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

Abstract

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‐C2B10H12 and closo‐1‐SB11H11, 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‐SB11H11 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 (ΔHf298) 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 ΔHf298 of a 13‐vertex nido‐thiaborane cluster has been computed to be more negative than that of the well‐known nido‐SB10H11− cluster (−6.7 and −5.6 kcal mol−1 per vertex, respectively).

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

“…Figure S1 illustrates the 11 B NMR spectrum of 1 ( along with the 11 B– 11 B COSY measurement) with three signals. Conceivably, those showing the resonances of B­(2,3,4,5,6,8,9,10,11) overlap and are deshielded with respect to those in closo- 1-SB 11 H 11 and, in the case of B(2–6), also with respect to 7,8,9,10,11,12-(CH 3 ) 6 - closo -1-SB 11 H 5 . This observation seems to be common in boron-cluster chemistry, in which CH 3 behaves as an electron acceptor. − Indeed, such an electron-withdrawing behavior leads to electron-density dilution, which is reflected in the prolongation of some of the bonds.…”

“…In contrast, when reacting closo- SB 11 H 11 with CH 3 I under milder conditions, the thiaborane icosahedral barrier is retained, and 7,8,9,10,11,12-(CH 3 ) 6 -1- closo -SB 11 H 5 is obtained. The upper hemisphere of this thiaborane does not contain any Me group . The methylation of nido -B 10 H 14 and 1,2,3,4-(CH 3 ) 4 - nido -B 10 H 10 has prompted us to methylate both 7,8,9,10,11,12-(CH 3 ) 6 - closo -1-SB 11 H 5 and closo -1-SB 11 H 11 at higher temperatures than those reported in ref .…”

“…The upper hemisphere of this thiaborane does not contain any Me group. 11 The methylation of nido-B 10 H 14 and 1,2,3,4-(CH 3 ) 4 -nido-B 10 H 10 has prompted us to methylate both 7,8,9,10,11,12-(CH 3 ) 6 -closo-1-SB 11 H 5 and closo-1-SB 11 H 11 at higher temperatures than those reported in ref 11. Indeed, 7,12-I 2 -2,3,4,5,6,8,9,10,11-(CH 3 ) 9 -closo-1-SB 11 (1, Figure 1) and 7,8,12-I 3 -2,3,4,5,6,9,10,11-(CH 3 ) 8 -closo-1-SB 11 were obtained.…”

“…In addition, trimethylaluminum is known as a very robust methylation agent; for instance, it methylates tertiary alcohols directly to 13 and, in the case of B(2−6), also with respect to 7,8,9,10,11,12-(CH 3 ) 6 -closo-1-SB 11 H 5 . 11 This observation seems to be common in boron-cluster chemistry, in which CH 3 behaves as an electron acceptor. 3−7 Indeed, such an electron-withdrawing behavior leads to electron-density dilution, which is reflected in the prolongation of some of the bonds.…”

The Influence of Halogenated Hypercarbon on Crystal Packing in the Series of 1-Ph-2-X-1,2-dicarba-closo-dodecaboranes (X = F, Cl, Br, I)

“…The thermodynamic stability has decreased with the increasing atomic number of the halogen atom and with the reduced size of the carborane cage. Note that the positive values of the heat of formation do not necessarily mean experimental unavailability, as exemplified by e.g., closo-SB9H9 with the computed ΔHf 298 value of 11.3 kcal mol −1 [14,15]. Moreover, we have computed the ΔHf 298 of 26.5 kcal mol −1 for closo-1,2-C2B8H10, which was previously prepared as well [16].…”