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)A, b = 6.5374(7)A, c = 17.668(2)A, α = 90.00°, β = 100.981(5)°, γ = 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.