SYNTHESIS AND ELECTROCHEMISTRY OF IRON(III) CORROLES CONTAINING A NITROSYL AXIAL LIGAND - SPECTRAL CHARACTERIZATION OF [(OEC)FE-III(NO)](N) WHERE N=0, 1, 2, OR -1 AND OEC IS THE TRIANION OF 2,3,7,8,12,13,17,18-OCTAETHYLCORROLE

The synthesis, structural and spectral characterization of(OEC)Fe(NO) and [(OEC)Fe(NO)](+) where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole, is reported. To our knowledge, (OEC)Fe-III(NO) is the first example of a neutral air-stable iron(III) nitrosyl tetrapyrrole. It undergoes up to three oxidations and two reductions in nonaqueous media. The first two oxidations and both reductions are reversible and are located at E(1/2) = +0.61, +1.14, -0.41, and -1.92 V vs SCE in benzonitrile containing 0.1 M tetra-n-butylammonium perchlorate. The two oxidations involve the stepwise abstraction of a single electron from the corrole macrocycle and generate a stable iron(III) pi-cation radical and dication, both of which still contain a coordinated NO axial ligand. The two one-electron reductions of (OEC)Fe-III(NO) are also reversible and proceed without loss of the axial NO ligand. The first two oxidations and the first reduction of (OEC)Fe(NO) give products which are sufficiently stable to be spectroscopically characterized by UV-visible, IR, and/or ESR spectroscopy after in situ generation in PhCN or CH2Cl2. The ESR spectrum of the singly reduced product shows three g values at 2.00, 2.04, and 2.08 with nitric oxide N-14 hyperfine splitting in each region. This spectrum is quite similar to the one reported for five-coordinate (TPP)Fe-II(NO) in toluene glass at 120 K and also resembles the spectra of various hemoproteins containing iron(II) with bound NO. This result is consistent with formation of a five-coordinate iron(II) macrocycle containing a bent MNO unit which is formulated as [(OEC)Fe-II(NO)](-). The thin-layer FTIR spectral changes obtained during the first electroreduction are also consistent with formation of [(OEC)Fe-II(NO)](-). The initial iron(III) compound has a nu(NO) band at 1767 cm(-1) in CH2Cl2 which disappears upon reduction as anew band diagnostic of a bent Fe(II)-NO unit grows in at 1585 cm(-1). The ESR and UV-visible spectral data of singly oxidized (OEC)Fe(NO) are also self-consistent and give clear evidence for formation of an iron(III) nitrosyl corrole pi-cation radical rather than an iron(IV) nitrosyl corrole. The ESR spectrum of electrogenerated [(OEC)Fe-III(NO)](.+) in benzonitrile glass at 120 K has rhombic symmetry with resonances at g = 2.02, 2.00, 1.98 and is typical for a low-spin iron(III) species, i.e. S = 1/2. This is the first example for the ESR characterization of an iron(III) tetrapyrrole pi-cation radical. The nu(NO) band of [(OEC)Fe-III(NO)](.+) is located at 1815 cm(-1) which is 48 cm(-1) higher than the nu(NO) band of (OEC)Fe-III(NO). X-ray crystallographic data shows that the neutral and singly oxidized corroles both contain a linear Fe-NO unit. It also indicates that the two corrole planes in [(OEC)Fe(NO)](+) are close to each other and thus implies that the singly oxidized corrole is best formulated as an iron(III) pi-cation radical rather than an iron(IV) nitrosyl.