5-(1-Cyclohexen-1-yl)-1,5-dimethylbarbituric acid (hexobarbitone): a low-temperature redetermination

A low-temperature redetermination of the title compound, C12H16N2O3, more commonly known as hexobarbitone, is reported, with significantly improved precision. The crystal packing reveals an infinite hydrogen-bonded hexobarbitone chain linked by a single N—H⋯O interaction, an extremely rare motif in barbiturate crystal packing. Unlike some other barbiturate crystal structures, there is no phase transition on cooling to 150 K.

A low-temperature redetermination of the title compound, C 12 H 16 N 2 O 3 , more commonly known as hexobarbitone, is reported, with significantly improved precision. The crystal packing reveals an infinite hydrogen-bonded hexobarbitone chain linked by a single N-HÁ Á ÁO interaction, an extremely rare motif in barbiturate crystal packing. Unlike some other barbiturate crystal structures, there is no phase transition on cooling to 150 K.

Comment
As part of our research on s-block complexes of barbituric acid and its derivatives, we have redetermined the crystal structures of the various ligands of interest at low temperatures for the purpose of having reference structures that are more precise than those previously published, most of which are over 30 years old. We found that at least two of these compounds actually undergo a phase transition at low temperatures (Nichol & Clegg, 2005a,b). We redetermined the structure of hexobarbitone at 150 K, but in this case no phase transition occurs.
The crystal structure of hexobarbitone [5-(1-cyclohex-en-1yl)-1,5-dimethylbarbituric acid], (I), was reported by Bideau et al. (1970). The structure refined to a final R = 0.09. The authors were unable to locate from a difference Fourier synthesis any H atoms; those which could be placed in calculated positions by means of well established geometry were added to the model; however, others (such as the methyl H atoms) were omitted. We have redetermined the crystal structure of compound (I) at 150 K. The structure refines to a final R value of 0.036. The precision of the structure is improved markedly. The unit-cell volume decreases by ca 34 Å 3 , as expected for a low-temperature determination. Molecular dimensions are unexceptional and are in general agreement with the room-temperature structure.
Compound (I) crystallizes from water in the space group P2 1 /c with one molecule in the asymmetric unit and no solvent molecules (Fig. 1). The steric hindrance of the N-methyl group prevents hydrogen bonding on that side of the molecule so, in contrast to the crystal structures of many other barbiturate compounds, there is only one hydrogen bond observed. This forms an infinite chain (Fig. 2) and two of the three carbonyl groups are not involved in hydrogen bonding. Such hydrogenbonding geometry is highly unusual in barbiturate crystal packing. A search of the Cambridge Structural Database (Version 5.26, plus one update; Allen, 2002) shows there to be only two other 5,5-disubstituted barbiturate chains formed by a single N-HÁ Á ÁO interaction. These are for 1-methyl-5,5diethylbarbituric acid (Wunderlich, 1973) and 1-methyl-5isopropyl-5--bromoallylbarbituric acid (Wilhelm & Fischer, 1976). Fig. 3 shows the positions of the chains relative to one another and the orientation of the cyclohexenyl rings in the crystal packing. The barbiturate rings are staggered rather than overlapping, as shown in Fig. 4.

Experimental
Hexobarbitone was obtained as a commercial crystalline compound and was not recrystallized.

Refinement
Refinement on F 2 R[F 2 > 2(F 2 )] = 0.036 wR(F 2 ) = 0.099 S = 1.06 2792 reflections 160 parameters H atoms treated by a mixture of independent and constrained refinement Nichol and Clegg C 12 H 16 N 2 O 3 o1005  The molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
The hydrogen-bonding motif observed in the crystal packing of (I). Hydrogen bonds are indicated in light blue, the red dotted lines indicate the hydrogen-bond continuation and H atoms not involved in hydrogen bonding have been omitted for clarity.
All H atoms were found in a difference map. Methyl H-atom positions were then idealized (C-H = 0.98 Å ) and refined as riding, with free rotation about the C-C bond, and with U iso (H) = 1.5U eq (C). CH 2 H atoms were also positioned geometrically (C-H = 0.99 Å ) and refined as riding, with U iso (H) = 1.2U eq (C). The H atom bonded to C8 was positioned geometrically (C-H = 0.95 Å ) and also refined as riding, with U iso (H) = 1.2U eq (C). The N-H H-atom position was refined freely, with U iso (H) = 1.2U eq (N). 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.