Acta Crystallographica Section B

Structural Science

Volume 57, Part 6 (December 2001)

research papers

Acta Cryst. (2001). B57, 859-865 [ doi:10.1107/S0108768101014963 ]

Associations of squaric acid and its anions as multiform building blocks of hydrogen-bonded molecular crystals

G. Gilli, V. Bertolasi, P. Gilli and V. Ferretti

Abstract: Squaric acid, H₂C₄O₄ (H₂SQ), is a completely flat diprotic acid that can crystallize as such, as well as in three different anionic forms, i.e. H₂SQ·HSQ^-, HSQ^- and SQ^2-. Its interest for crystal engineering studies arises from three notable factors: (i) its ability of donating and accepting hydrogen bonds strictly confined to the molecular plane; (ii) the remarkable strength of the O-HO bonds it may form with itself which are either of resonance-assisted (RAHB) or negative-charge-assisted [(-)CAHB] types; (iii) the ease with which it may donate a proton to an aromatic base which, in turn, back-links to the anion by strong low-barrier N-H⁺O^1/2- charge-assisted hydrogen bonds. Analysis of all the structures so far known shows that, while H₂SQ can only crystallize in an extended RAHB-linked planar arrangement and SQ^2- tends to behave much as a monomeric dianion, the monoanion HSQ^- displays a number of different supramolecular patterns that are classifiable as -chains, -chains, -dimers and -tetramers. Partial protonation of these motifs leads to H₂SQ·HSQ^- anions whose supramolecular patterns include ribbons of dimerized -chains and chains of emiprotonated -dimers. The topological similarities between the three-dimensional crystal chemistry of orthosilicic acid, H₄SiO₄, and the two-dimensional one of squaric acid, H₂C₄O₄, are finally stressed.

Keywords: multiform building blocks; hydrogen bonding; squaric acid; supramolecular patterns.