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Issue 10, 2015
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Taming the beast: fluoromesityl groups induce a dramatic stability enhancement in boroles

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The electron-deficient pentaarylborole 1-(2′,4′,6′-tris(trifluoromethyl)phenyl)-2,3,4,5-tetraphenylborole (1) has been synthesised with the long-term aim of developing borole-based optoelectronic materials. The bulky 2,4,6-tris(trifluoromethyl)phenyl (FMes) group on the boron atom of 1 significantly improves (>600 times) its air stability relative to its mesityl analogue. Moreover, 1 shows good thermal stability without undergoing the dimerisation or isomerisation reactions reported for some other boroles. A triarylborole analogue (2), belonging to a new class of borole with the 3- and 4-positions of the BC4 ring linked by a –(CH2)3– group, has also been synthesised to elucidate the influence of carbon-bonded substituents on the stability of boroles. Both boroles were prepared through the reaction of Li[FMesBF3] and divinyldilithium reagents, a new and general method for borole syntheses. Compound 2 was found to isomerise through a [1,3]-H shift and double-bond rearrangement to an s-trans-butadienylborane species under highly basic (NaOH) conditions. The increased steric crowding at the boron centre through incorporation of the FMes group does not preclude binding of Lewis bases to either 1 or 2, as demonstrated by their fully reversible binding of pyridine. Interestingly, 1 exhibits a blue-shifted absorption spectrum, as compared with its mesityl analogue, a result contrary to previous understanding of the influence of substituent electronics on the absorption spectra of boroles. Most importantly, these boroles exhibit much greater air-stability than previously reported analogues without sacrificing the strong electron-accepting ability that makes boroles so attractive; indeed, 1 and 2 have very low reduction potentials of −1.52 and −1.69 eV vs. Fc/Fc+, respectively.

Graphical abstract: Taming the beast: fluoromesityl groups induce a dramatic stability enhancement in boroles

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The article was received on 18 Jun 2015, accepted on 10 Jul 2015 and first published on 13 Jul 2015

Article type: Edge Article
DOI: 10.1039/C5SC02205C
Citation: Chem. Sci., 2015,6, 5922-5927
  • Open access: Creative Commons BY-NC license
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    Taming the beast: fluoromesityl groups induce a dramatic stability enhancement in boroles

    Z. Zhang, R. M. Edkins, M. Haehnel, M. Wehner, A. Eichhorn, L. Mailänder, M. Meier, J. Brand, F. Brede, K. Müller-Buschbaum, H. Braunschweig and T. B. Marder, Chem. Sci., 2015, 6, 5922
    DOI: 10.1039/C5SC02205C

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