Tetrahydroxydiboron Explained

Tetrahydroxydiboron is a chemical reagent which can be used to prepare boronic acids.^[1]

Synthesis

The reaction of boron trichloride with alcohols was reported in 1931, and was used to prepare dimethoxyboron chloride, B(OCH₃)₂Cl.^[2] 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.^[3] The methanol used in this process can be recycled:

BCl₃   ->[\text{+ } \ce{CH3OH}][- \text{ } \ce{HCl}]   B(OCH₃)₂Cl   ->[\text{+ } \ce{Na}][- \text{ } \ce{NaCl}]   B₂(OCH₃)₄   ->[\text{+ } \ce{H2O}][- \text{ } \ce{CH3OH}]   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.^[4]

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

Notes and References

1. Encyclopedia: Encyclopedia of Reagents for Organic Synthesis. Little. Sarah. Trice. Jane. Tetrahydroxydiboron. 2001. John Wiley & Sons. 9780470842898. 10.1002/047084289x.rn01181.
2. Wiberg. Egon. Sütterlin. Walther. Zur Kenntnis einiger Verbindungen vom Typus BCl_3−n(OR)_n. (Über alkoxyl-substituierte Borchloride). German. Notes on some compounds of type BCl_3−n(OR)_n. (Via alkoxy-substituted boron chlorides). Z. anorg. allg. Chem.. 1931. 202. 1. 1–21. 10.1002/zaac.19312020102.
3. Wiberg. Egon. Ruschmann. Wilhelm. Über eine neue Borsäure ('Unterborsäure'︁) der Formel H₄B₂O₄ und ihre Ester. Ber. Dtsch. Chem. Ges. A/B. 1937. 70. 6. 1393–1402. 10.1002/cber.19370700636. de. On a new boric acid ('Sub-boric acid') of the formula H₄B₂O₄ and its esters.
4. Wartik. Thomas. Apple. Eugene F.. A New Modification of Boron Monoxide. J. Am. Chem. Soc.. 1955. 77. 23. 6400–6401. 10.1021/ja01628a116.