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

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Zhang, Zuolun and Edkins, Robert M. and Haehnel, Martin and Wehner, Marius and Eichhorn, Antonius and Mailänder, Lisa and Meier, Michael and Brand, Johannes and Brede, Franziska and Müller-Buschbaum, Klaus and Braunschweig, Holger and Marder, Todd B. (2015) Taming the beast : fluoromesityl groups induce a dramatic stability enhancement in boroles. Chemical Science, 6 (10). pp. 5922-5927. ISSN 2041-6539 (https://doi.org/10.1039/c5sc02205c)

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Abstract

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.

ORCID iDs

Zhang, Zuolun, Edkins, Robert M. ORCID logoORCID: https://orcid.org/0000-0001-6117-5275, Haehnel, Martin, Wehner, Marius, Eichhorn, Antonius, Mailänder, Lisa, Meier, Michael, Brand, Johannes, Brede, Franziska, Müller-Buschbaum, Klaus, Braunschweig, Holger and Marder, Todd B.;
CORE (COnnecting REpositories)