Preparation of quaternary pyridinium salts as possible proton conductors

Abstract

On basis of earlier experimental experience, the transfer of protons in salts of the organic cation-inorganic anion type occurs primarily through directional arrangement of the anion-anion type short hydrogen bonds. The submitted work presents the preparation of quaternary pyridinium salts of inorganic hydrogen anions in the absence of solvent molecules in their crystal structure. These substances can form only the above-described anion-anion type hydrogen bonds; in addition, the absence of solvate anions increases the stability of the prepared compounds. A total of six substituted pyridinium salts were prepared, four of which have not been described yet: 1,2,4,6-tetraphenylpyridinium perchlorate, 1-benzyl-2,4,6-trimethylpyridinium perchlorate, 1,4-dimethylpyridinium hydrogen sulphate, 1,4-dimethylpyridinium dihydrogen phosphate, 1,4-dimethylpyridinium hydrogen sulphate, and 1,2-dimethyl-5-ethylpyridinium dihydrogen phosphate. Three of these substances were characterised by X-ray structural analysis: 1,2,4,6-tetraphenylpyridinium perchlorate crystallises in the orthorhombic system, space group Pbca; 1-benzyl-2,4,6-trimethylpyridinium perchlorate crystallises in the monoclinic system, space group P21/c; and 1,4-dimethylpyridinium dihydrogen phosphate crystallises in the monoclinic system, space group C2/c. This structure contains an oriented anion network bond by short anion-anion type hydrogen bonds with the donor acceptor lengths of 2.567(3)Å and 2.557(3)Å and thus fulfils the requirements of a good proton conductor.