Photochemistry of Nitrosyl Porphyrins in the Temperature Range 180-300 K and the Effects of Pyridine on Photodenitrosylation of Nitrosyliron Tetraphenylporphyrin

Article abstract of DOI:10.1021/j100351a032

Nitrosylmetal tetraphenylporphyrin (NOMTPP, M=Fe,Co, and Mn) in benzene solutions undergoes facile photodenitrosylation to give NO and metal(II) tetraphenylporphyrin, M^IITPP.The photodenitrosylation yields of NOFeTPP and NOCoTPP are respectively determined as 0.50 +/- 0.05 and 1.0 +/- 0.05 at both excitation wavelengths 355 and 532 nm.The yield, Φ, in toluene is expressed as a function of temperature: Φ = (1 + 1.56 x 10^-2 exp(2400/RT))^-1 for NOFeTPP and Φ = (1 + 1.02 x 10^-6 exp(5200/RT))^-1 for NOCoTPP.Contrary to NOFeTPP and NOCoTPP, the photodenitrosylation yields, Φ, of NOMnTPP are independent of temperature in the range 300-180 K and exhibit excitation wavelength dependence: Φ = 0.78 +/- 0.05 at 355 nm and Φ + 0.62 +/- 0.05 at 532 nm.The effects of the central metal on photodenitrosylation are described.The laser photolysis of NOFeTPP in the presence of low concentration of pyridine revealed that the Fe^IITPP initially produced reacts with a pyridine molecule, Py, to form five-coordinate (Py)Fe^IITPP.The five-coordinate (Py)Fe%II%TPP further reacts with Py to give six-coordinate (Py)2Fe^IITPP, which recombines with NO to regenerate NOFeTPP.The kinetic study has demonstrated that the five-coordinate (Py)Fe^IITPP which is in equilibrium with (Py)2Fe^IITPP is responsible for the recombination reaction of (Py)2Fe^IITPP and NO.