Photochemical Reactions of Nitrocobalt(III) Tetraphenylporphyrin and Its Pyridine Complex Studied by Laser Flash Photolysis

The photochemical reactions of nitrocobalt(III) tetraphenylporphyrin, (NO 2 )Co III TPP, and its six-coordinated complex with 4-(dimethylamino)pyridine, (NO 2 )Co III TPP(DMAPy), in benzene were studied by using a laser flash technique. The structures solved by X-ray analysis show that both compounds are the nitro form (O 2 N−Co III ) instead of the nitrito form (ONO−Co III ). The transient absorption spectrum observed 50 ns after laser excitation of (NO 2 )Co III TPP(DMAPy) in benzene revealed that the pyridinate complex undergoes photodissociation of the N−Co III bond to yield NO 2 and Co II TPP(DMAPy). These products recombine and regenerate the original complex with a second-order rate constant of 2.5 × 10 9 dm 3 mol -1 s -1 . In the case of (NO 2 )Co III TPP, the transient absorption spectrum detected 50 ns after the laser flash changes to another spectrum which has a longer lifetime and decays according to first-order kinetics with the rate constant of 3.0 × 10 2 s -1 . The former spectrum corresponds to formation of Co II TPP from the photodissociation of the N−Co III bond. The change from the former to latter spectrum is found to follow second-order kinetics with the rate constant of 3.6 × 10 9 dm 3 mol -1 s -1 . On the basis of the results obtained, it was concluded that the metastable transient giving the latter spectrum is the nitrito species, (ONO)Co III TPP, which undergoes a nitrito → nitro linkage isomerization.