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 followed a different course in which two distinct steps occurred. The first step formed the paramagnetic intermediates which, in a slower step, reacted with a second equivalent of the metal to give the diamagnetic [(SiMe3)4C4B8H8]2-. For the lighter metals, this dianion picked up a proton to give the products [(THF)4M][(SiMe3)4C4B8H9] (M = Li (VIII), Na (IX), K (X)) in 35−54% yield. In the case of Cs, no protonation occurred and the final product was a polymeric dicesiacarborane, [exo-Cs(TMEDA)-1-Cs-(SiMe3)4C4B8H8] n (VII), isolated in 41% yield. All were characterized by chemical analysis and infrared and 1H, 11B, and 13C NMR spectroscopy; VII and VIII were additionally characterized by single-crystal X-ray diffraction studies. In VIII−X the group 1 metal was solvated by four THF molecules and was not involved in the cage, while in VII one Cs occupied an apical position above a C3B3 open face of one carborane and bonded to a B3 face of a neighboring carborane. The second Cs, solvated by a TMEDA molecule, occupies an exo-polyhedral position and was not part of the polymeric chain. One “carbons adjacent” magnesacarborane, exo-(μ -H)3Mg(THF)3(SiMe3)2(Me)2C4B8H8 (XII), was also synthesized, in 81% yield, by the reaction of the metal with the (SiMe3)2(Me)2C4B8H8 precursor. Single-crystal X-ray diffraction studies showed the compound to be composed of an exo-polyhedral [Mg(THF)3]2+ that is loosely bound to a [(SiMe3)2(Me)2C4B8H8]2- cage. The carborane is best describe