Structure and FTIR Spectra of 3 : 2 Complexes of Trimethylamine N-oxide and 4-Dimethylamine-2,6-dimethylpyridine N-oxide with Perchloric Acid

Trimethylamine N-oxide and 4-dimethylamine-2,6-dimethylpyridine N-oxide form three types of crystalline complexes with perchloric acid, with the base-to-acid ratio 1:1, 2:1 and 3:2, which are easily distinguished by their IR absorption. The asymmetric unit of the P1 unit cell contains two (Me3NO)3·(HClO4)2 formula units. Each of the formula units assembles into a (Me3NO)32H+ dication and two independent ClO−4 anions which show no H-bond interactions with the cationic components. The oxygen atom of a central trimethylamine N-oxide molecule accepts hydrogen bonds from two protonated trimethylamine N-oxide cations with O...O distances in the range 2.537(5) – 2.562(7) Å. The O...HO bonds are linear and formed along direction of the two electron lone-pairs on the oxygen atom. In acetonitrile solutions the 3:2 complexes exist as mixtures of the 1:1 and 2:1 complexes.