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, a

摘要

The“carbons apart”tetracarbon carborane nido-2,6-(R)_2-4,12-(SiMe_3)_2-2,4,6,12-C_4B_8H_8(R = SiMe_3(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(SiMe_3)_4(B-Me)C_4B_7H_7 (IV) and the other in (L)_2Mg(SiMe_3)_2(R)_2(B-Y)C_4B_7H_7 (L = THF, R = SiMe_3, Y = t-Bu (V); L = THF, R = SiMe_3, 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 dnesity functional ab initio molecular orbital calculations showed that the dianionic C_4B_8 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 [(SiMe_3)_4C_4B_8H_8]~2-. For the lighter metals, this dianion picked up a proton to give the products [(THF)_4M][SiMe_3)_4C_4B_8H_9](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-(SiMe_3)_4C_4B_8H_8]_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 hcaracterized 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 C_3B_3 open face of one carborane and bonded to a B_3 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-(#mu#-H)_3Mg(THF)_3(SiMe_3)_2(Me)_2C_4B_8H_8 (XII), was also synthesized, in 81% yield, by the reaction of the metal with the (SiMe_3)_2(Me)_2C_4B_8H_8 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 [(SiMe_3)_2(Me)_2C_4B_8H_8]~2- cage. The carbornae is best described as an 10-vertex arachno-(SiMe_3)_2C_2B_8H_8 cage that subtends an electron-precise MeC=CMe fragment.