Tuning of the Optical Properties and Lewis Acidity of Dibenzopnictogenaborins by Modification on Bridging Main Group Elements

Dibenzophosphaborin 1 and dibenzoazaborine 7 have been synthesized from the corresponding dibromides and
MesB(OMe)2. Dibenzophosphaborin P-sulfide 2, the P-selenide 3, and the phosphonium salt 4 were obtained by
the reaction of 1 with S8, elemental selenium, and benzyl bromide, respectively. Crystallographic analysis of 1−3
and 7 showed that the dibenzophosphaborin framework does not have a planar structure, which is caused by the
pyramidalization around the phosphorus atoms, unlike in 7. Compound 1 showed a blue-shifted and weak UV−vis
absorption relative to 7, indicating a weak electronic interaction between the phosphorus lone pair electrons and
the π-orbitals on the dibenzopnictogenaborin framework. The longest absorption bands of 2 and 3 are attributable
to the intramolecular electron transfer from the lone pair electrons on the chalcogen atoms to the π* orbitals on
the dibenzophosphaborin framework, and the existence of these electronic transitions is supported by TD-DFT
calculations. The UV−vis and 11B NMR spectra of 4 showed a temperature-dependent change due to the coordination
of a counter bromide anion to the boron center, indicating a strong Lewis acidity of the boron center. We also
investigated the complex formation ability of dibenzopnictogenaborin to fluoride or chloride anions. The complex
formation constants increased in accordance with the decrease in LUMO energy levels calculated using the DFT
method. Compound 4 exhibited an anomalously high complex formation ability among these compounds, due to
the strong electron-withdrawing effect of the phosphonium cation moiety.