Synthesis and Structural Manipulation of a Polyanionic Zincoborate [Zn(H2O)6](NO3)2·[Zn{B3O3(OH)4}2(H2O)4]·2H2O by the Molten Salt Method and its Potential Application as a Flame Retardant

A polyanionic borate [Zn(H2O)6](NO3)2??[Zn{B3O3(OH)4}2(H2O)4]??2H2O (1) have been synthesized by a molten salt method in this work. Its crystal structure consists of [B3O3(OH)4]??? cluster and NO3??? anions. Zn2+ cations are covalently bonded to the polyanionic clusters and water molecules leading to a [Zn(H2O)6] octahedron and a [Zn{B3O3(OH)4}2(H2O)4] cluster, which are connected by hydrogen bonds (H-Bs) to form a fictitious 3D structure, in which the NO3??? anion are filled in the void. Crystal data for 1 is as following: Monoclinic, P21/c, a???=???11.2817(12)??, b???=???6.5374(7)??, c???=???17.668(2)??, ?????=???90.00??, ?????=???100.981(5)??, ?????=???90.00??, Z???=???2. Two isostructural compounds, [Co(H2O)6](NO3)2??[Co{B3O3(OH)4}2(H2O)4]??2H2O (2) and [Ni(H2O)6](NO3)2??[Ni{B3O3(OH)4}2(H2O)4]??2H2O (3) have been obtained by similar molten salt method, indicating the structure of 1 can be manipulated by replacement of Zn2+ with Co2+ or Ni2+. Structures of the borates have been further confirmed by element analysis, FTIR, PXRD, TG, and UV???vis diffusion reflectance spectra. The bandgap of the borates showed a tunable performance with a change of metallic ions. Furthermore, we studied that flame retardant property of 1 by mixing it with Acrylonitrile Butadiene Styrene (ABS). The cone calorimetry tests confirmed that ABS/1 composites with 15 wt% loading of 1 exhibited superior performance.