Supramolecular aggregation by means of charge-assisted hydrogen bonds in acid-base adducts containing amidinium cations
Ferretti, Valeria; Bertolasi, Valerio; Pretto, Loretta; Ferretti Valeria; Centro di Strutturistica Diffrattometrica and Dipartimento di Chimica, University of Ferrara; Bertolasi Valerio; Centro di Strutturistica Diffrattometrica and Dipartimento di Chimica, University of Ferrara; Pretto Loretta; Centro di Strutturistica Diffrattometrica and Dipartimento di Chimica, University of Ferrara
Журнал:
New Journal of Chemistry
Дата:
2004
Аннотация:
Amidinium cations are molecular building blocks widely used in crystal engineering studies in view of their capacity to form strong and directional charge-assisted Nâ Hâ ¯O bonds. The X-ray crystallographic structures of three acid-base complexes, amidiniumâ acetate and amidiniumâ (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<sup>+</sup>â H and O<sup>â </sup> functionalities via an Oâ ¯H bond is highly conserved and dominates the crystalâ s three-dimensional arrangement. Calculations of the two polymorphsâ crystal energies revealed that the total energy difference between the polymorphs is less than 1 kcalâ mol<sup>â 1</sup>, 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 â a prioriâ 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.
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