10,11-Dihydrocarbamazepine formic acid solvate

10,11-Dihydrocarbamazepine (DHC) is a recognized impurity in carbamazepine (CBZ), a dibenzazepine drug used to control seizures (Cyr et al., 1987). DHC is known to crystallize in three polymorphic forms: monoclinic form I (Bandoli et al., 1992), orthorhombic form II (Harrison et al., 2006) and triclinic form III (Leech et al., 2007a). The title compound, (I), was produced during an automated parallel crystallization search (Florence, Johnston, Fernandes et al., 2006) on DHC as part of a wider study into the predicted and experimental structures of CBZ (Florence, Johnston, Price et al., 2006; Florence, Leech et al., 2006) and related molecules (Leech et al., 2007b). The sample was identified as a new form using multi-sample foil transmission X-ray powder diffraction analysis (Florence et al., 2003). Subsequent manual recrystallization from a saturated formic acid solution by slow evaporation at 298 K yielded samples of (I) suitable for singlecrystal diffraction (Fig. 1).

Comment 10,11-Dihydrocarbamazepine (DHC) is a recognized impurity in carbamazepine (CBZ), a dibenzazepine drug used to control seizures (Cyr et al., 1987). DHC is known to crystallize in three polymorphic forms: monoclinic form I (Bandoli et al., 1992), orthorhombic form II (Harrison et al., 2006) and triclinic form III (Leech et al., 2007a). The title compound, (I), was produced during an automated parallel crystallization search (Florence, Johnston, Fernandes et al., 2006) on DHC as part of a wider study into the predicted and experimental structures of CBZ (Florence, Johnston, Price et al., 2006;Florence, Leech et al., 2006) and related molecules (Leech et al., 2007b). The sample was identified as a new form using multi-sample foil transmission X-ray powder diffraction analysis (Florence et al., 2003). Subsequent manual recrystallization from a saturated formic acid solution by slow evaporation at 298 K yielded samples of (I) suitable for singlecrystal diffraction (Fig. 1).

Experimental
DHC was used as received from Sigma-Aldrich and a single-crystal sample of the title compound was obtained by slow evaporation of a saturated formic acid solution at 298 K. Symmetry code: (i) Àx þ 1; Ày þ 1; Àz.
The three H-atoms attached to N2 and O2 were located in a difference map and refined isotropically [N-H = 0.88 (3) and 0.89 (3) Å ; O-H = 1.04 (4) Å ]. All other H atoms were constrained to idealized geometries and included in the refinement using the riding-model approximation: U iso (H) = 1.2U eq (C) and C-H = 0.95 or 0.99 Å .

Figure 2
Plot showing the hydrogen-bonded dimer arrangement in (I) with two R 2 2 (8) dimers joined in a centrosymmetric arrangement via an R 2 4 (8) motif.

Special details
Experimental. Sample crystals twinned. "Single" small piece cut out from a larger, twinned sample. 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.