Chemistry of C-Trimethylsilyl-Substituted Heterocarboranes. 26. Further Investigation of Oxidative Cage Closure, Cage Fusion, and Cage Isomerizations: Synthetic, Structural, and Bonding Studies on “Carbons Adjacent” and “Carbons Apart” Tetracarba-nido-dodecaborane(12) Derivatives†

A number of tetracarbon carboranes of the general form (SiMe3)2(R)2C4B8H8 (R = SiMe3 (III and IV) Me (XIII and XIV), n-Bu (VII, XI, and XII), t-Bu (VIII)) were synthesized by the reactions of the corresponding mono- or dianions of nido-2-(SiMe3)-3-(R)-2,3-C2B4H6 with NiCl2 in n-hexane. The nido-carboranes where R = n-Bu (I), t-Bu (II) are newly reported and were obtained from the reaction of Me3SiC⋮CR with B5H9. The tetracarbon compounds were minor products of the reactions, with yields ranging from 13% for VIII to 37% for XI; the major products were the corresponding closo-1-(SiMe3)-2-(R)-1,2-C2B4H4 compounds, which were obtained in yields from 69% (R = t-Bu (VI)) to 32% (R = SiMe3). The closo-carboranes where R = n-Bu (V), t-Bu (VI) could be converted to the more stable closo-1-(SiMe3)-6-(R)-1,6-C2B4H4 (R = n-Bu (IX), t-Bu(X)) by heating to 250−255 °C for 4−5 h; this agrees with previous studies on the compounds where R = SiMe3, Me. To aid in cage structure determination, a monobrominated, tetracarbon c...