American Chemical Society
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Oxidation and Oxygenation of Iron Complexes of 2-Aza-21-carbaporphyrin

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posted on 2004-04-07, 00:00 authored by Krystyna Rachlewicz, Sian-Ling Wang, Jia-Ling Ko, Chen-Hsiung Hung, Lechosław Latos-Grażyński
Oxidation and oxygenation of (HCTPPH)FeIIBr an iron(II) complex of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH)H2 have been followed by 1H and 2H NMR spectroscopy. Addition of I2 or Br2 to the solution of (HCTPPH)FeIIBr in the absence of dioxygen results in one-electron oxidation yielding [(HCTPPH)FeIIIBr]+. 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)FeIIIBr. In the subsequent step an insertion of the oxygen atom into the preformed FeIII−C(21) bond has been detected to produce [(CTPPO)FeIIIBr]-. Protonation at the N(2) atom affords (HCTPPO)FeIIIBr. The considered mechanism of (HCTPPH)FeIIBr oxygenation involves the insertion of dioxygen into the Fe−C bond. The 1H NMR and 2H 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 1H NMR spectrum of intermediate-spin (HCTPP)FeIIIBr: β-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)FeIIIBr and (HCTPPO)FeIIIBr, 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) Å), and the pyrrolic trigonal carbon (Fe−C:  1.981(8) Å). The iron(III) of (HCTPPO)FeIIIBr forms bonds to three pyrrole nitrogen atoms (Fe−N distances 2.104(5), 2.046(5), 2.102(5) Å). The Fe−O 2.041(5) Å and Fe−C(21) 2.192(5) Å distances suggests a direct interaction between the iron center and the π electron density on the carbonyl group in a η2 fashion.