Comment

The antiepileptic compound carbamazepine (CBZ) is known to crystallize in at least four anhydrous polymorphic forms (Grzesiak et al., 2003) and the crystal structures of several solvates and co-crystals have also been reported (Fleischman et al., 2003). The title compound, (I), was produced during an automated parallel crystallization polymorph screen on CBZ. The sample was identified as a novel form using multisample X-ray powder diffraction analysis of all recrystallized samples (Florence et al., 2003). Subsequent manual recrystallization from a saturated furfural solution by slow evaporation at 278 K yielded samples of the carbamazepine furfural hemisolvate suitable for synchrotron-based single-crystal X-ray analysis (Cernik et al., 1997).

The asymmetric unit of (I) contains two molecules of CBZ and one of furfural (Fig. 1). Pairs of CBZ molecules are connected by two N-HO hydrogen bonds (contacts 1 and 2, Fig. 2) to form the R₂²(8) dimer motif. This motif is observed in all of the known polymorphs and the majority of CBZ solvate crystal structures (Fleischman et al., 2003). In all other CBZ solvate crystal structures, each of the NH donor groups is involved in hydrogen-bonding interactions; the syn-oriented NH group of CBZ forms the dimer motif and the anti-oriented NH donors connect to molecules of solvent. In (I), however, only one of the anti-oriented NH groups is utilized in a hydrogen bond between CBZ and solvent (contact 3, Fig. 2). The structure also contains four C-HO interactions: contacts 4, 6 and 7 connect CBZ and furfural molecules, and contact 5 connects molecules of CBZ. The molecules pack such that the polar groups (furfural and CBZ carboxamide moiety) and hydrophobic azepine rings are segregated into alternating polar and non-polar layers in the ac plane, which are stacked in the direction of the b axis.

Figure 1 View of the asymmetric unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2 A packing diagram of (I). Dashed lines indicate hydrogen bonds, which produce the two ring motifs, viz. A [the R22(8) CBZ dimer] and B [an R32(9) motif linking one solvent molecule to the dimer].

Experimental

A single-crystal sample of the title compound was recrystallized from a furfural solution of carbamazepine (used as supplied from Sigma-Aldrich) by slow evaporation at 278 K.

Crystal data

C15H12N2O·0.5C5H4O2 Mr = 284.31 Monoclinic, P21/n a = 5.1815 (4) Å b = 26.0450 (19) Å c = 20.5735 (15) Å = 91.302 (2)° V = 2775.7 (4) Å3 Z = 8 Dx = 1.361 Mg m-3 Synchrotron radiation, = 0.6902 Å Cell parameters from 4821 reflections = 2.5-29.5° = 0.11 mm-1 T = 120 (2) K Plate, brown 0.04 × 0.04 × 0.01 mm

Data collection

Bruker SMART APEX2 CCD diffractometer Fine-slice scans Absorption correction: none 28844 measured reflections 8144 independent reflections 5329 reflections with I > 2(I) Rint = 0.053 max = 29.5° h = -7 7 k = -37 35 l = -29 28

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.057 wR(F2) = 0.151 S = 1.01 8144 reflections 424 parameters H atoms treated by a mixture of independent and constrained refinement w = 1/[2(Fo2) + (0.0702P)2 + 0.8667P] where P = (Fo2 + 2Fc2)/3 (/)max < 0.001 max = 0.30 e Å-3 min = -0.25 e Å-3

Table 1 Hydrogen-bonding geometry (Å, °) D-HA D-H HA DA D-HA N2-H1NO1Ai 0.91 (2) 1.91 (2) 2.817 (2) 175 (2) N2A-H3NO1ii 0.93 (2) 1.95 (2) 2.876 (2) 175 (2) N2A-H4NO3 0.89 (2) 2.13 (2) 2.961 (2) 156 (2) C3A-H3AO3i 0.95 2.45 3.250 (2) 142 C13-H13O1ii 0.95 2.55 3.449 (2) 159 C16-H16O1Aiii 0.95 2.48 3.108 (2) 124 C18-H18O1 0.95 2.56 3.283 (2) 133 Symmetry codes: (i) 1+x,y,z; (ii) x-1,y,z; (iii) .

The H atoms of the six- and five-membered rings of carbamazepine and furfural were positioned geometrically at distances of 0.95 Å (CH) from the parent C atoms; a riding model was used during the refinement process. The U_iso(H) values were constrained to be 1.2 times U_eq of the carrier atom. The remaining H atoms were located in a difference synthesis and were refined isotropically [C-H = 0.95 (2)-1.01 (2) Å and N-H = 0.85 (3)-0.93 (2) Å].

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97.