Oxidation and oxygenation of iron complexes of 2-aza-21-carbaporphyrin

Oxidation and oxygenation of (HCTPPH)(FeBr)-Br-II an iron(II) complex of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH)H-2 have been followed by H-1 and H-2 NMR spectroscopy. Addition of I-2 or Br-2 to the solution of (HCTPPH)(FeBr)-Br-II in the absence of dioxygen results in one-electron oxidation yielding [(HCTPPH)(FeBr)-Br-III](+). One electron oxidation with dioxygen, accompanied by deprotonation of a C(21)H fragment and formation of an Fe-C(21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (HCTPP)(FeBr)-Br-III. In the subsequent step an insertion of the oxygen atom into the preformed Fe-III-C(21) bond has been detected to produce [(CTPPO)(FeBr)-Br-III](-). Protonation at the N(2) atom affords (HCTPPO)(FeBr)-Br-III. The considered mechanism of (HCTPP)(FeBr)-Br-II oxygenation involves the insertion of dioxygen into the Fe-C bond. The H-1 NMR and H-2 NMR spectra of paramagnetic iron(III) complexes were examined. Functional group assignments have been made with use of selective deuteration. The characteristic patterns of pyrrole and 2-NH resonances have been found diagnostic of the ground electronic state of iron and the donor nature localized at C(21) center as exemplified by the H-1 NMR spectrum of intermediate-spin (HCTPP)(FeBr)-Br-III: beta-H 7.2, -10.6, -19.2, -20.6, -23.2, -24.9, -43.2; 2-NH -76.6 (ppm, 298 K). The structures of two compounds (HCTPP)(FeBr)-Br-III and (HCTPPO)(FeBr)-Br-III, were determined by X-ray diffraction studies, In the first case, the iron(III) is five-coordinate with bonds to three pyrrole nitrogen atoms (Fe-N distances: 1.985(8), 2.045(7), 2.023(8) Angstrom), and the pyrrolic trigonal carbon (Fe-C: 1.981(8) Angstrom). The iron(III) of (HCTPPO)(FeBr)-Br-III forms bonds to three pyrrole nitrogen atoms (Fe-N distances 2.104(5), 2.046(5), 2.102(5) Angstrom). The Fe-O 2.041(5) Angstrom and Fe-C(21) 2.192(5) Angstrom distances suggests a direct interaction between the iron center and the r electron density on the carbonyl group in a eta(2) fashion.