In 1940, the synthesis of a new boron oxide material with the empirical formula BO was reported. This was followed by an improved synthesis in 1955 from the condensation of tetrahydroxydiboron. Materials characterization methods of the day did not permit the elucidation of the material’s structure, although a few candidates were postulated. Fast forward 68 years and the only known structural information on the material stems from its reactivity with halogens which reveals that the B-B bonds from the precursor are preserved. A number of 0D, 1D, 2D, and 3D structures have been postulated for the material using density functional theory studies, but none of these has obtained experimental backing.

With recent interest in two-dimensional boron materials, we sought to determine BO’s chemical structure once and for all. Using multidimensional ¹¹B solid-state NMR spectroscopy we probed the symmetry of the B-B bond, as well as the local 6-membered repeating unit. Using a ¹¹B J-resolved experiment we could determine that bonded boron atoms are magnetically equivalent and that the O₂B-BO₂ unit is planar and symmetrical. This result was corroborated by a ¹¹B double-quantum single-quantum correlation (2Q/1Q) experiment. Using the first of its king 3Q/1Q correlation experiment correlating three boron atoms, we could further determine that neighboring B₂ pairs are aligned and that the primary building unit is a B₄O₂ ring. ¹¹B{¹H} double-resonance experiments revealed that the BO sheets are OH-terminated and have hydroxyl-free domains on several tens of microns in scale. Powder diffraction experiments revealed only sparse reflections originating primarily from the c dimension, indicating that the BO sheets have disordered stacking.

Additional heating of the material at a temperature exceeding 600 °C leads to a phase transition and the formation of a dark and hard material. Using ¹¹B NMR and XPS experiments, we identified that the phase transition originates from a disproportionation of the compound and the formation of a B/B₂O₃ glass.

Ranjan Behera

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