Synthesis and Structural Characterization of Pentaarylboroles and Their Dianions

Abstract

A series of l-aryl-2,3,4,5-tetraphenylboroles [Ph 4 C 4 BAr] [Ar ) p-MeC 6 H 4 (2), p-Me 3 SiC 6 H 4 (3), p-FC 6 H 4 (4)] were synthesized. The X-ray crystallography of 2-4 revealed that they have short distances between the borole boron atom and the phenyl rings of the neighboring molecules, suggesting the existence of certain intermolecular interaction. Despite this interaction, the borole rings in 2-4 remain planar and contain substantial bond alternation in the butadiene skeleton, which is consistent with the theoretically optimized structures of the singlet boroles. These results demonstrated that the boroles still have antiaromatic character in the crystalline state. Compound 3 and pentaphenylborole [Ph 4 C 4 BPh] (5) were further reacted with potassium or potassium graphite to form potassium borole dianion salts [K 2 {Ph 4 C 4 B(p-Me 3 SiC 6 H 4 )}] ( 6) and [K 2 (Ph 4 C 4 BPh)] (7), respectively. In their X-ray crystal structures, compounds 6 and 7 are polymeric and have two K + ions lying on both sides of the borole plane with η 5 -coordination.

A theoretical study of the aromaticity in neutral and anionic borole compounds

“…In addition, we have investigated corresponding NICS values for the negatively charged derivatives. As previously reported by the groups of Herberich, [52] Yamaguchi [18] and Braunschweig, [15,17,[53][54][55] the synthesis and isolation of various dianionic and a monoanionic species has been successfully achieved ( Figure 4). The addition of one electron noticeably decreases the antiaromatic character of boroles, as judged by both the NICS and multicenter indices.…”

“…[9] Nevertheless, isolable boroles are typically highly reactive species and follow various pathways to reduce their antiaromaticity, namely by activation of H−H and Si−H bonds, [10][11][12] [4+2] cycloaddition reactions, [13,14] adduct formation with Lewis bases [15,16] or one-and two-electron reductions to form the corresponding radical anions and dianions, respectively. [17,18] Besides their rich reactivity profile, boroles display chromophoric properties, i.e. they absorb light strongly in the UV-vis region of the spectrum, and are highly Lewis acidic due to the presence of a vacant p-orbital on boron.…”

Perfluoropentaphenylborole

“…Starting from an input file derived from the crystallographic pdb file, the optimized structure features the localized bonding expected for the 4p antiaromatic borole heterocycle. [3,14] Thus, while the general structural features are maintained, the B1ÀC1 and C2ÀC2* bonds elongate to 1.585 and 1.525 , respectively, and the C1ÀC2 distance contracts to 1.356 in the computed structure. These parameters are in line with those found in Yamaguchi and co-workers less symmetrical pentaaryl boroles.…”

“…These parameters are in line with those found in Yamaguchi and co-workers less symmetrical pentaaryl boroles. [14] The triplet structure of 4 was also optimized and found to be higher in energy than the singlet by 16.9 kcal mol À1 , somewhat larger than the singlettriplet gaps of 15.4-15.9 kcal mol À1 found for the nonfluorinated compounds.…”

“…[12] Although pentaphenylborole (II) was first synthesized in 1986, it was only recently that its structure was determined; [13] several related complexes were also recently reported and structurally characterized. [14] The high Lewis acidity of these rather sterically encumbered heterocycles was manifested in significant interactions observed in the solid state between the boron center and the p system of a phenyl group on a neighboring molecule. In all examples save II itself, localized bonding within the borole ring was observed.…”