Unveiling Borane Complexes With N, O Chelation: Synthesis, Reactivity, and Theoretical Studies

Abstract

We synthesized a series of 2-aminotropone (N, O)-based borane complexes (2a–2c) in a five-membered scaffold that exhibits strong fluorescence in solution. The photoluminescence lifetimes of these complexes, measured in dichloromethane, range from 3.40 to 3.91 ns. Correspondingly, fluorescence quantum yields of boranes (2a-2c) range from 10% to 15%, indicating the quantum yields and fluorescence decay times have minimal impact of the substituents present on the nitrogen atom. Treatment of these borane complexes with trimethylsilyl trifluoromethanesulfonate (TMS-OTf) results in the formation of mixtures of borenium (three coordinated) and boronium (four coordinated) cations in solution. In contrast, reacting 2-aminotropones with BBr<inf>3</inf> at room temperature selectively afforded boronium cations (4a–4c). All these neutral and cationic complexes were fully characterized using multinuclear NMR spectroscopy and single-crystal x-ray diffraction analysis. Furthermore, Density Functional Theory (DFT) studies were performed to analyze the electronic properties of both neutral and cationic boron species. © 2025 Wiley-VCH GmbH.