Pentamethylcyclopentadienyl Cobaltaboranes Derived from the B5H8(-) and B9H14(-) Ions: Studies in Synthesis and Structure

The reactions of of B5H8(-) and B9H14(-) ions (both generated from B5H9 and NaH in THF solution under different conditions) with CoCl2 and Li(+)(C5(CH3)5)(-) in THF, were examined. The two reaction systems generate entirely different cobaltaborane products, which were isolated as air-stable, colored crystalline solids and characterized by B11 and H1FT NMR spectroscopy at 115.5 and 360 MHz, respectively, and by unit- and high-resolution mass spectrometry, infrared spectra, and (in five cases) by X-ray diffraction studies. From the B9H14(-) reaction four products were characterized, all of which are 10-vertex CoB9 or Co2B8 nido cages analogous to B10H14; the major species, 6-(C5(CH3)5)CoB9H13, was obtained in 25% yield. Minor products were 6,9-(C5(CH3)5)2Co2B8H12'5,7-(C5(CH3)5)2CoB8H12, and the 6-chloro derivative of the latter compound. The B5H8(-) reaction generates a larger and structurally more diverse series of products, none in greater than 5% yield. The major products obtained after a 2-hr reaction period at room temperature are 2-(C5(CH3)5)CoB4H8, 1,2-(C5(CH3)5)2Co2B4H6, and 1,2,3-(C5(CH3)5)3Co3B4H4 all of which are analogous to cyclopentadienyl complexes obtained in the reaction of B5H8(-) with CoCl2 and C5H5C5H--5 reported earlier.