Laser photolysis studies of the photochemical reactions of chromium(III) octaethylporphyrin and tetramesitylporphyrin complexes in toluene solution

A nanosecond laser photolysis study was carried out for the Cr(III) porphyrin complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin, Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin, Cr(TMP)(Cl)(L)], in toluene containing water and an excess amount of L (L = axial ligand). The laser photolysis generates the triplet excited state of the parent complex as well as a five-coordinate complex, Cr(porphyrin)(Cl)], produced by the photodissociation of the axial ligand L. The yields for the formation of the triplet state and the photodissociation of L are found to markedly depend on the nature of both L and porphyrin ligand. The five-coordinate Cr(porphyrin)(Cl)] readily reacts with both H(2)O and L in the bulk solution to give Cr(porphyrin)(Cl)(H(2)O)] and Cr(porphyrin)(Cl)(L)], respectively. The axial H(2)O ligand in Cr(porphyrin)(Cl)(H(2)O)] is then substituted by the ligand L to regenerate the original complex Cr(porphyrin)(Cl)(L)]. In principle, the substitution reaction takes place by the dissociative mechanism: the first step is the dissociation of H(2)O from Cr(porphyrin)(Cl)(H(2)O)], followed by the reaction of the five-coordinate Cr(porphyrin)(Cl)] with the ligand L to regenerate Cr(porphyrin)(Cl)(L)]. The rate constants for this ligand substitution reaction are found to exhibit bell-shaped ligand concentration dependence. The detailed kinetic analysis revealed that both ligands L and H(2)O in toluene make a hydrogen bond with the axial H(2)O ligand in Cr(porphyrin)(Cl)(H(2)O)] to yield dead-end complexes for the substitution reaction. The reaction mechanisms are discussed on the basis of the substituent effects of the porphyrin peripheral groups and the kinetic parameters determined from the temperature dependence of the rate constants.