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 we re characterized by chemical analysis and infrared and H-1, B-11, and C-13 NMR spectroscopy and by single-crystal X-ray diffraction. Two types of cage s were found, one in (THF)(2)Mg(SiMe3)(4)(B-Me)C4B7H7 (IV and the other in (L)(2)Mg(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 or bital calculations showed that the dianionic C4B8 cage can exist in a numbe r of energy-equivalent isomeric forms that can be trapped by a metal ion su ch as Mg. The reactions of I with the group 1 metals followed st different course in which two distinct steps occurred. The first step formed the para magnetic intermediates which, in a slower step, reacted with a second equiv alent of the metal to give the diamagnetic [(SiMe3)(4)C4B8H8](2-). For the lighter metals, this dianion picked up a proton to give the products [(THF) (4)M][(SiMe3)(4)C4B8H9] (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 polyme ric dicesiacarborane, [exco-Cs(TMEDA)-1-Cs-(SiMe3)(4)C4B8H8](n) (VIII), iso lated in 41% yield. All were characterized by chemical analysis and infrare d and H-1,B-11, and 13C NMR spectroscopy; VII and VIII were additionally ch aracterized by single-crystal X-ray diffraction studies. In VIII-X the grou p 1 metal was solvated by four THF molecules and was not involved in the ca ge, while in VII one Cs occupied an apical position above a C3B3 Open face of one carborane and bonded to a Bg face of a neighboring carborane. The se cond Cs, solvated by a TMEDA molecule, occupies an exe-polyhedral position and was not part of the polymeric chain. One "carbons adjacent" magnesacarb orane, exo-(mu-H)(3)Mg(THF)(3)(SiMe3)(2)(Me)(2)C4B8H8 (XII), was also synth esized, in 81% yield, by the reaction of the metal with the (SiMe3)(2)(Me)( 2)C4B8H8 precursor. Single-crystal X-ray diffraction studies showed the com pound to be composed of an exe-polyhedral [Mg(THF)(3)](2+) that is loosely bound to a [(SiMe3)(2)(Me)(2)C4B8H8](2-) cage. The carborane is best descri bed as an 10-vertex arachno-(SiMe3)(2)C2B8H8 cage that subtends an electron -precise MeC=CMe fragment.