Supramolecular aggregation by means of charge-assisted hydrogen bonds in acid-base adducts containing amidinium cations

Amidinium cations are molecular building blocks widely used in crystal engineering studies in view of their capacity to form strong and directional charge-assisted N-H...O bonds. The X-ray crystallographic structures of three acid-base complexes, amidinium-acetate and amidinium-(2-hydroxyethoxy)acetate in two polymorphic forms, are reported, together with a systematic analysis of acid-base adducts containing amidinium cations found in the Cambridge Crystallographic Database. A detailed discussion on the preferred packing patterns adopted by these adducts shows that the supramolecular synthon characterized by a dimeric coupling of RN+-H and O- functionalities via an O...H bond is highly conserved and dominates the crystal's three-dimensional arrangement. Calculations of the two polymorphs' crystal energies revealed that the total energy difference between the polymorphs is less than 1 kcal mol(-1), H-bonding contributions being strictly comparable and the main difference being imputed to van der Waals and angular and torsional intramolecular terms, in agreement with the fact that in the two crystals the flexible anion adopts completely different conformations. This is in keeping with the idea that, for an 'a priori' predictability of crystal packing motifs, it is important not only to perfectly match the number of H-bond donors and acceptors but also to have a relative rigidity of the molecular fragments constituting the co-crystal.