Structural and Stereochemical Studies of Tris[2-(trifluoromethyl)phenyl]borane: Spontaneous Resolution, Stereodynamics, and Intramolecular C-F···B Interactions

Abstract

The structure and stereochemistry of the title organoboron compound, prepared from the corresponding organolithium compound and BCl3, were studied by spectroscopic methods and molecular orbital calculations. The X-ray crystal structure analysis shows that molecules take a propeller-shaped structure of nearly C3 symmetry, in which three CF3 groups lie at the same side of the boron sp2 plane. This compound crystallizes as conglomerates to give single crystals consisting of one or the other enantiomeric forms, both of which are CD active in the solid state. In solutions, molecules exist as a mixture of two conformational isomers, the C3 and C1 symmetric forms. Interconversion between the two isomers was observed by the variable temperature 19F NMR, the barriers to isomerization being 8 and 6 kcal mol-1 for two independent processes in the two ring flip mechanism. The presence of C-F···B interactions was supported by the short F···B contacts (ca. 2.80 Å) in the X-ray structure and by the chemical shift of 19F NMR in solutions.

Electronically Driven Regioselective Iridium‐Catalyzed C−H Borylation of Donor‐π‐Acceptor Chromophores Containing Triarylboron Acceptors

“…The polarized ground state also results in a quinoidal distortion of the phenylene spacer, as the c and f bonds in 1 and 1‐(Bpin) 2 (Figure , Table ) (1.373(3)–1.374(3) Å) are significantly shorter than the a, b, d, and e bonds (1.402(3)–1.414(3) Å). Similar to previously reported triarylboranes bearing ortho ‐trifluoromethyl groups, the molecular structures of 1 and 1‐(Bpin) 2 show B⋅⋅⋅F contacts (2.769(3)–2.876(3) Å), which are shorter than the sum of their van der Waals radii (3.39 Å), indicating a stabilizing interaction between the lone pairs of the CF 3 fluorine atoms and the empty p z ‐orbital of the boron center.…”

“…This can be avoided by using bulky substituents or by fixing the boron center in a rigid scaffold . Only recently the use of ortho ‐trifluoromethylated aryls in triarylboranes has been established as a strategy to improve acceptor strength as well as stability . The improved acceptor strength can be attributed to the electron‐withdrawing nature of the trifluoromethyl groups, while the increase in stability is due to steric shielding of the boron center and its empty p z ‐orbital, as well as a direct interaction of the fluorine lone pairs with the empty p z ‐orbital on boron.…”

Electrochemistry and Photophysics of Donor‐Substituted Triarylboranes: Symmetry Breaking in Ground and Excited State

“…Since triphenylborane and related TABs have been shown to adopt a propeller-shaped ground-state geometry with threefold symmetry, [20][21][22][23] we initially expected TAB 3 also to possess a similar ground-state structure with D 3 symmetry. On the other hand, it has been demonstrated recently that the ground-state symmetry of various triphenylmethane dyes, like parafuchsin or crystal violet, is lowered by an interaction of one amino group with the solvent or counterion.…”

“…Although the electrochemistry [14][15][16] and photophysics [17][18][19] of simple TABs, such as triphenylborane or trimesitylborane, have been the subject of various publications in the past decades, some aspects of the properties of their donor-substituted analogues still demand further investigation. Especially, the symmetry of the ground state, which has been studied intensively for triphenylborane and related model compounds [18,[20][21][22][23] and for triphenylmethane dyes, [24,25] is still a controversial topic. We synthesized the carbazole-substituted TAB 3, which consists of three equivalent 1D subchromophores, and its 1D model compounds 1 and 2.…”

Insights into the Optical Properties of Triarylboranes with Strongly Electron‐Accepting Bis(fluoromesityl)boryl Groups: when Theory Meets Experiment

“…Alternative bulky groups with higher electron affinities should enhance acceptor strength while maintaining or even enhancing the desired chemical stability of the systems. [6,12,28,[36][37][38][57][58][59][60][61][62][63][64][65][66][67] Exploiting this favorable property, some of us recently reported a series of air-stable bis (fluoromesityl)boryl ((FMes) 2 B) derivatives, namely compounds 1-3 (Scheme 1) with different electron-donating groups for comparison, i. e., phenyl, 4-tert-butylphenyl and 4-N,N-diphenylaminophenyl, respectively, attached to the electron acceptor group (FMes) 2 B through BÀ C bonds. [12] Comparison of these (FMes) 2 B-containing donor-acceptor compounds with compound 4, which contains a (Mes) 2 B group (Scheme 1), confirms that (FMes) 2 B is a much stronger acceptor, leading to: (i) a larger quinoidal distortion in the ground state structure; (ii) significantly red-shifted emission in solution and in the solid state; (iii) stronger emission solvatochromism; and (iv) significantly lower reduction potentials.…”