Chemistry of<i>C</i>-Trimethylsilyl-Substituted Heterocarboranes. 23. Synthetic, Spectroscopic, and Structural Investigation on Half- and Full-Sandwich Magnesacarboranes of 2,3- and 2,4-C₂B₄Carborane Ligands

Abstract

The reaction of nido -1-Na(L) n -2-(SiMe 3 )-3-(R)-2,3-C 2 B 4 H 5 ( n = 2, L = THF, R = SiMe 3; n = 1, L = TMEDA, R = SiMe 3 or Me) or closo-exo- 5,6-[(μ-H) 2 Li(L) n ]-1-Li(L) n -2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 ( n = 2, L = THF; n = 1, L = TMEDA) with various magnesium reagents produced a number of different magnesacarboranes. The product of a 1:1 molar ratio reaction of the 2,3-C 2 B 4 monosodium compound (R = SiMe 3 and L = TMEDA) with MeMgBr was the half-sandwich magnesacarborane closo -1-Mg(TMEDA)-2,3-(SiMe 3 ) 2 -2,3-C 2 B 4 H 4 ( I ), while when R = Me, the same reaction conditions gave the novel full-exo-sandwich complex commo-exo- 4,4‘,5,5‘-Mg(TMEDA)[2-(SiMe 3 )-3-(Me)-2,3-C 2 B 4 H 5 ] 2 ( II ). The full-endo-sandwich magnesacarboranes [Na(THF) 2 ] 2 [ commo -1,1‘-Mg{2,3-(SiMe 3 ) 2 -2,3-C 2 B 4 H 4 } 2 ] ( IV ) and [Na(TMEDA)] 2 [ commo- 1,1‘-Mg{2,3-(SiMe 3 ) 2 -2,3-C 2 B 4 H 4 } 2 ] ( V ) were the exclusive products when the appropriate monosodium compound reacted with Mg(Bu) 2 in 2:1 molar ratios. Reaction of closo-exo- 5,6-[(μ-H) 2 Li(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 with MgBr 2 in a 1:1 molar ratio produced closo -1-Mg(TMEDA)-2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 ( III ), while a 2:1 molar ratio gave [Li(TMEDA) 2 ] 2 [ commo- 1,1‘-Mg{2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 } 2 ] ( VI ). The yields ranged from 73% for III to 94% for II . On the other hand, the 1:1 molar ratio reaction of the THF-solvated 2,4-C 2 B 4 disodium compound with MeMgBr, followed by the addition of 1 equiv of the THF-solvated monosodium compound of the 2,3-C 2 B 4 carborane did not give the expected mixed-ligand complex but produced a 50:50 mixture of IV and [Li(THF) 2 ] 2 [ commo -1,1‘-Mg{2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 } 2 ] ( VII ) in nearly quantitative yields. The magnesacarboranes were characterized by their infrared spectra, chemical analysis, H 1, 11 B, and 13 C NMR spectra, and, in the case of VII, by its 7 Li NMR spectrum. Compounds I, II, and IV were further characterized by single-crystal X-ray analysis. Compound I crystallizes as a dimer in which a Mg occupies the apical position above the pentagonal face of one carborane and is bonded to the unique boron of the other carborane in the dimer 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-C 2 B 4 monoanionic ligands through a pair of Mg−H−B bridges, while in IV, the two carborane dianions are η 5 -bonded to a Mg in a more conventional endo-sandwich complex. The reactions with MeMgBr are thought to proceed through the formation of a methylmagnesacarborane intermediate which undergoes further reaction to give the final products. The 11 B NMR spectra of I − VII were analyzed with the aid of ab initio GIAO molecular orbital calculations.