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

Squaric acid, H2C4O4 (H(2)SQ), is a completely flat diprotic acid that can crystallize as such, as well as in three different anionic forms, i.e. H(2)SQ . 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-H . . .O 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+. . .O1/2- charge-assisted hydrogen bonds. Analysis of all the structures so far known shows that, while H(2)SQ 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 beta -chains, alpha -chains, alpha -dimers and alpha -tetramers. Partial protonation of these motifs leads to H(2)SQ . HSQ(-) anions whose supramolecular patterns include ribbons of dimerized beta -chains and chains of emiprotonated alpha -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.