Iron Complexes of 5,10,15,20-Tetraphenyl-21-oxaporphyrin

The iron complexes of 5,10,15,20-tetraphenyl-21-oxaporphyrin (OTPP)H have been investigated. Insertion of iron(II) followed by one-electron oxidation yielded a high-spin, six-coordinate (OTPP)FeIIICl2 complex. The reduction of
(OTPP)FeIIICl2 has been accomplished by means of moderate reducing reagents producing high-spin five-coordinate
(OTPP)FeIICl. The molecular structure of (OTPP)FeIIICl2 has been determined by X-ray diffraction. The iron(III)
21-oxaporphyrin skeleton is essentially planar. The furan ring coordinates in the η1 fashion through the oxygen
atom, which acquires trigonal geometry. The iron(III) apically coordinates two chloride ligands. Addition of potassium
cyanide to a solution of (OTPP)FeIIICl2 in methanol-d4 results in its conversion to a six-coordinate, low-spin complex
[OTPP)FeIII(CN)2] which is spontaneously reduced to [OTPP)FeII(CN)2]- by excess cyanide. The spectroscopic
features of [OTPP)FeIII(CN)2] correspond to the common low-spin iron(III) porphyrin (dxy)2(dxzdyz)3 electronic
configuration. Titration of (OTPP)FeIIICl2 or (OTPP)FeIICl with n-BuLi (toluene-d8, 205 K) resulted in the formation
of (OTPP)FeII(CH2CH2CH2CH3). (OTPP)FeII(n-Bu) decomposes via homolytic cleavage of the iron−carbon bond to
produce (OTPP)FeI. The EPR spectrum (toluene-d8, 77 K) is consistent with a (dxy)2(dxz)2(dyz)2(dz2)1(dx2-y2)0 ground
electronic state of iron(I) oxaporphyrin (g1 = 2.234, g2 = 2.032, g3 = 1.990). The 1H NMR spectra of (OTPP)FeIIICl2, (OTPP)FeIII(CN)2, {[(OTPP)FeIII)]2O}2+, and (OTPP)FeIICl have been analyzed. There are considerable
similarities in 1H NMR properties within each iron(n) oxaporphyrin−iron(n) regular porphyrin or N-methylporphyrin
pair (n = 2, 3). Contrary to this observation, the pattern of downfield positions of pyrrole resonances at 156.2,
126.5, 76.3 ppm and furan resonance at 161.4 ppm (273 K) detected for the two-electron reduction product of
(OTPP)FeIIICl2 is unprecedented in the group of iron(I) porphyrins.