Imidazolidin-2-one: pseudosymmetry and twinning

The title compound, C₃H₆N₂O, crystallizes with imposed twofold symmetry in the space group I4₁/a. The five-membered ring displays a half-chair conformation. N—H...O hydrogen bonds connect the mol­ecules to form R₂²(8) rings and thence ribbons parallel to the a and b axes. These inter­sect via O₂H₂ rings involving longer H...O contacts. The crystal was merohedrally twinned. Preliminary indications of the higher symmetry space group I4₁/amd, which would require the ring to be planar, proved to be incorrect. A previous brief report of the structure in Fdd2 is also probably incorrect.

S1. Introduction

We are inter­ested in the structures of derivatives and solvent adducts of urea (see e.g. Jones et al., 2013; Taouss et al., 2013, and references therein). Initial attempts to obtain adducts of imidazolidin-2-one (ethyl­eneurea) with various solvents yielded only crystals of the uncomplexed starting material. However, to our surprise, the structure of this simple substance, the title compound, (I), has never been reported in detail. Some adducts such as the hemihydrate (Kapon & Reisner, 1989) and the 1:1 urea adduct [Deutsch & Bernstein, 2008; Cambridge Structural Database (CSD; Allen, 2002) refcode HOJLIC] are known, but the structure of imidazolidin-2-one is only mentioned as part of the Supplementary Material to a study of deep eutectic solvents (Zhang et al., 2009), where it is presented solely as a displacement ellipsoid plot. The authors provided the CSD with information concerning the space group (Fdd2) and cell constants (a = 11.140, b = 10.530 and c = 13.270 Å, with standard uncertainties > 0.1 Å), but requests for the atomic coordinates do not seem to have been complied with. The R value is also high (0.14).

S2. Experimental

S2.1. Synthesis and crystallization

The title compound, obtained from Acros and used without further purification, was dissolved in a 1:1 mixture of 1,4-dioxane and methanol (by volume) and overlayered with di­ethyl ether. Colourless blocks formed overnight.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. N-bound H atoms were refined freely. The treatment of methyl­ene H atoms used a riding model starting from calculated positions, with C—H = 0.99 Å and U_iso(H) = 1.2U_eq(C).

The structure was refined as a merohedral twin; the occupation of the minor component refined to 0.208 (2).

A rigid-body libration correction (Schomaker & Trueblood, 1968) gave an R_lib value of 0.062 and corrected bond lengths (Å) as follows: O1—C1 = 1.240, C1—N2 = 1.371, N2—C3 = 1.461 and C3—C3ⁱ = 1.547 Å.

S3. Comment

In our hands, (I) displayed an I-centred tetra­gonal cell. Initial analysis of the data suggested the space group I4₁/amd, but this was rejected for three reasons: (i) the R_int value was too high at 0.167; (ii) some reflections corresponding to d-glide absences were present (mean I/σ 2.8, cf. 0.6 for the a-glide absences); and (iii) although the structure could be solved, all atoms lay exactly in the mirror plane at x = 0, which is unexpected for the unsaturated imidazolidine ring system. A refinement of this structure led to poor R values and high U components parallel to the molecular plane. All further calculations were therefore performed in the lower symmetry space group I4₁/a. We note in passing that the matrix [1 1 0, 1 1 0, 0 0 1] generates a metrically F-centred orthorhombic cell with a = b = 10.779 and c = 13.163 Å, cf. the cell of Zhang et al. (2009). The structure was refined as a merohedral twin; as is well known, such twinning lowers the R_int value in the higher Laue group [see (i) above)].

The molecule of I) is shown in Fig. 1. It displays crystallographic twofold symmetry, with atoms C1 and O1 lying along the twofold axis. Because of the symmetry, standard ring numbering is not used. The molecular dimensions (Table 2) can be regarded as normal, e.g. the N—Csp² bond length is 1.3652 (11) Å, cf. average 1.366 Å for some 16000 values extracted from the CSD (Version?). However, it should be noted that the librational corrections are significant (e.g. 0.008 Å for the C—C bond). The ring displays a slightly distorted half-chair conformation.

The main feature of the molecular packing of I) is a classical N—H···O hydrogen bond (Table 3), which connects the molecules to form ribbons parallel to the a and the b axes. Rings of the well known graph set R₂²(8) (Etter, 1990) are clearly recognisable (Fig. 2) and are a well known feature in the packing of many urea derivatives, e.g. the closely related structure of the analogous six-membered ring system 3,4,5,6-tetra­hydro­pyrimidin-2(1H)-one (Rizal et al., 2008). The mutual orientation of the ribbons is such that those parallel to a pass through the inter­stices between those parallel to b, and vice versa. However, there are further N—H···O contacts, although they are quite long and have angles of only 118 (1)°, between the two sets of ribbons. At the junctions of the two sets, four-membered O₂H₂ rings can be recognized (Fig. 3), with transannular O···O distances of 2.775 (2) Å (symmetry operator from atom O1 to its transannular equivalent: -1/4 + y, 5/4 - x, 5/4 - z).

