ORGANOTRANSITION-METAL METALLACARBORANES .19. INDENYLIRON(II) AND INDENYLIRON(III) COMPLEXES AND RELATED SPECIES - ETA-6-]ETA-5 HAPTOTROPIC REARRANGEMENT, ELECTROCHEMISTRY, AND POLYHEDRAL EXPANSION OF (ARENE)FE(ET2C2B4H4) CLUSTERS

Thermal displacement of the cyclooctatriene ligand in (eta-6-C8H10)Fe(II)(Et2C2B4H4) by indene at 180-degrees-C afforded orange air-stable (eta-6-C9H8)Fe(II)(Et2C2B4H4) (1) in high yield together with a minor product (2), which is evidently a dimer of 1. The eta-6-coordination of indene to iron in 1 was confirmed by X-ray diffraction. Treatment of 1 with tetramethylethylenediamine (TMEDA) gave the yellow decapitated product (eta-6-C9H8)Fe(II)(Et2C2B3H5) (3). Deprotonation of 1 at -78-degrees-C gave a violet anionic intermediate; reaction of that species with dry HCl at -78-degrees-C regenerated 1. However, when the anion was warmed to 0-degrees-C or higher, proton NMR data indicated that a haptotropic rearrangement occurred to produce the (eta-5-C9H7)Fe(Et2C2B4H4)- anion 4-. Treatment of the latter species with HCl.Et2O gave the Fe-protonated complex (eta-5-C9H7)FeH(Et2C2B4H4) (5), which on reaction with NaH regenerated 4-. Exposure of 4- or 5 to oxygen gave magenta, paramagnetic (eta-5-C9H7)Fe(III)(Et2C2B4H4) (4), whose ESR spectrum supports the assignment of an Fe(III) oxidation state; reduction of 4 with potassium metal afforded 4- quantitatively. The reaction of the 4- anion with LiCp*/NiBr2, and with the cobalt-diborole complex CpCo(Et2MeC3B2Et2)- and NiBr2, generated, respectively, the compounds (eta-5-C9H7)Fe-(Et2C2B4H4)NiCp* (6) and (eta-5-C9H7)Fe(Et2C2B4H4)Ni(Et2MeC3B2Et2)CoCp (7). An X-ray diffraction study of 6 established that these species incorporate eight-vertex FeNiC2B4 clusters, representing the first clear examples of cage expansion of a seven-vertex metallacarborane via metal insertion. In 7, the nickel atom also participates in a seven-vertex NiCoC3B2 cluster. In an analogous reaction, Cp*Fe(Et2C2B4H4)- (8-, a C5Me5 counterpart of 4-) and LiCp*/NiBr2 gave Cp*Fe(Et2C2B4H4)NiCp* (9), which was assigned an FeNiC2B4 cluster geometry corresponding to that of 6. A similar reaction of 8- with the cobalt-diborole complex CpCo(Et2HC3B2Me2)- produced Cp*Fe(Et2C2B4H4)Ni(Et2HC3B2Me2)CoCp (10), whose proposed structure is analogous to that of 7. The new compounds were isolated via column and/or plate chromatography on silica and characterized from their H-1, B-11, and (in some cases) C-13 NMR spectra; infrared and mass spectra; ESR spectra on 4; and crystallographic studies on 1 and 6. In addition, cyclic voltammetry was conducted on the compounds 1, 3, 4, 5, and 7. The electrochemical behavior of most species examined was complex, with oxidation and/or reduction leading in several cases to chemical reactions whose products generate further signals during cyclic voltammetry. Crystal data for 1: mol wt 301.4; space group P2(1)/c; Z = 4; a = 8.267 (3), b = 14.459 (6), c = 13.512 (5) angstrom; beta = 103.04 (3)-degrees; V = 1574 angstrom-3; R = 0.041 for 2081 reflections having F(o)2 > 2.0-sigma(F(o)2). Crystal data for 6: mol wt 494.3; space group P2(1)/a; Z = 4; a = 14.529 (8), b = 8.507 (5), c = 20.374 (12) angstrom; beta = 93.09 (4)-degrees; V = 2514 angstrom-3; R = 0.038 for 3754 reflections having F(o)2 > 2.0-sigma(F(o)2).