Chemistry of C-Trimethylsilyl-Substituted Heterocarboranes. 28. Selective Alkylation and Reactivity of \dqCarbons Adjacent\dq and \dqCarbons Apart\dq 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

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Organometallics, 19 (4): 497--508 (2000)
DOI: 10.1021/om990677z


The carbons apart tetracarbon carborane nido-2,6-(R)2-4,12-(SiMe3)2-2,4,6,12-C4B8H8 R = SiMe3 (I), Bu (II) and several of its B-alkylated derivs. react with Mg metal in THF solvent to produce magnesacarboranes e.g., (THF)2Mg(SiMe3)4(B-Bu-t)B4B7H7 (V) in yields ranging from 57\% to 74\%. The magnesacarboranes were characterized by chem. anal. and IR 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 = Bu, Y = H (XI). Both cages showed electron-precise C and B atoms, as well as electron-deficient fragments. Approx. d. functional ab initio MO calcns. showed that the dianionic C4B8 cage can exist in a no. of energy-equiv. isomeric forms that can be trapped by a metal ion such as Mg. The reactions of I with the Group IA metals followed a different course in which two distinct steps occurred. The 1st step formed the paramagnetic intermediates which, in a slower step, reacted with a 2nd equiv. of the metal to give the diamagnetic (SiMe3)4C4B8H82-. For the lighter metals, this dianion picked up a proton to give (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)4C4B8H8n (VII), isolated in 41\% yield. All were characterized by chem. anal. and IR and 1H, 11B, and 13C NMR spectroscopy; VII and VIII were addnl. characterized by single-crystal x-ray diffraction studies. In VIII-X the Group IA metal was solvated by four THF mols. 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 2nd Cs, solvated by a TMEDA mol., occupies an exo-polyhedral position and was not part of the polymeric chain. One carbons adjacent magnesacarborane, exo-(m-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 compd. to be composed of an exo-polyhedral Mg(THF)32+ that is loosely bound to a (SiMe3)2(Me)2C4B8H82- cage. The carborane is best described as an 10-vertex arachno-(SiMe3)2C2B8H8 cage that subtends an electron-precise MeC:CMe fragment. on SciFinder(R)



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