Tetrahydroxydiboron

Tetrahydroxydiboron is a chemical reagent which can be used to prepare boronic acids.

Synthesis

The reaction of boron trichloride with alcohols was reported in 1931, and was used to prepare dimethoxyboron chloride, B(OCH₃)₂Cl. Egon Wiberg and Wilhelm Ruschmann used it to prepare tetrahydroxydiboron by first introducing the boron–boron bond by reduction with sodium and then hydrolysing the resulting tetramethoxydiboron, B₂(OCH₃)₄, to produce what they termed sub-boric acid. The methanol used in this process can be recycled:

BCl₃   $\underset{-\mathrm{H}\mathrm{C}\mathrm{l}}{\overset{\text{+ }\mathrm{C}\mathrm{H}{\phantom{A}}_3\mathrm{O}\mathrm{H}}{\to }}$   B(OCH₃)₂Cl   $\underset{-\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}}{\overset{\text{+ }\mathrm{N}\mathrm{a}}{\to }}$   B₂(OCH₃)₄   $\underset{-\mathrm{C}\mathrm{H}{\phantom{A}}_3\mathrm{O}\mathrm{H}}{\overset{\text{+ }\mathrm{H}{\phantom{A}}_2\mathrm{O}}{\to }}$   B₂(OH)₄

Overall: 2 BCl₃ + 2 Na + 4 H₂O → B₂(OH)₄ + 2 NaCl + 4 HCl

Reactions

When heated to over 90 °C, tetrahydroxydiboron dehydrates to a polymeric boron(II) oxide. The temperature must rise to 220 °C to be totally free from water.

Tetrahydroxydiboron is a reducing agent. A water solution slowly gives off hydrogen gas.