PHOTOCHEMICAL-REACTIONS OF NITROCOBALT(III) TETRAPHENYLPORPHYRIN AND ITS PYRIDINE COMPLEX STUDIED BY LASER FLASH-PHOTOLYSIS

The photochemical reactions of nitrocobalt(III) tetraphenylporphyrin, (NO2)(CoTPP)-T-III, and its six-coordinated complex with 4-(dimethylam ino)pyridine, (NO2)(CoTPP)-T-III(DMAPy), in benzene were studied by us ing a laser flash technique. The structures solved by X-ray analysis s how that both compounds are the nitro form (O2N-Co-III) instead of the nitrito form (ONO-Co-III). The transient absorption spectrum observed 50 ns after laser excitation of (NO2)(CoTPP)-T-III(DMAPy) in benzene revealed that the pyridinate complex undergoes photodissociation of th e N-Co-III bond to yield NO2 and (CoTPP)-T-III(DMAPy). These products recombine and regenerate the original complex with a second-order rate constant of 2.5 x 10(9) dm(3) mol(-1) s(-1). In the case of (NO2)(CoT PP)-T-III, the transient absorption spectrum detected 50 ns after the laser flash changes to another spectrum which has a longer lifetime an d decays according to first-order kinetics with the rate constant of 3 .0 x 10(2) s(-1). The former spectrum corresponds to formation of (CoT PP)-T-II from the photodissociation of the N-Co-III bond. The change f rom the former to latter spectrum is found to follow second-order kine tics with the rate constant of 3.6 x 10(9) dm(3) mol(-1) s(-1). On the basis of the results obtained, it was concluded that the metastable t ransient giving the latter spectrum is the nitrito species, (ONO)(CoTP P)-T-III, which undergoes a nitrito(-->)nitro linkage isomerization.