Structure and Properties of Sodium Enneaborate, Na₂[B₈O₁₁(OH)₄]·B(OH)₃·2H₂O

Millions of tons of sodium borates are used annually by global industries in diverse applications important to modern society. The Na₂O–B₂O₃–H₂O phase diagram in the 0–100 °C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products. Structures were previously reported for each of these except for that having the highest boron content, known as sodium enneaborate, Na₄B₁₈O₂₉·11H₂O or 2Na₂O·9B₂O₃·11H₂O (1). Here we report the single-crystal structure of 1, revealing the structural formula Na₂[B₈O₁₁(OH)₄]·B­(OH)₃·2H₂O, and describe some of its properties and relationships to other sodium borates. The structure of 1 features linear polyborate chains composed of the repeating [B₈O₁₁(OH)₄]^2– fundamental building blocks with interstitial water and boric acid molecules integrated by extensive H bonding. Interstitial sodium cations occur in groups of four with interatomic distances of 3.7830(6) and 3.7932(8) Å. Upon heating, 1 initially becomes amorphous and then crystallizes as α-Na₂B₈O₁₃ along with amorphous B₂O₃. Notably, α-Na₂B₈O₁₃ contains octaborate fundamental building blocks that are topologically equivalent to those in 1. Compound 1 crystallizes in the monoclinic space group P2₁/n with a = 10.2130(8) Å, b = 12.940(1) Å, c = 12.457(1) Å, β = 93.070(2)°, V = 1644.0(2) ų, and Z = 2.