On the Synergy of Coulombic and Chelate Effects in Bidentate Diboranes: Synthesis and Anion Binding Properties of a Cationic 1,8-Diborylnaphthalene

As part of our investigation in the chemistry of bidentate
Lewis acids for anion complexation, we have carried out the reaction
of 1-(dimesitylboryl)-8-(1′-bora-9′-thia-anthryl)­naphthalene
(1) with methyltriflate. This reaction proceeds via alkylation
of the sulfur atom to afford a bidentate diborane ([2]+) decorated by a peripheral sulfonium unit. This new
diborane, which has been isolated as the triflate salt, reacts with
both fluoride and azide anions to form the corresponding anion chelate
complexes 2-μ2-F and 2-μ2-N3, respectively. Titration experiments carried
out in chloroform indicate that the fluoride binding constant of [2]+ exceeds that of 1 by at least
4 orders of magnitude. These results, which are supported by spectroscopic,
structural, and computational data, show that chelate and Coulombic
effects are additive and can be combined to boost the anion affinity
of bidentate Lewis acids.