Lewis Acidity and Anion‐Complexation Ability of Oxo‐ and Phenylimino‐Bridged Planar Triphenylboranes as Lithium‐Ion‐Conducting Solid‐State Electrolytes

Boron-containing π-conjugated molecules are promising candidates for the development of advanced Lewis-acidic materials. Planar triphenylboranes containing heteroatoms instead of the methylene groups at the 8-, 14-, and 22-positions of 1-borahexacyclodocosanonaene are particularly interesting because they are stable in the absence of sterically protecting groups around the boron center. However, further investigation of their complexation behavior is required to gain insight into their Lewis acidity and to develop functional materials. Here, the Lewis acidity and anion-complexation ability of oxo- and aza-bridged planar triphenylboranes are reported. UV-vis titration experiments showed that oxo- and aza-bridged planar triphenylboranes exhibit higher binding constants with fluoride ions than trimesitylborane, a conventional triarylborane bearing a sterically protecting group on the boron center. Density-functional-theory (DFT) studies indicated that the sterically less crowded environment around the boron center in planar triphenylboranes is most likely responsible for their high binding constants, and that the six-membered bridging structure does not hinder complexation. The oxo-bridged planar triphenylborane, which shows the highest binding constant toward fluoride ions among the three analogs evaluated, also exhibits high complexation ability toward diverse soft halide anions and molecular anions. A solid-state electrolyte prepared using the oxo-bridged planar triphenylborane and lithium triflate possesses very high lithium-ion conductivity.