Synthesis, Characterization, and Properties of Three-Dimensional Analogues of 9-Borafluorenes

Abstract

Three-dimensional (3D) analogues of 9-borafluorenes, (C2B10H10)2BR (R = Cl (1), Br (2), H (3, merely as ether or silane adduct), phenyl (4), mesityl (5)), were synthesized and fully characterized. Gutmann–Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies confirmed the Lewis superacidity of 1–4, with the Lewis acidity of 1–3 being higher than that of the corresponding 3D analogues of 9,10-diboraanthracenes (DBA). The phenyl group and the C4B borole unit are nearly coplanar in the solid state, while the rotation of the mesityl group of 5 is hindered, forcing 5 to adopt a large C4B/Mes dihedral angle, which leads to distinct properties: air-stable and bluish-violet fluorescence arising from the intramolecular charge-transfer (ICT) transition. The 29Si NMR spectra indicated a high silylium cation character of 3·Et3SiH. The reaction of 4 with Ph3CBr yielded (Ph3C)+[(C2B10H10)2B­(Ph)­Br]− (6). The 4/PPh3 Lewis pair was capable of splitting dihydrogen and Si–H bond of triethylsilane.

The Chemistry of [1,1′‐bis(o‐Carboranyl)]Borane η2‐σ‐Silane Adduct

“…29 Si NMR showed a signal at δ=37.16 ppm. Overall, spectral data for 1‐H‐SiEt 3 were in good agreement with previous report by Ye [13] …”

“…Interestingly, there are structural differences in the X‐ray molecular structure of 1‐H‐SiEt 3 obtained by us compared to the previously reported structure by Ye, [13] which could be the result of a higher crystal quality obtained in our case (R factor =4.74 % in our case compared to R factor =8.41 % in the previous case with a slight disorder in B−H−Si fragment [13] ). For instance, Si1−H1 bond length in our case is longer than in a previously reported (1.635 vs. 1.620 Å, respectively) as well as B1−H1 bond length is shorter in our structure compared to previously reported (1.229 vs. 1.310 Å, respectively).…”

“…Inspired by this chemistry, we decided to prepare a 3D analogue of 9‐borafluorene, [12] [1,1′‐bis( o‐ carboranyl)]borane, ( 1 ) and see whether it forms a stable adduct with hydrosilanes. Noteworthy, during the work on this project, Ye and co‐workers reported a series of [1,1′‐bis( o‐ carboranyl)]boranes including 1‐H‐SiEt 3 adduct (Figure 1), which was synthesized by the reaction of Et 3 SiH with a 1 ‐Br in rather low yields (16 %) [13] . We report here an alternative synthesis of 1‐H‐SiEt 3 in 85 % yields.…”

Persistent and Predominantly Localized Boron Radical from the Reduction of a Three‐Dimensional Analogue of NHC‐Stabilized Borafluorenium

“…In view of the notably weakened B−Cl bonds in 2 , the Lewis superacidity of 1 and its ability to form weakly coordinating anions, [17e] we became curious whether the chloride of 2 could be abstracted by 1 to yield the first 3D 9‐borafluorenium cation. To this end, 0.5 equivalents of Me I i Pr was reacted with 1 .…”

“…The use of carborane units to replace the benzene units in conventional boron‐doped polycyclic π‐conjugated hydrocarbons for the construction of 2D/3D fused molecular systems has gained significant attention in recent years [17] . Owing to the strong electron‐withdrawing effect at the C‐positions of o ‐carborane, the 3D analogues of benzoborirene, [17a,b,i] 9,10‐diboraanthracene (DBA), 17c,d and 9‐borafluorene [17e,h] exhibit boosted Lewis acidity, which gives them applications in forming Lewis acid‐base adducts, 17e,i inert bond activation, [18] as well as serving as electron acceptors in luminescent molecules [17e,19] . In addition, this has sparked our interest in their reduction chemistry.…”