PHOTOINDUCED DECARBONYLATION FROM CARBONYLIRON TETRAPHENYLPORPHYRIN IN ETHANOL - EFFECTS OF AXIAL IMIDAZOLE STUDIED BY LASER FLASH-PHOTOLYSIS

Photochemical reactions of carbonyliron(II) tetraphenylporphyrins, (CO)(L)FeIITPP (L = ethanol and imidazole) and bisimidazoleiron(II) tetraphenylporphyrin, (Im)2FeIITPP, in ethanol were systematically investigated by laser flash photolysis. The yields for photodissociation of CO from (CO)(C2H5OH)FeIITPP and (CO)(Im)FeIITPP were determined to be 0.53 and 0.9 at room temperature, respectively. On the basis of the quantum yield measurements in the temperature range 180-300 K, the activation energies for decarbonylation obtained for (CO)(C2H5OH)FeIITPP and (CO)(Im)FeIITPP were 0 and 4.9 kcal mol-1, respectively. No dissociation of the ligand L from (CO)(L)FeIITPP was detected. The photochemical dissociation of the axial imidazole from (IM)2FeIITPP was also studied. The yield for photodissociation of imidazole obtained was 0. 1 8 at room temperature, and the activation energy was evaluated to be 2.6 kcal mol-1. It was found that (Im)(C2H5OH)FeIITPP produced by photolysis of both (CO)(Im)FeIITPP and (IM)2FeIITPP undergoes spontaneous base elimination to give (C2H5OH)2FeIITPP and imidazole, Im. The reactive excited states responsible for photodissociation of the axial ligands as well as the spontaneous base elimination reaction are discussed in detail.