Quantitative syntheses of permethylated closo-1,10-R2C2B8Me8 (R = H, Me) carboranes. Egg-shaped hydrocarbons on the Frontier between inorganic and organic chemistry

Thiaborane Icosahedral Barrier Increased by the Functionalization of all Terminal Hydrogens in closo-1-SB11H11

The electrophilic substitution of icosahedral closo-1-SB₁₁H₁₁ with methyl iodide has resulted in two B-functionalized thiaboranes, 7,12-I₂-2,3,4,5,6,8,9,10,11-(CH₃)₉-1-closo-SB₁₁ and 7,8,12-I₃-2,3,4,5,6,9,10,11-(CH₃)₈-closo-1-SB₁₁, with the former being significantly predominant. These two icosahedral thiaboranes are the first cases of polysubstituted polyhedral boron clusters with another vertex that differs from B and C. Such polyfunctionalizations have increased the earlier observed thiaborane icosahedral barrier, not exhibiting any reactivity toward bases, unlike the parent thiaborane. The search for methylation pathways has revealed that the complete B₁₁-methylation is impossible, like in the case of decaborane(14), where this seems to be a result of the positively charged upper parts of these two molecules.

Swollen Polyhedral Volume of the anti-B18H22 Cluster via Extensive Methylation: anti-B18H8Cl2Me12

Methylation of anti-B₁₈H₂₂ (1) affords the first example of alkyl substitution of terminal hydrogen atoms on the fluorescent octadecaborane-22 molecule to give highly methylated 2,2'-Cl₂-1,1',3,3',4,4',7,7',8,8',10,10'-Me₁₂-anti-B₁₈H₈ (2). This extensive chemical substitution leads to a swelling in the polyhedral volume of the 18-vertex boron atomic skeleton of the molecule and an enhancement of the absorption and solubility characteristics of this highly fluorescent molecule. We propose this "swollen polyhedral volume" to be the result of a marked increase in the relative positivity of the "cluster-only total charge" of the boron atomic skeleton caused by the combined electron-withdrawing capacity of the 12 methyl groups. Enhancement in the absorption and solubility properties may be crucial in the design of new borane-based laser materials.

Thiaboranes on Both Sides of the Icosahedral Barrier: Retaining and Breaking the Barrier with Carbon Functionalities

The concept of icosahedral barrier has been expanded from the chemistry of carbaboranes to the area of thiaboranes. Both representatives of this barrier, i. e., closo-1,2-C₂ B₁₀ H₁₂ and closo-1-SB₁₁ H₁₁ , are similar in their electron distribution, which is dominated by positive charge in the midpoint of the C-C vector and on the sulfur atom with experimentally determined dipole moments of 4.50 D and 3.64 D, respectively. This is a driving force for their reactivity as exemplified by their reactions with different carbon functionalities. Icosahedral closo-1-SB₁₁ H₁₁ reacts both with an electron sextet containing carbon (in the form of N-heterocyclic carbenes), reported earlier, and with methyl iodide with an electron octet on the carbon. The latter reaction provides hexamethylated thiaborane on the basis of methylation so far unknown in this area of heteroborane chemistry. The computations of the heat of formation (ΔH_f ²⁹⁸ ) make it possible to estimate the height of the barrier as well as to propose closo-thiaboranes beyond the barrier. Eleven and twelve vertex thiaboranes with nido electron count are known experimentally for breaking the barrier. These computations also suggest that the larger nido-thiaboranes are promising candidates for the corresponding experimental availability, i. e., the ΔH_f ²⁹⁸ of a 13-vertex nido-thiaborane cluster has been computed to be more negative than that of the well-known nido-SB₁₀ H₁₁ ^- cluster (-6.7 and -5.6 kcal mol^-1 per vertex, respectively).

Synthesis of closo-1,2-H2C2B8Me8 and 1,2-H2C2B8Me7X (X = I and OTf) Dicarbaboranes and Their Rearrangement Reactions

Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-H₂C₂B₈Me₈ have been prepared in high yields either from nido-5,6-H₂C₂B₈H₁₀ or closo-1,2-H₂C₂B₈H₈ via electrophilic methylation reactions and cluster-rearrangement methods. Prepared were also monosubstituted derivatives of general formulation closo-H₂C₂B₈Me₇-X (X = I or OTf). The permethylated compounds exhibit extreme air stability in comparison to unprotected counterparts as a consequence of rigid, egg-shaped hydrocarbon structures incorporating inner C₂B₈ carborane scaffolding. The structures of all compounds isolated were confirmed unambiguously by multinuclear (¹¹B, ¹H, ¹³C, and ¹⁹F) NMR measurements, supported by X-ray diffraction analyses and geometry optimization methods on several compounds.