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

Abstract

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 f nitro linkage isomerization.

Linkage Isomerization in Heme−NOx Compounds: Understanding NO, Nitrite, and Hyponitrite Interactions with Iron Porphyrins

“…31,35,36 Demonstration of this prevalent N -binding mode can be obtained from the results of laser flash photolysis experiments performed on the five-coordinate (TPP)Co(NO 2 ) (Figure 12A). 37 Photolysis of this compound in benzene resulted in the formation of (TPP)Co II (determined spectroscopically) via photodissociation of the NO 2 ligand. The photodissociated NO 2 ligand then recombined with (TPP)Co II to give initially the metastable (TPP)Co(ONO) compound; a nitrito-to-nitro isomerization occurs to regenerate the thermally stable nitro (TPP)Co(NO 2 ) compound.…”

Crystal structures of manganese- and cobalt-substituted myoglobin in complex with NO and nitrite reveal unusual ligand conformations

“…To the best of our knowledge, there is no exception to this observation for model cobalt porphyrin compounds. Indeed, it was demonstrated that during the recombination of NO 2 with (TPP)Co II (generated from flash photolysis of (TPP)Co(NO 2 )), an intermediate forms which decays to (TPP)Co(NO 2 ); the intermediate was assigned as the nitrito linkage isomer (TPP)Co(ONO) [124].…”

Laser Photolysis of Chromium(lll) Porphyrins in Toluene Solutions. Effects of Oxygen on the Photodissociation Yields of the Axial Ligands

“…If (Y is appropriate, the plot of DT(A) versus A should give the absorption spectrum of the 6TI state. Here we assume that the absorption band of the 6T, state in the Soret band region is not distorted (16,26). When a = 0.40, the plot of DT(A) versus A gave a smooth absorption band with a peak centered at 480 nm.…”