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 S-8, 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 pi-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 pi* orbitals on the dibenzophosphaborin framework, and the existence of these electronic transitions is supported by TD-DFT calculations. The UV-vis and B-11 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.