Borate compounds have attracted attention due to their rich structural chemstry and wide range of industrial uses, including emerging applications in the energy sector. Ammonium pentaborate and ammonium tetraborate are well characterized commercial products having important uses in electrolytic capacitors, metallurgy and fire retardant materials. An obscure "ammonium octaborate" having the proposed composition (NH4)2O●4B2O3.6H2O was prepared in the laboratories of the U.S. Borax company more than 40 years ago, but was not structurally characterized until now. Prepared by solid state synthesis, this compound was studied by single crystal X-ray crystallography and found to be an ammonium salt of a heptaborate dianion, B7O9(OH)52-, with interstitial disordered boric acid and water molecules. The compound has the structural formula (NH4)2B7O9(OH)5●3/4B(OH)3●5/4H2O, which can be written in oxide form as (NH4)2O●3.9B2O3●4.9H2O, differring somewhat from the originally assigned octaborate composition. Synthesis, structure and properties of this compound are presented. The hepaborate dianion in this compound is one of two known isomers; and comparisons are made to the few other known heptaborates.

Synthesis and structural characterization of a unique ammonium borate containing a heptaborate anion

251st National Meeting (2016)

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