Synthesis, Characterization, and Reactivity of Aminotroponiminate-Based Difluoroboranes: A Pathway toward Bore(boro)nium Cations

Aminotroponiminates (ATIs), a well-known monoanionic bidentate ligand, display a wide range of coordination chemistry. To exploit their electronic and steric factors in achieving boron cations, a series of symmetrical, unsymmetrical, and bis(ATI) borane complexes [(C₇H₅)(N^<i>i</i>Pr)₂BF₂] (<b>3a</b>), [(C₇H₅)(N^<i>t</i>Bu)₂BF₂] (<b>3b</b>), [(C₇H₅)(N^<i>i</i>Bu)₂BF₂] (<b>3c</b>), [(C₇H₅)(N^<i>i</i>Bu)(N^<i>t</i>Bu)BF₂] (<b>3d</b>), and [(C₇H₅)(N^<i>i</i>Pr)(NCH₂)BF₂]₂ (<b>3e</b>) were synthesized in this work. All the ATI borane complexes are highly blue luminescent in the solution, and their fluorescence decay time was recorded in DCM. The observed decay time range was found to exist between 1.7 and 2.8 ns. Complexes <b>3a</b> and <b>3b</b> when treated with trimethylsilyl triflate (TMS-OTf) enabled the isolation of tricoordinated borenium cations [(C₇H₅)(N^<i>i</i>Pr)₂BOH]⁺ (<b>4a</b>) and [(C₇H₅)(N^<i>t</i>Bu)₂BOH]⁺ (<b>4b</b>), respectively. Interestingly, the reaction of bis(ATI) borane <b>3e</b> and TMS-OTf resulted in a boron dication <b>4e</b> in which the borenium and boronium cations are bridged via an oxygen atom in a seven-membered ring scaffold. All of the ATI boranes and cations were well characterized by various spectroscopic techniques in solution and X-ray structure diffraction analysis in the solid state. Further, theoretical calculations using density functional theory (DFT) were conducted to understand the electronic structure and bonding scenario of these complexes.