Structure and Properties of Sodium Enneaborate, Na 2 [B 8 O 11 (OH) 4 ]·B(OH) 3 ·2H 2 O.

Millions of tons of sodium borates are used annually by global industries in diverse applications important to modern society. The Na2 O-B2 O3 -H2 O 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, Na4 B18 O29 ·11H2 O or 2Na2 O·9B2 O3 ·11H2 O (1). Here we report the single-crystal structure of 1, revealing the structural formula Na2 [B8 O11 (OH)4 ]·B(OH)3 ·2H2 O, and describe some of its properties and relationships to other sodium borates. The structure of 1 features linear polyborate chains composed of the repeating [B8 O11 (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-Na2 B8 O13 along with amorphous B2 O3 . Notably, α-Na2 B8 O13 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) Å3 , and Z = 2.