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

Abstract

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−...