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Hexol is the name for various salts of a coordination complex that has historical significance. The salts were the first synthetic non-carbon-containing chiral compounds. The sulfate salt has the formula {[Co(NH3)4(OH)2]3Co}(SO4)3.
Contents
Preparation and optical resolution
Salts of hexol were first described by Jorgenson. The salt is prepared by heating [Co(NH3)4(H2O)2]3+ with dilute base such as ammonia followed by precipitation of the sulfate salt:
4 [Co(NH3)4(H2O)2]3+ → {[Co(NH3)4(OH)2]3Co}6+ + 4 NH4+ + 2 H+ + 2 H2ODepending on the conditions one obtains the 9-hydrate, the 6-hydrate, and the 4-hydrate. These salts exists as dark brownish-violet or black tabular crystals. The salts has low solubility in water. The cation can be quantitatively precipitated from its yellow-gray chromate and hexachloroplatinate salts. When treated with concentrated hydrochloric acid, hexol converts to cis-diaquotetramminecobalt(III) sulfate. In boiling dilute sulfuric acid, hexol degrades with evolution of oxygen and nitrogen.
Optical resolution
In a historic set of experiments, Alfred Werner obtained chiral resolution by fractional crystallisation of the diastereomeric D-(+)-bromocamphorsulfonate salt. This ion has a high specific rotation of 2640°. More efficient methods involve the bis(tartrato)diantimonate(III) salt.
The "second hexol"
Werner also described a second achiral hexol (a minor byproduct from the production of Fremy's salt) that he incorrectly identified as a linear trimer. The second hexol is hexanuclear (contains six cobalt centres in each ion), not tetranuclear.