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

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 SQ²⁻. 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 SQ²⁻ tends to behave much as a monomeric dianion, the monoanion HSQ⁻ displays a number of different supramolecular patterns that are classifiable as β-chains, α-chains, α-dimers and α-tetramers. Partial protonation of these motifs leads to H₂SQ·HSQ⁻ anions whose supramolecular patterns include ribbons of dimerized β-chains and chains of emiprotonated α-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.