Structure and vibrational spectra of squaric acid complexes with 4,4′- and 5,5′-dimethyl-2,2′-bipyridine

The complexes formed by squaric acid with 4,4′- and 5,5′-dimethyl-2,2′-bipyridine were prepared (abbreviation 4-DMBPH+·HSQ− and 5-DMBPH+·HSQ−·H2O, respectively). Their crystal structure shows that ionization of squaric acid leads to formation of dimeric double ionized (HSQ)2−2 units and monoprotonated bipyridines. The N+H groups form bifurcated hydrogen bonds with the CO group of squarate anion and with the second non-protonated nitrogen atom. In the case of 5,5′-DMBP the complex crystallizes with one water molecule attached to the charged N+H group, with OH groups forming hydrogen bonds to carbonyl groups of squaric acid. Infrared and Raman spectra reflect sufficiently well the packing. Most spectacular is the continuous absorption ascribed to the (HSQ)2−2 units with two very short OH⋯O hydrogen bonds. The analysis has been performed for the 4-DMBPH+·HSQ− complex by comparison of the experimental infrared spectra with the calculated ones for separated ions and the crystalline state.