Switching polymorph stabilities with impurities provides a thermodynamic route to benzamide form III

Almost 200 years ago, benzamide was reported as polymorphic with two of its forms (II and III) found to be difficult to crystallise. In a recent study, it was shown that benzamide form I can easily convert into benzamide form III using mechanochemistry in the presence of nicotinamide. Here we show, experimentally and computationally, that this transformation is the result of a thermodynamic switch between these two polymorphic forms driven by the formation of solid solutions with small amounts of nicotinamide. The presence of nicotinamide in the crystallisation environment promotes the robust and exclusive crystallisation of the elusive form III. These results represent a promising route to the synthesis and utilisation of elusive polymorphs of pharmaceutical interest.


Materials
Benzamide (BZM) and Nicotinamide (NCM) were purchased from Sigma Aldrich ( ≥ 99 %, United Kingdom) and used without further purification. Deuterated acetone (acetone D6, 99.8%) was purchased from VWR International Ltd. and used without any further purification for NMR analysis.

Crystal structure models
The crystal structures of benzamide forms I and III were retrieved from the Cambridge Structural Database (CSD refcodes BZAMID05 1 and BZAMID08 2 respectively). Basic crystallographic information for these two forms is provided in Supplementary Table 2. Supplementary

Geometry optimisations
All crystal lattices were subjected to the same optimisation procedure using the software VASP (version 5.3.3 3-6 ). For this, the PBE functional 7 with PAW pseudopotentials 8,9 was used together with the Grimme's van der Waals corrections. 10 For the planewaves, a kinetic energy cut-off of 520 eV was used. The Brillouin zone was sampled using the Monkhorst-Pack approximation 11 using k-points separated by approximately 0.05 Å (see Supplementary Table 3). For cell lengths of 5-6 Å four K-points were used in such direction, for cell lengths of ~9-11 Å two K-points and for cell lengths of > 20 Å one The optimisation cycle performed consisted of two steps: i) a full geometry optimisation allowing for the unit-cell parameters to change followed by, ii) a geometry optimisation with the unit cell fixed.
Structural relaxations were halted when the calculated force on every atom was less than 0.003 eV Å -1 .
The energy obtained from this process is the electronic energy of the supercell being simulated (E e supercell).

Calculation of gas-phase energy of BZM and NCM
A single molecule of BZM was placed in a 20 Å x 20 Å x 20 Å supercell. The molecule was then allowed to geometry optimise freely (the cell parameters being fixed). The VASP energy model described above was used and the electronic energy of BZM was calculated (E e BZM).
For NCM (configuration A), the same process was repeated and thus the electronic energy of a single NCM molecule was calculated (E e NCM).

Calculation of lattice energies
The lattice energies of forms I and III with its various levels of NCM incorporated were calculated from the electronic energies of the supercell and the molecules in the gas-phase using the equation

Isopropanol based slurries
All slurry experiments were conducted using a total amount of solid mixture of 2. were stirred continuously for 1 week by magnetic stirrers to ensure that solid-liquid equilibrium is achieved. After a week the slurries were filtered under vacuum and the resulting powder was immediately characterised via PXRD.

Ethanol based slurries
Slurries were performed in a jacketed vessel at 25 °C and at 45°C in 10 g of ethanol using a total load of 5 g of solid.

Optical Microscopy
Samples from slurries and crash cooling were analysed using Zeiss Axioplan 2 microscope and images were obtained using the INFINITY Analyse and Capture Software version 6.5.6.

BZM
The crystalline structures of BZM powders/crystals were identified by comparing the experimental PXRD patterns with the calculated patterns from the single crystal structures obtained from CSD: BZAMID05 (form I) 1 and BZAMID08 (from III) 2 .

NCM
The crystalline structure of commercial NCM powder was confirmed by comparing the experimental PXRD pattern with the calculated patterns from the single crystal structures obtained from CSD: NICOAM05 (form I) 15 .

Crystal packing comparison of Form I and Form III
A crystal structure comparison of BZM I and III has been carried out using the XPac procedure 16 using the whole molecule of BZM I as the common ordered set of points (COSP). Results of the comparison, including the dissimilarity index (χ), are reported in Supplementary Table 6. Supplementary Figure 2 shows the structural similarity and a description of the crystal packing of BZM I and III.

Neat grinding
Supplementary Figure 6 shows a comparison of the PXRD patterns of BZM obtained from neat grinding with BZM form I and BZM form III patterns calculated from the single crystal-structures (BZAMID05 1 and BZAMID08 2 , respectively). The results clearly show that neat grinding of pure BZM promotes the conversion of BZM form I to BZM form III.
Supplementary Figure 6. The PXRD pattern of BZM exposed to neat grinding, and its comparison to BZM form I and BZM form III patterns obtained from CSD.

Slurries in isopropanol
In order to ascertain whether a different solvent also promotes the conversion of BZM form I to BZM form III in the presence of NCM, slurry experiments in isopropanol were carried out. Supplementary

Crash Cooling
Supplementary Figure 15 shows PXRD patterns of samples obtained from crash cooled solutions of BZM in the presence of 10%, 20% and 30 % of NCM (wt.%). In all cases the resulting PXRD pattern shows a good fit with that of BZM form III.
Supplementary Figure 15. PXRD patterns of crash cooling samples obtained from mixtures of BZM and NCM in isopropanol.

Incorporation levels/ Segregation Coefficients
Supplementary Figure 16 shows the differences between the incorporation of NCM in the crystal lattice BZM during slurry and crash cooling experiments in isopropanol. Higher incorporations of NCM occur during slurry experiments, corresponding to a higher segregation coefficient, which is indicated by the slope of the trendline.
Supplementary Figure 17 demonstrates the differences between the incorporation of NCM in BZM form III and BZM form I at slurring conditions (isopropanol slurries). It was observed that more NCM incorporates in BZM form III lattice than in form I lattice at equilibrium conditions. This is also demonstrated by the estimated segregation coefficients, being 0.8 for NCM in BZM form III and 0.7 for NCM in BZM form I, which are however very close.

Optical Microscopy images
Supplementary Figures 18 and 19 show microscopy images of crystallites obtained from slurries in isopropanol and crash cooling crystallisation, respectively. The initial solution concentration for both samples had 20 wt.% NCM (with respect to total solid added).
Supplementary Figure 18. Optical microscopy image of crystallites obtained from slurry in isopropanol doped with 20wt.% NCM [initial concentration added to the solution -corresponding to 16% NCM in the crystal -BZM form III solid solution].