CHEMISTRY OF C-TRIMETHYLSILYL-SUBSTITUTED HETEROCARBORANES - 23 - SYNTHETIC, SPECTROSCOPIC, AND STRUCTURAL INVESTIGATION ON HALF-SANDWICH AND FULL-SANDWICH MAGNESACARBORANES OF 2,3-C2B4 AND 2,4-C2B4 CARBORANE LIGANDS

The reaction of nido-1-Na(L)(n)-2-(SiMe3)-3-(R)-2,3-C2B4H5 (n = 2, L = THF, R = SiMe3; n = 1, L = TMEDA, R = SiMe3 or Me) or )(2)Li(L)(n)]-1 -Li(L)(n)-2,4-(SiMe3)(2)-2,4-C2B4H4 (n = 2, L = THF; n 1, L = TMEDA) w ith various magnesium reagents produced a number of different magnesac arboranes. The product of a 1:1 molar ratio reaction of the 2,3-C2B4 m onosodium compound (R = SiMe3 and L = TMEDA) with MeMgBr was the half- sandwich magnesacarborane closo-1-Mg(TMEDA)-2,3-(SiMe3)(2)-2,3-C2B4H4 (I), while when R = Me, the same reaction conditions gave the novel fu ll-exo-sandwich complex ,4',5,5'-Mg(TMEDA)[2-(SiMe3)-3-(Me)-2,3-C2B4H5 ](2) (II). The full-endo-sandwich magnesacarboranes )](2)[commo-1,1'-M g{2,3-(SiMe3)(2)-2,3-C2B4H4}(2)] (IV) and )](2)[commo-1,1'-Mg{2,3-(SiM e3)(2)-2,3-C2B4H4}(2)] (V) were the exclusive products when the approp riate monosodium compound reacted with Mg(Bu)(2) in 2:1 molar ratios. Reaction of 2)Li(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe3)(2)-2,4-C2B4H4 with Mg Br2 in a 1:1 molar ratio produced close-1-Mg(TMEDA)-2,4-(SiMe3)(2)-2,4 -C2B4H4 (III), while a 2:1 molar ratio gave commo-1,1'-Mg{2,4-(SiMe3)( 2)-2,4-(C)2B(4)H(4)}(2)] (VI). The yields ranged from 73% for III to 9 4% for II. On the other hand, the 1:1 molar ratio reaction of the THF- solvated 2,4-C2B4 disodium compound with MeMgBr, followed by the addit ion of 1 equiv of the THF-solvated monosodium compound of the 2,3-C2B4 carborane did not give the expected mixed-ligand complex but produced a 50:50 mixture of IV and )](2)[commo-1,1'-Mg{2,4-(SiMe3)(2)-2,4-C2B4 H4}(2)] (VII) in nearly quantitative yields. The magnesacarboranes wer e characterized by their infrared spectra, chemical analysis, H-1, B-1 1, and C-13 NMR spectra, and, in the case of VII, by its Li-7 NMR spec trum. Compounds I, II, and IV were further characterized by single-cry stal X-ray analysis. Compound I crystallizes as a dimer in which a Mg occupies the apical position above the pentagonal face of one carboran e and is bonded to the unique boron of the other carborane in the dime r through a Mg-H-B bridge. The structure of II is one in which a TMEDA -solvated Mg is exo-polyhedrally bonded to two 2,3-C2B4 monoanionic li gands through a pair of Mg-H-B bridges, while in IV, the two carborane dianions are eta(5)-bonded to a Mg in a more conventional endo-sandwi ch complex. The reactions with MeMgBr are thought to proceed through t he formation of a methylmagnesacarborane intermediate which undergoes further reaction to give the final products. The B-11 NMR spectra of I -VII were analyzed with the aid of ab initio GIAO molecular orbital ca lculations.