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 th e first example of a neutral air-stable iron(III) nitrosyl tetrapyrrol e. It undergoes up to three oxidations and two reductions in nonaqueou s media. The first two oxidations and both reductions are reversible a nd 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 t wo oxidations involve the stepwise abstraction of a single electron fr om the corrole macrocycle and generate a stable iron(III) pi-cation ra dical and dication, both of which still contain a coordinated NO axial ligand. The two one-electron reductions of (OEC)Fe-III(NO) are also r eversible and proceed without loss of the axial NO ligand. The first t wo oxidations and the first reduction of (OEC)Fe(NO) give products whi ch 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 spl itting in each region. This spectrum is quite similar to the one repor ted for five-coordinate (TPP)Fe-II(NO) in toluene glass at 120 K and a lso resembles the spectra of various hemoproteins containing iron(II) with bound NO. This result is consistent with formation of a five-coor dinate iron(II) macrocycle containing a bent MNO unit which is formula ted as [(OEC)Fe-II(NO)](-). The thin-layer FTIR spectral changes obtai ned during the first electroreduction are also consistent with formati on 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 a lso self-consistent and give clear evidence for formation of an iron(I II) nitrosyl corrole pi-cation radical rather than an iron(IV) nitrosy l 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 [(O EC)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 lin ear 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 o xidized corrole is best formulated as an iron(III) pi-cation radical r ather than an iron(IV) nitrosyl.