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<sup>III</sup>Cl<sub>2</sub> complex. The reduction of (OTPP)Fe<sup>III</sup>Cl<sub>2</sub> has been accomplished by means of moderate reducing reagents producing high-spin five-coordinate (OTPP)Fe<sup>II</sup>Cl. The molecular structure of (OTPP)Fe<sup>III</sup>Cl<sub>2</sub> has been determined by X-ray diffraction. The iron(III) 21-oxaporphyrin skeleton is essentially planar. The furan ring coordinates in the η<sup>1</sup> 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<sup>III</sup>Cl<sub>2</sub> in methanol-<i>d</i><sub>4</sub> results in its conversion to a six-coordinate, low-spin complex [OTPP)Fe<sup>III</sup>(CN)<sub>2</sub>] which is spontaneously reduced to [OTPP)Fe<sup>II</sup>(CN)<sub>2</sub>]<sup>-</sup> by excess cyanide. The spectroscopic features of [OTPP)Fe<sup>III</sup>(CN)<sub>2</sub>] correspond to the common low-spin iron(III) porphyrin (d<i><sub>xy</sub></i>)<sup>2</sup>(d<i><sub>xz</sub></i>d<i><sub>yz</sub></i>)<sup>3</sup> electronic configuration. Titration of (OTPP)Fe<sup>III</sup>Cl<sub>2</sub> or (OTPP)Fe<sup>II</sup>Cl with <i>n-</i>BuLi (toluene-<i>d</i><sub>8</sub>, 205 K) resulted in the formation of (OTPP)Fe<sup>II</sup>(CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>3</sub>). (OTPP)Fe<sup>II</sup>(<i>n</i>-Bu) decomposes via homolytic cleavage of the iron−carbon bond to produce (OTPP)Fe<sup>I</sup>. The EPR spectrum (toluene-<i>d</i><sub>8</sub>, 77 K) is consistent with a (d<i><sub>xy</sub></i>)<sup>2</sup>(d<i><sub>xz</sub></i>)<sup>2</sup>(d<i><sub>yz</sub></i>)<sup>2</sup>(d<i><sub>z</sub></i><sup><sub>2</sub></sup>)<sup>1</sup>(d<i><sub>x</sub></i><sup><sub>2</sub></sup><sub>-</sub><i><sub>y</sub></i><sup><sub>2</sub></sup>)<sup>0</sup> ground electronic state of iron(I) oxaporphyrin (<i>g</i><sub>1</sub> = 2.234, <i>g</i><sub>2</sub> = 2.032, <i>g</i><sub>3</sub> = 1.990). The <sup>1</sup>H NMR spectra of (OTPP)Fe<sup>III</sup>Cl<sub>2</sub>, (OTPP)Fe<sup>III</sup>(CN)<sub>2</sub>, {[(OTPP)Fe<sup>III</sup>)]<sub>2</sub>O}<sup>2+</sup>, and (OTPP)Fe<sup>II</sup>Cl have been analyzed. There are considerable similarities in <sup>1</sup>H NMR properties within each iron(<i>n</i>) oxaporphyrin−iron(<i>n</i>) regular porphyrin or <i>N</i>-methylporphyrin pair (<i>n</i> = 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<sup>III</sup>Cl<sub>2</sub> is unprecedented in the group of iron(I) porphyrins.