Crystal Structure and Twisted Aggregates of Oxcarbazepine Form III

Polymorphism and crystal habit play vital roles in dictating the properties of crystalline materials. Here, the structure and properties of oxcarbazepine (OXCBZ) form III are reported along with the occurrence of twisted crystalline aggregates of this metastable polymorph. OXCBZ III can be produced by crystallization from the vapor phase and by recrystallization from solution. The crystallization process used to obtain OXCBZ III is found to affect the pitch, with the most prominent effect observed from the sublimation-grown OXCBZ III material where the pitch increases as the length of aggregates increases. Sublimation-grown OXCBZ III follows an unconventional mechanism of formation with condensed droplet formation and coalescence preceding nucleation and growth of aggregates. A crystal structure determination of OXCBZ III from powder X-ray diffraction methods, assisted by crystal structure prediction (CSP), reveals that OXCBZ III, similar to carbamazepine form II, contains void channels in its structure with the channels, aligned along the c crystallographic axis, oriented parallel to the twist axis of the aggregates. The likely role of structural misalignment at the lattice or nanoscale is explored by considering the role of molecular and closely related structural impurities informed by crystal structure prediction.

An Efficient, Large-Scale Synthesis of Cytenamide

Dibenzosuberenone (5 H-dibenzo[a,d]cyclohepten-5-one) was reduced to the corresponding alcohol by sodium borohydride/MeOH and converted to the corresponding 5-chloro compound by thionyl chloride/benzene, treatment of which with CuCN/toluene gave the corresponding nitrile. Hydrolysis by ethanolic KOH yielded the corresponding amide, cytenamide (5 H-dibenzo[a,d]cycloheptene-5-carboxamide).

Specific energy contributions from competing hydrogen-bonded structures in six polymorphs of phenobarbital

Background

In solid state structures of organic molecules, identical sets of H-bond donor and acceptor functions can result in a range of distinct H-bond connectivity modes. Specifically, competing H-bond structures (HBSs) may differ in the quantitative proportion between one-point and multiple-point H-bond connections. For an assessment of such HBSs, the effects of their internal as well as external (packing) interactions need to be taken into consideration. The semi-classical density sums (SCDS-PIXEL) method, which enables the calculation of interaction energies for molecule–molecule pairs, was used to investigate six polymorphs of phenobarbital (Pbtl) with different quantitative proportions of one-point and two-point H-bond connections.

Results

The structures of polymorphs V and VI of Pbtl were determined from single crystal data. Two-point H-bond connections are inherently inflexible in their geometry and lie within a small PIXEL energy range (−45.7 to −49.7 kJ mol⁻¹). One-point H-bond connections are geometrically less restricted and subsequently show large variations in their dispersion terms and total energies (−23.1 to −40.5 kJ mol⁻¹). The comparison of sums of interaction energies in small clusters containing only the strongest intermolecular interactions showed an advantage for compact HBSs with multiple-point connections, whereas alternative HBSs based on one-point connections may enable more favourable overall packing interactions (i.e. V vs. III). Energy penalties associated with experimental intramolecular geometries relative to the global conformational energy minimum were calculated and used to correct total PIXEL energies. The estimated order of stabilities (based on PIXEL energies) is III > I > II > VI > X > V, with a difference of just 1.7 kJ mol⁻¹ between the three most stable forms.

Conclusions

For an analysis of competing HBSs, one has to consider the contributions from internal H-bond and non-H-bond interactions, from the packing of multiple HBS instances and intramolecular energy penalties. A compact HBS based on multiple-point H-bond connections should typically lead to more packing alternatives and ultimately to a larger number of viable low-energy structures than a competing one-point HBS (i.e. dimer vs. catemer). Coulombic interaction energies associated with typical short intermolecular C–H···O contact geometries are small in comparison with dispersion effects associated with the packing complementary molecular shapes.Graphical abstract

A strategy for producing predicted polymorphs: catemeric carbamazepine form V

A computationally assisted approach has enabled the first catemeric polymorph of carbamazepine (form V) to be selectively formed by templating the growth of carbamazepine from the vapour phase onto the surface of a crystal of dihydrocarbamazepine form II.

Cytenamide-formic acid (1/1)

IN THE CRYSTAL STRUCTURE OF THE TITLE COMPOUND [SYSTEMATIC NAME: 5H-dibenzo[a,d]cyclo-hepta-triene-5-carboxamide-meth-anoic acid (1/1)], C(16)H(13)NO·CH(2)O(2), the cytenamide and solvent mol-ecules form a hydrogen-bonded R(2) (2)(8) dimer motif, which is further connected to form a centrosymmetric double-motif arrangement. The asymmetric unit contains two formula units.

Cytenamide-1,4-dioxane (2/1)

IN THE CRYSTAL STRUCTURE OF THE TITLE COMPOUND [SYSTEMATIC NAME: 5H-dibenzo[a,d]cyclo-hepta-triene-5-carboxamide-1,4-dioxane (2/1)], 2C(16)H(13)NO·C(4)H(8)O(2), the cytenamide mol-ecules form a hydrogen-bonded R(2) (2)(8) dimer. The solvent mol-ecule is located between two adjacent cytenamide dimers and forms N-H⋯O hydrogen bonds with one cytenamide mol-ecule from each dimer.

Cytenamide-butyric acid (1/1)

Cytenamide forms a 1:1 solvate with butyric acid [systematic name: 5H-dibenzo[a,d]cyclo-hepta-triene-5-carboxamide-butanoic acid (1/1)], C(16)H(13)NO·C(4)H(8)O(2). The title compound crystallizes with one mol-ecule of cytenamide and one of butyric acid in the asymmetric unit; these mol-ecules are linked by N-H⋯O and O-H⋯O hydrogen bonds to form an R(2) (2)(8) heterodimer motif. Pairs of adjacent motifs are further connected via N-H⋯O inter-actions to form a discrete centrosymmetric assembly.