Laser Photolysis Studies on Photodissociation of Axial Ligands from Isocyanide Complexes of Cobalt(III) and Rhodium(III) Porphyrins in Toluene Solutions. A Comparison with the Photochemistry of Carbonylrhodium(III) Porphyrin

Chlorocobalt(III) and iodorhodium(III) tetraphenylporphyrins, (Cl)CoIIITPP and (I)RhIIITPP, react with 2,6-dimethylphenyl isocyanide (R−NC) in toluene solutions to give the isocyanide complexes, (Cl)(R−NC)CoIIITPP and (I)(R−NC)RhIIITPP, respectively. The 355-nm laser photolysis studies have shown that these complexes photodissociate the axial ligand, R−NC. The quantum yield, φ, for the photodissociation of R−NC from (Cl)(R−NC)CoIIITPP is determined as 0.091 in both degassed and oxygen-saturated toluene. On the other hand, (I)(R−NC)RhIIITPP gives φ = 0.19 in degassed toluene and φ = 0.07 in oxygen-saturated toluene. The dissociation of R−NC from (I)(R−NC)RhIIITPP was confirmed to occur from both the excited singlet and triplet states by transient spectroscopic measurements. The small quantum yield obtained with the oxygen-saturated solution is explained by the quenching of the triplet state, 3(π,π*) of (I)(R−NC)RhIIITPP. The triplet state decays with a rate constant 9.5 × 104 s-1 in degassed toluene and is effectively quenched by oxygen with a bimolecular rate constant 6.7 × 108 M-1 s-1. In comparison with (I)(R−NC)RhIIITPP, the lifetime of the 3(π,π*) state of (Cl)(R−NC)CoIIITPP is too short to be detected by the present laser photolysis system, probably because of the very fast radiationless process, 3(π,π*) → 3(π,d z 2) → 3(dπ,d z 2). The dissociation of R−NC from (Cl)(R−NC)CoIIITPP is assumed to occur from the 3(dπ,d z 2) state. The long lifetime of the 3(π,π*) state of (I)(R−NC)RhIIITPP is interpreted on the assumption that the 3(dπ,d z 2) is located higher in energy than the 3(π,π*) state. The dissociation of R−NC from (I)(R−NC)RhIIITPP at the triplet state is suggested to occur from the thermally populated 3(dπ,d z 2) state. The laser photolysis studies of carbonylrhodium(III) tetraphenylporphyrin are also described to elucidate the ligand dissociation mechanism of rhodium(III) porphyrins.