Synthesis, structural characterization and scalable preparation of new amino-zinc borates

Zinc borates are very important industrial materials with applications that include fire retardants and preservatives in wood. In this work, we report the preparation of three novel zinc borates: [Zn(NH₃)₃B₄O₅(OH)₄]·H₂O (ZB1), Zn₃(H₂B₃O₇)₂·2NH₃·4H₂O (ZB2), and [Zn(NH₃)₄][B₄O₅(OH)₄]·4H₂O (ZB3). The solid phases were characterized by elemental analysis, X-ray diffraction, infrared and Raman spectroscopy, scanning electron microscopy and thermal analysis. The crystal structures of ZB1 (monoclinic, Cc, a = 12.1972(8), b = 7.8314(5), c = 12.1441(8) Å and β = 107.404(5)°) and ZB3 (orthorhombic, Pbca, a = 15.0796(9), b = 11.8853(5) and c = 16.7606(8) Å) were determined. They are novel neutral Zn-polyborate complexes [Zn(NH₃)₃B₄O₅(OH)₄] hydrate and [Zn(NH₃)₄][B₄O₅(OH)₄] tetrahydrate salt, respectively. The complete assignment of infrared and Raman spectra was performed theoretically using DFT calculations. For ZB2 (obtained as a polycrystalline phase), powder X-ray diffraction confirmed a single phase and allowed the determination of the unit cell parameters and lattice type (rhombohedral, a = 36.78076 (6), c = 12.20052 (3) Å) with an expected formula Zn₃(H₂B₃O₇)₂·2NH₃·4H₂O suggesting that this compound is a complex triborate. Furthermore, a low-cost scalable synthetic procedure for ZB1 starting from zinc oxide and boric acid in ammonia solution with a high yield is also reported.