Structural Characterization and Chemistry of the Industrially Important Zinc Borate, Zn[B₃O₄(OH)₃]

Several unique crystalline zinc borates are known, a few of which find industrial use in significant tonnages. Although the most important of these has been a commercial product for more than 3 decades, it was never before structurally characterized. The structure of Zn[B₃O₄(OH)₃] (1) was determined for the first time by single-crystal X-ray diffraction, revealing it to be a complex network consisting of infinite polytriborate chains cross-linked by coordination with zinc and further integrated by hydrogen bonding. The structure of 1 bears similarities to certain borate minerals, most notably, studenitsite (Ca[B₃O₄(OH)₃]) and colemanite (Ca[B₃O₄(OH)₃]·H₂O); however, significant differences are described. Hydrolytic and thermochemical properties of 1 are discussed. This compound illustrates the important role played by metal cations in directing the spatial arrangement of anionic polyborate structural units in metal borates. This new structural information leads to a revision in the chemical formula, 2ZnO·3B₂O₃·3.5H₂O, typically used to describe this material as an article of commerce, to 2ZnO·3B₂O₃·3H₂O. Compound 1 crystallizes in the monoclinic space group P2₁/n with a = 6.845(2) Å, b = 9.798(2) Å, c = 7.697(2) Å, β = 106.966(4)°, V = 493.8 (2) ų, and Z = 4.