Compounds of chromium(VI): the pyridine-chromic anhydride complex, benzimidazolinium dichromate, and three 2-alkyl-1H-benzimidazolinium dichromates

摘要

Pyridine, when allowed to react with chromic anhydride under strictly anhydrous conditions, gives the known, very air-sensitive, crystalline 2:1 Lewis acid-base complex 1. The crystal structure has now been successfully determined. When benzimidazole and three 2-alkyl-1H-benzimidazoles were treated with chromium trioxide in aqueous acetic acid, crystalline dichromate salts (2-5) were readily formed. These salts consist of dichromate anions linked to the cations by hydrogen bonds of the type N-H…O. The chromium atoms have distorted tetrahedral environments, with the Cr-O distances being typical for dichromate anions. In the cases of the 2-methyl and 2-ethyl salts, the anions are disordered about a centre of inversion. The hydrogen bonding arrangements are discussed and the structures are compared with other oxygenated chromium(VI) species. The dichromate salts are useful selective oxidants for a range of primary and secondary alcohols; examples of these reactions are reported.