Related literature

For related literature, see: Allen (2002); Deutsch & Bernstein (2008); Etter (1990); Jones et al. (2013); Kapon & Reisner (1989); Rizal et al. (2008); Schomaker & Trueblood (1968); Taouss et al. (2013); Zhang et al. (2009).

Computing details

Data collection: CrysAlis PRO (Agilent, 2013); cell refinement: CrysAlis PRO (Agilent, 2013); data reduction: CrysAlis PRO (Agilent, 2013); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level and only the asymmetric unit is numbered.
	Fig. 2. A packing diagram for (I), viewed parallel to the a axis, showing three hydrogen-bonded ribbons. Hydrogen bonds are indicated by dashed lines. H atoms not involved in hydrogen bonds have been omitted.
	Fig. 3. The junction point of two ribbons parallel to b (right) and a (left), showing in each case two eight-membered rings. Contacts between perpendicular ribbons are indicated as thin dashed lines and thick dashed lines indicate hydrogen bonds.

Imidazolidin-2-one

Crystal data

C3H6N2O	Dx = 1.496 Mg m^−3
Mr = 86.10	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/a	Cell parameters from 2735 reflections
a = 7.6219 (3) Å	θ = 3.1–29.2°
c = 13.1632 (9) Å	µ = 0.12 mm^−1
V = 764.69 (7) Å^3	T = 100 K
Z = 8	Block, colourless
F(000) = 368	0.20 × 0.20 × 0.18 mm

Data collection

Oxford Xcalibur Eos diffractometer	546 reflections with I > 2σ(I)
Radiation source: Enhance (Mo) X-ray Source	Rint = 0.043
Graphite monochromator	θmax = 30.9°, θmin = 3.1°
Detector resolution: 16.1419 pixels mm^-1	h = −10→11
ω scans	k = −10→10
10301 measured reflections	l = −18→18
583 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.079	H atoms treated by a mixture of independent and constrained refinement
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0439P)^2 + 0.2482P] where P = (Fo^2 + 2Fc^2)/3
583 reflections	(Δ/σ)max < 0.001
34 parameters	Δρmax = 0.24 e Å^−3
0 restraints	Δρmin = −0.21 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.5000	0.7500	0.51960 (7)	0.0143 (2)
C1	0.5000	0.7500	0.42554 (9)	0.0114 (3)
N2	0.52887 (11)	0.60730 (11)	0.36512 (5)	0.0136 (2)
H02	0.5135 (18)	0.504 (2)	0.3903 (10)	0.024 (3)*
C3	0.49143 (14)	0.64940 (12)	0.25925 (6)	0.0148 (2)
H3A	0.3717	0.6120	0.2397	0.018*
H3B	0.5777	0.5944	0.2129	0.018*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0157 (4)	0.0181 (5)	0.0090 (4)	0.0003 (4)	0.000	0.000
C1	0.0092 (5)	0.0135 (6)	0.0115 (5)	−0.0004 (5)	0.000	0.000
N2	0.0199 (5)	0.0113 (4)	0.0096 (3)	0.0009 (3)	−0.0010 (3)	0.0001 (3)
C3	0.0193 (4)	0.0155 (4)	0.0096 (4)	0.0001 (4)	−0.0009 (3)	−0.0007 (3)

Geometric parameters (Å, º)

O1—C1	1.2382 (15)	C3—C3^i	1.5391 (18)
C1—N2	1.3652 (11)	N2—H02	0.859 (16)
C1—N2^i	1.3652 (11)	C3—H3A	0.9900
N2—C3	1.4583 (11)	C3—H3B	0.9900
O1—C1—N2	125.63 (5)	C3—N2—H02	122.8 (9)
O1—C1—N2^i	125.63 (5)	N2—C3—H3A	111.4
N2—C1—N2^i	108.75 (11)	C3^i—C3—H3A	111.4
C1—N2—C3	110.48 (8)	N2—C3—H3B	111.4
N2—C3—C3^i	101.69 (5)	C3^i—C3—H3B	111.4
C1—N2—H02	118.7 (9)	H3A—C3—H3B	109.3
O1—C1—N2—C3	170.90 (5)	C1—N2—C3—C3^i	21.92 (12)
N2^i—C1—N2—C3	−9.10 (5)	N2—C3—C3^i—N2^i	−25.20 (13)

Symmetry code: (i) −x+1, −y+3/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H02···O1^ii	0.859 (16)	2.275 (15)	3.1253 (10)	170.0 (12)
N2—H02···O1^iii	0.859 (16)	2.509 (15)	3.0076 (9)	117.8 (10)

Symmetry codes: (ii) −x+1, −y+1, −z+1; (iii) −y+5/4, x−1/4, z−1/4.