Lutidine adducts of urea: molecular mechanisms for twinning effects on cooling

Abstract

The structures of three adducts between urea and 2,6- or 3,5-lutidine have been determined: urea : 2,6-lutidine (1 : 1) (1), urea : 2,6-lutidine (2 : 1) (2) and urea : 3,5-lutidine (2 : 1) (3). Adducts 1 and 3 each crystallize as different polymorphs depending on the temperature. In each of the five structures, the lutidine nitrogen acts as a hydrogen bond acceptor for both NH groups of a urea molecule, and the lutidine molecules form infinite stacks. The known structure of 1 in space group C2/c (1m), previously known only from photographic data in projection, was redetermined at 200 K. Both residues display crystallographic twofold symmetry. The urea substructure is a ribbon of [R²₂(8)] rings. Further cooling results in a reduction in symmetry to P (structure 1t); crystals are non-merohedrally twinned by 180° rotation about (b*–c*), which is the same direction as the b axis of 1m. The qualitative structure remains almost unchanged, but the lutidine rings tilt slightly away from the twofold axis (which is lost on twinning). The three structures involving 2 : 1 adducts all display a urea substructure in the form of a corrugated layer, from which urea molecules project and form hydrogen bonds to the lutidines. The urea layers are formed from ribbons of [R²₂(8)] rings that are linked by lateral hydrogen bonds. However, the linkages are in all three cases subtly different. Adduct 2 crystallizes in Pnma with the lutidine and both ureas displaying mirror symmetry. Adduct 3 crystallizes in space group Abm2 (3o) at 173 K, with two lutidines and four ureas all displaying crystallographic mirror symmetry; on cooling to 100 K, the space group changes to Cc (3m) and the crystals form reticular pseudo-merohedral twins by 180° rotation about a*, probably as a consequence of the mutual displacement of the urea ribbons, parallel to their direction of propagation, to accommodate a change in hydrogen bonding.