Expanding the chemistry of borates with functional [BO(2)](−) anions

More than 3900 crystalline borates, including borate minerals and synthetic inorganic borates, in addition to a wealth of industrially-important boron-containing glasses, have been discovered and characterized. Of these compounds, 99.9 % contain only the traditional triangular BO(3) and tetrahedral BO(4) units, which polymerize into superstructural motifs. Herein, a mixed metal K(5)Ba(2)(B(10)O(17))(2)(BO(2)) with linear BO(2) structural units was obtained, pushing the boundaries of structural diversity and providing a direct strategy toward the maximum thresholds of birefringence for optical materials design. (11)B solid-state nuclear magnetic resonance (NMR) is a ubiquitous tool in the study of glasses and optical materials; here, density functional theory-based NMR crystallography guided the direct characterization of BO(2) structural units. The full anisotropic shift and quadrupolar tensors of linear BO(2) were extracted from K(5)Ba(2)(B(10)O(17))(2)(BO(2)) containing BO(2), BO(3), and BO(4) and serve as guides to the identification of this powerful moiety in future and, potentially, previously-characterized borate minerals, ceramics, and glasses.