Urea–N,N-dimethylacetamide (1/1)

Urea forms a 1:1 solvate with N,N-dimethylacetamide (DMA) [systematic name: diaminomethanal–N,N-dimethylacetamide (1/1), C4H9NO·CH4N2O] with both molecules positioned on a twofold axis, giving rise to rotational disorder of the DMA molecule. The molecules display a layered structure in which urea molecules form hydrogen-bonded ribbons bounded by molecules of solvent.

Urea forms a 1:1 solvate with N,N-dimethylacetamide (DMA) [systematic name: diaminomethanal-N,N-dimethylacetamide (1/1), C 4 H 9 NOÁCH 4 N 2 O] with both molecules positioned on a twofold axis, giving rise to rotational disorder of the DMA molecule. The molecules display a layered structure in which urea molecules form hydrogen-bonded ribbons bounded by molecules of solvent.

Comment
The crystal structure of urea has been widely studied (see for example, Vaughan and Donohue (1952) and references therein; Swaminathan et al. (1984), Pryor and Sanger (1970), Guth et al. (1980) and Weber et al. (2002)). This previously unreported crystalline solvate of urea was discovered during an investigation into the influence of different crystallization solvents on urea crystal morphology (see also Fernandes et al., 2007). The sample was obtained by slow evaporation from a saturated N,N-dimethylacetamide (DMA) solution at 298 K and identified by using multi-sample foil transmission X-ray powder diffraction analysis (Florence et al., 2003). Subsequent recrystallization produced a single-crystal suitable for X-ray diffraction at 120 K ( Fig. 1).
Both molecules lie over a two fold rotation axis resulting in the DMA being disordered (see refinement section for details).
Each urea molecule interacts with adjacent urea molecules via contact 1 (Fig. 2, entry 1, Table 1), forming a hydrogen bonded ribbon that runs in the direction [−1 0 1]. Molecules of DMA lie on the edge of the ribbons, connected through a second hydrogen bond (contact 2), (entry 2, Table 1)·The DMA-bordered ribbons of urea pack side-by-side to form a two-dimensional sheet.

Experimental
The compound was sourced from Sigma-Aldrich and used as supplied. A single-crystal sample of the 1/1 solvate was recrystallized from a saturated N,N-dimethylacetamide solution by isothermal solvent evaporation at room temperature (298 K).

Refinement
The DMA moiety was found to be disordered over a 2-fold rotation axis, with atoms C7 and O9 sitting on this axis. The site occupancies of N4 and C8 were consequently fixed to 1/2, whilst that of C6 was fixed to 1.0 as this atom acts as a methyl carbon both attached to N4 and to C8 in the disordered model. All non-H-atoms were modelled with anisotropic displacement parameters. H-atoms attached to N3 were located in a difference Fourier map and their positions were freely refined. H-atoms attached to C6 and C7 were positioned geometrically, taking into account disorder and occupancy of the parents atoms, and their positions were fixed during refinement. U iso (H) were assigned in the range 1.2-1.5 times U eq of the parent atom.
Note that both the (1 1 0) and the (−2 0 2) reflections were excluded from the final refinement as they were significant outliers on the F o versus Fc plot.
supplementary materials sup-2 Figures   Fig. 1. The molecular structure of the title compound showing 50% probablility displacement ellipsoids. Hydrogen atoms have been omitted for clarity. A twofold axis runs through C 1 , O 2 of urea and O 9 , C 7 of DMA, giving rise to the rotational disorder of the DMA molecule. Symmetry codes: (i) −x, y, 1/2 − z. (ii) −x, y, 3/2 − z. Fig. 2. Selected molecular packing, viewed down the a axis, of the title compound illustrating the hydrogen bonded network. Urea molecules (green) form an R 2 2 (8) motif (Etter, 1990) involving contact 1 (entry 1, Table 1) that propagates to form an infinite ribbon. DMA molecules (shown in blues with rotational disorder) are hydrogen bonded via N-H···O contacts 2 (entry 2, Table 1