ORGANOTRANSITION-METAL METALLACARBORANES .25. REDOX CHEMISTRY AND ELECTRONIC STUDIES OF MONONUCLEAR AND DINUCLEAR IRON(II)/IRON(III) SANDWICH COMPLEXES

The mononuclear 7-vertex closo-ferracarborane clusters (C5Me5)Fe(II)H(Et2C2B4H4) (1) and (C5Me5)Fe(III)(Et2C2B4H4) (2) and the phenylene-linked diiron species (Et2C2B4H4)M(C5Me4-C6H4-C5Me4)M'(Et2C2B4H4) (3, M = M' = Fe(II)H; 4, M = Fe(II)H, M' = Fe(III); and 5, M = M' = Fe(III)) were prepared and isolated via column and/or plate chromatography on silica or Al2O3. Their chemical interconversion was examined, and the air-stable products were characterized from their H-1, C-13, and B-11 NMR, infrared, UV-visible, mass spectra, and elemental analysis and an X-ray diffraction study of 3. The crystallographic data established the location of the Fe-H hydrogen atom in each of the two equivalent ferracarborane clusters; this hydrogen is coordinated both to iron and to the neighboring boron atoms in the carborane ligand and can be described as capping an Fe-B-B triangular face. The paramagnetic species 2, 4, and 5 were investigated via ESR spectroscopy in toluene and THF between 4.5 and 298 K. Magnetic susceptibility measurements conducted on 4 and 5 showed one and two unpaired electrons, respectively, with strong spin-orbit couplings. The H-1 and C-13 NMR spectra of paramagnetic 2 were assigned by recording a series of spectra during quantitative stepwise reduction of the complex in THF-d8, via a potassium mirror in a sealed NMR tube, to form diamagnetic 2-. The latter anion was characterized from its multinuclear NMR spectra. Cyclic voltammetry was conducted on compounds 1-5. Although 2 exhibits reversible reduction, oxidation and/or reduction of 1, 3, or 4 leads to hydrogen abstraction and the formation of Fe(III) species, detectable from their reversible signals and ESR spectra. Crystal data for 3: M = 689.0; monoclinic, space group P2(1)/c; Z = 2; a = 8.698 (2), b = 14.949 (4), c = 14.931 (4) angstrom; beta = 106.54 (2)-degrees; V = 1861 angstrom3; R = 0.045 for 4135 reflections having I > 2-sigma(I).