Photochemical Reduction of Nitrate and Nitrite by Manganese and Iron Porphyrins

Kenneth S. Suslick, Randall A. Watson

Research output: Contribution to journalArticlepeer-review

Abstract

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=MnIV(TPP) quantitatively, with quantum yields of 1.58 × 10-4 and 5.30 × 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=FeIV(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 Å, with the metal 0.21 Å 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 Å. Mn(TPP)(NO2) is the first metalloporphyrin complex with oxygen-bound nitrite. The average Mn-pyrrole nitrogen distance is 2.012 Å, with the metal 0.23 Å 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 Å. Crystal data for Mn(TPP)(NO3)-2C6H6 at -76 °C: space group P1, a = 13.271 (4) Å, b = 13.610 (5) Å, c = 12.880 (3) Å, α = 111.44 (2)°, β = 95.71 (2)°, γ = 85.50 (3)°, V= 2152 (2) Å3, Z = 2, RF = 0.063, RwF = 0.086 for 408 variables and 4890 unique data with I > 2.58σ(I). Crystal data for Mn(TPP)(NO2)-C6H6: space group Pccn, a = 21.631 (11) Å, 6 = 19.941 (12) Å, c = 17.991 (6) Å, V= 7760 (7) Å3, Z = 8, RF = 0.062, Rwf = 0.096 for 543 variables and 3820 unique data with I > 2.58σ(I).

Original languageEnglish (US)
Pages (from-to)912-919
Number of pages8
JournalInorganic Chemistry
Volume30
Issue number5
DOIs
StatePublished - Mar 1 1991

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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