PHOTOCHEMICAL REDUCTION OF NITRATE AND NITRITE BY MANGANESE AND IRON PORPHYRINS

New nitrate and nitrite complexes of metalloporphyrins have been synthesized and crystallographically characterized, and their photochemistry has been examined. Irradiation of Mn(TPP)(NO3) and Mn(TPP)(NO2) (where TPP = 5,10,15,20-tetraphenylporphyrinate(2-)) produces the high-valent metal-oxo species O = Mn(IV)(TPP) quantitatively, with quantum yields of 1.58 x 10(-4) and 5.30 x 10(-4), respectively. This metal-oxo species is capable of oxidizing substrates, as demonstrated in reactions with styrene or triphenylphosphine. Mn(TPP)(NO2) is formed as an intermediate in the complete photolysis of Mn(TPP)(NO3). Similarly, the photochemistry of Fe(TPP)(NO3) produces substrate oxidation, including C-H hydroxylation, which suggests the photochemical formation of O = Fe(IV)(TPP.+) as the active oxidant. Remarkably, all three oxygen atoms of the initially bound NO3- can be used for substrate oxidation. The X-ray crystal structures of Mn(TPP)(NO3).2C6H6' and Mn(TPP)(NO2).C6H6 have been solved. In the nitrate complex Mn(TPP)(NO3), the average Mn-pyrrole N distance is 2.007 angstrom, with the metal 0.21 angstrom above the mean plane of the nitrogen atoms. The nitrate ion is coordinated in a unidentate fashion with a Mn-O bond length of 2.101 angstrom. Mn(TPP)(NO2) is the first metalloporphyrin complex with oxygen-bound nitrite. The average Mn-pyrrole nitrogen distance is 2.012 angstrom, with the metal 0.23 angstrom above the mean plane of the nitrogen atoms. The nitrite ion is coordinated through one of the oxygens, with a Mn-O bond length of 2.059 angstrom. Crystal data for Mn(TPP)(NO3).2C6H6 at -76-degrees-C: space group P1BAR, a = 13.271 (4) angstrom, b = 13.610 (5) angstrom, c = 12.880 (3) angstrom, alpha = 111.44 (2)-degrees, beta = 95.71 (2)-degrees, gamma = 85.50 (3)-degrees, V = 2152 (2) angstrom-3, Z = 2, R(F) = 0.063, R(wF) = 0.086 for 408 variables and 4890 unique data with I > 2.58-sigma(I). Crystal data for Mn(TPP)(NO2).C6H6: space group Pccn, a = 21.631 (11) angstrom, b = 19.941 (12) angstrom, c = 17.991 (6) angstrom, V = 7760 (7) angstrom-3, Z = 8, R(F) = 0.062, R(wF) = 0.096 for 543 variables and 3820 unique data with I > 2.58-sigma-(I).