 dichloromethane and toluene containing water. The five-coordinate [Cr(TPP)(Cl)] produced by the photoinduced dissociation of pyridine from [Cr(TPP)(Cl)(Py)] initially reacts with H2O to
give [Cr(TPP)(Cl)(H2O)], which eventually exchanges the axial H2O 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 exhibit a bell-shaped
pyridine-concentration dependence. Kinetic studies revealed that at a high Py concentration, the exogenous Py
probably makes a hydrogen bond with the axial H2O of [Cr(TPP)(Cl)(H2O)] to yield [Cr(TPP)(Cl)(HO−H···Py)]
as a dead-end complex. A similar structure of the Cr−TPP complex having 2-methylpyridine molecules bound
to the coordinated H2O ligand by a hydrogen bond was determined by X-ray structure analysis. The exchange
reaction 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(TPP)(Cl)(HO−H···Py)], and the second step is the dissociation of
H2O. The five-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 in [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 reaction 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).