Chemistry ofC-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-(SiMe3)-3-(R)-2,3-C2B4H5 (n = 2, L = THF, R = SiMe3; n = 1, L = TMEDA, R = SiMe3 or Me) or closo-exo-5,6-[(μ-H)2Li(L) n ]-1-Li(L) n -2,4-(SiMe3)2-2,4-C2B4H4 (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-C2B4 monosodium 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 full-exo-sandwich complex commo-exo-4,4‘,5,5‘-Mg(TMEDA)[2-(SiMe3)-3-(Me)-2,3-C2B4H5]2 (II). The full-endo-sandwich magnesacarboranes [Na(THF)2]2[commo-1,1‘-Mg{2,3-(SiMe3)2-2,3-C2B4H4}2] (IV) and [Na(TMEDA)]2[commo-1,1‘-Mg{2,3-(SiMe3)2-2,3-C2B4H4}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)2Li(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe3)2-2,4-C2B4H4 with MgBr2 in a 1:1 molar ratio produced closo-1-Mg(TMEDA)-2,4-(SiMe3)2-2,4-C2B4H4 (III), while a 2:1 molar ratio gave [Li(TMEDA)2]2[commo-1,1‘-Mg{2,4-(SiMe3)2-2,4-C2B4H4}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-C2B4 disodium compound with MeMgBr, followed by the addition 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 [Li(THF)2]2[commo-1,1‘-Mg{2,4-(SiMe3)2-2,4-C2B4H4}2] (VII) in nearly quantitative yields. The magnesacarboranes were characterized by their infrared spectra, chemical analysis, H1, 11B, and 13C NMR spectra, and, in the case of VII, by its 7Li 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-C2B4 monoanionic ligands through a pair of Mg−H−B bridges, while in IV, the two carborane dianions are η5-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 11B NMR spectra of I−VII were analyzed with the aid of ab initio GIAO molecular orbital calculations.

Boron: Metallacarbaboranes

“…This means that CpCoC 2 B 9 H 11 is a closo-metallacarbaborane with an icosahedral skeleton ( Figure 1 can be represented either as commo-metallacarbaboranes comprising two vertex-fused icosahedra (1) or as organometallic-type sandwich complexes (2) (see Section 1). Similarly, two approaches can be taken when considering the bonding in such compounds: they may be described in terms of the two separate polyhedra by using PSEPT or in terms of the 18 Bonding within multiple stacks such as (20) and (21) is best dealt with in terms of the organometallic model, extended to give electron counts for the stack as a whole. Triple-decker stacks (three ligands and two metal atoms) usually require − (23), each half-cluster has 27 electrons (for each half-cluster, 2 CH = 6 electrons, 9 BH = 18 electrons, Au − contributes three electrons per cluster after six electrons have been accommodated in nonbonding orbitals).…”

“…Syntheses of metallacarbaboranes from neutral carbaborane precursors, that is reaction type {M} + {CB}, are exemplified in equations (17) to (20). Reaction conditions often involve thermolysis or photolysis.…”

“…This type of fragment replacement is called subrogation. Finally, equation (20) illustrates the insertion of a metal fragment into a nido cluster; the CpNi unit is a source of three electrons and one H atom is lost from the precursor in order to retain an even electron count for the cluster.…”

Recent Advances in the Chemistry of Carboranes and Metallacarboranes

“…depending on the ratios of reactants involved [22][23][24][25][26][27][28][29][30][31]. The structures of the representive compounds are depicted in Figs.…”

“…The Ba-C distances of substituted barocenes vary between 290 and 300 pm [42]. This coordination of the carborane to the barium center is quite different than that observed for the carborane complexes of magnesium and calcium [22,[43][44][45]. In the magnesacarborane complex, the metal center bonds to the hydride substituents but not to the carbon atoms.…”