General rules for the packing of hydrogen-bonded crystals as derived from the analysis of squaric acid anions: aminoaromatic nitrogen base co-crystals.

Preparation and single-crystal X-ray structure determination of three co-crystals of hydrogen squarate, HSQ(-), with 2-aminopyrimidine, 3-aminopyridine and 4-aminopyridine, and one of squarate, SQ(2-), with 8-aminoquinoline are reported. Their crystal packings are analyzed and discussed in terms of the intermolecular O--H...O, N--H...O/N and C--H...O hydrogen bonds formed. Although the fine details of the supramolecular architecture are barely rationalizable, the comparative analysis of the data makes it possible to suggest some simple rules that may be of general application for the packing of hydrogen-bonded crystals, i.e. Rule 1: 'All hydrogen-bond acceptors available in a molecule will be engaged in hydrogen bonding as far as there are available donors'; Rule 2: 'The hydrogen-bond acceptors will be saturated in order of decreasing strength of the hydrogen bonds formed'.