Chemistry of C-Trimethylsilyl-Substituted Heterocarboranes. 28. Selective Alkylation and Reactivity of “Carbons Adjacent” and “Carbons Apart” Tetracarba-nido-dodecaborane(12) Derivatives toward Group 1 and Group 2 Metals. Synthetic, Spectroscopic, and Structural Investigations on Lithium-, Sodium-, Potassium-, Cesium-, and Magnesium-Complexed C4B8 Carboranes

The “carbons apart” tetracarbon carborane nido-2,6-(R)2-4,12-(SiMe3)2-2,4,6,12-C4B8H8 (R = SiMe3 (I), n-butyl (II)) and several of its B-alkylated derivatives react with Mg metal in THF solvent to produce magnesacarboranes (IV−VI and XI) in yields ranging from 57% to 74%. The magnesacarboranes were characterized by chemical analysis and infrared and 1H, 11B, and 13C NMR spectroscopy and by single-crystal X-ray diffraction. Two types of cages were found, one in (THF)2Mg(SiMe3)4(B-Me)C4B7H7 (IV) and the other in (L)2Mg(SiMe3)2(R)2(B-Y)C4B7H7 (L = THF, R = SiMe3, Y = t-Bu (V); L = THF, R = SiMe3, Y = H (VI); (L)2 = TMEDA, R = n-Bu, Y = H (XI)). Both cages showed the presence of electron-precise C and B atoms, as well as electron-deficient fragments. Approximate density functional ab initio molecular orbital calculations showed that the dianionic C4B8 cage can exist in a number of energy-equivalent isomeric forms that can be trapped by a metal ion such as Mg. The reactions of I with the group 1 metals followe...