Laser Photolysis Studies of the Reaction of Chromium(III) Octaethylporphyrin Complex with Triphenylphosphine and Triphenylphosphine Oxide

The photoreaction of the chromium(III) octaethylpoprhyrin complex, [Cr(OEP)(Cl)(L)] (L = H2O, Py, OPPh3), in
dichloromethane was studied using laser flash photolysis technique. Laser irradiation causes the generation of a
coordinately unsaturated intermediate [Cr(OEP)(Cl)], which reacts with ligands in solution to give the parent complex,
[Cr(OEP)(Cl)(L)], or a transient species, [Cr(OEP)(Cl)(H2O)], when L = Py or OPPh3. Once produced [Cr(OEP)(Cl)(H2O)] eventually exchanges the axial H2O ligand with L to regenerate [Cr(OEP)(Cl)(L)]. The kinetics of the
axial ligand substitution reaction was followed spectrophotometrically, and the ligand-concentration dependence of
the ligand exchange rate revealed that the reaction occurs via a limited dissociative mechanism. The photoreaction
of [Cr(OEP)(Cl)(OPPh3)] containing excess PPh3 in the bulk solution leads to the unfavorable coordination of the
PPh3 molecule to the chromium ion to give a transient complex, [Cr(OEP)(Cl)(PPh3)]. The dynamic and thermodynamic
properties of [Cr(OEP)(Cl)(PPh3)] in solution are discussed on the basis of the kinetic parameters of the dissociation
and association reactions of the PPh3 ligand together with the steric aspect of the molecular structure of the
related complexes.