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)Fe^IIICl₂ complex. The reduction of (OTPP)Fe^IIICl₂ has been accomplished by means of moderate reducing reagents producing high-spin five-coordinate (OTPP)Fe^IICl. The molecular structure of (OTPP)Fe^IIICl₂ 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)Fe^IIICl₂ in methanol-d₄ results in its conversion to a six-coordinate, low-spin complex [OTPP)Fe^III(CN)₂] which is spontaneously reduced to [OTPP)Fe^II(CN)₂]^- by excess cyanide. The spectroscopic features of [OTPP)Fe^III(CN)₂] correspond to the common low-spin iron(III) porphyrin (d_xy)²(d_xzd_yz)³ electronic configuration. Titration of (OTPP)Fe^IIICl₂ or (OTPP)Fe^IICl with n-BuLi (toluene-d₈, 205 K) resulted in the formation of (OTPP)Fe^II(CH₂CH₂CH₂CH₃). (OTPP)Fe^II(n-Bu) decomposes via homolytic cleavage of the iron−carbon bond to produce (OTPP)Fe^I. The EPR spectrum (toluene-d₈, 77 K) is consistent with a (d_xy)²(d_xz)²(d_yz)²(d_z²)¹(d_x²-y²)⁰ ground electronic state of iron(I) oxaporphyrin (g₁ = 2.234, g₂ = 2.032, g₃ = 1.990). The ¹H NMR spectra of (OTPP)Fe^IIICl₂, (OTPP)Fe^III(CN)₂, {[(OTPP)Fe^III)]₂O}²⁺, and (OTPP)Fe^IICl 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)Fe^IIICl₂ is unprecedented in the group of iron(I) porphyrins.