Solid-State Synthesis and Structure of the Enigmatic Ammonium Octaborate: (NH4)2[B7O9(OH)5]·3/4B(OH)3·5/4H2O

Area of Science:

• Inorganic Chemistry
• Solid-State Chemistry
• Crystallography

Background:

• Ammonium octaborate, a compound known since the 19th century, has been structurally characterized.
• Previous understanding of its composition and structure was limited.

Purpose of the Study:

• To elucidate the precise crystal structure of ammonium octaborate.
• To understand the formation mechanism of this borate compound under ambient conditions.

Main Methods:

• Single-crystal X-ray diffraction was employed for structural characterization.
• Solid-state synthesis involving mixtures of ammonium tetraborate and ammonium pentaborate was utilized.

Main Results:

• The compound was identified as the ammonium salt of the ribbon isomer of the heptaborate anion, [B7O9(OH)5](2-), with associated boric acid and water molecules.
• It crystallizes in the monoclinic space group P21/c with specific lattice parameters.
• The structure features ordered ammonium cations and heptaborate anions, alongside disordered boric acid and water molecules linked by hydrogen bonding.

Conclusions:

• The structural characterization reveals a heptaborate anion, [B7O9(OH)5](2-), rather than an octaborate structure.
• Spontaneous formation of the heptaborate anion under ambient conditions has significant implications for developing practical syntheses of industrially relevant borates.