Structural and spectroscopic studies of selectively B-alkylated derivatives in the dicarbahexaborane (8) cage system

The sodium salt of nido-2,3-(SiMe3)-2,3-C2B4H6 (I) reacts with a number of alkylating agents to give, exclusively, nido-5-(R)-2,3-(SiMe3)2-2,3-C2B4H5 (R=CH3 (II), i-C4H9 (III), CH2CH2Cl, and CH2CH2Br). Further alkylation of III could be accomplished only after removing one of the SiMe3 groups with HCl(g) to produce nido-5-(i-C4H9)-2-(SiMe3)-2,3-C2B4H6 (IV), which could then be deprotonated with NaH and further reacted with the methylating agents MeI or CF3SO3Me to give the dialkylated product, nido-4-(CH3)-5-(i-C4H9)-2-(SiMe3)-2,3-C2B4H5 (V). Further alkylation of the sodium compound of V proved unsuccessful, either no reaction was found or inseparable mixtures were obtained. The overall yield of V was about 20%, based on the starting I. Compound III was found to undergo the typical reactions of deprotonation, to give 1-Na(THF)-5-(i-C4H9)-2,3-(SiMe3)2-2,3-C2B4H4 (VII) and oxidative cage closure, to produce closo-3-(i-C4H9)-1,2-(SiMe3)2-1,2-C2B4H3 (VI). The compounds were characterized by infrared spectroscopy, and by 1H, 11B and 13C NMR spectroscopy. Compound III was further characterized by single crystal X-ray diffraction. Irrespective of the nature of the alkyl group and its location, the 11B NMR spectra showed the same general pattern, namely, the alkylated boronʹs resonance shifts downfield by approximately 10–15 ppm, while the other resonance positions show little change. Theoretical chemical shifts using GIOA-HF/6-311G** methods on B3LYP/6-31G* optimized geometries of I–V, as well as several model compounds, were used in interpreting the 11B NMR spectra.