Polymorphic ROYalty: The 14th ROY Polymorph Discovered via High-Throughput Crystallization

Abstract

Polymorphism, when a substance can exist in more than one crystalline form yet return to the same liquid or solution phase, is characterized by differences in packing or molecular conformation. Polymorphs often exhibit differing physical properties, and are therefore particularly important in the development of materials and pharmaceuticals. However, gaining a thorough understanding of the solid-state landscape of a molecule requires exhaustive experimental screening of crystallization conditions, a particular challenge when using classical crystallization methods. We show that high-throughput Encapsulated Nanodroplet Crystallization (ENaCt) can enable the rapid and efficient exploration of the solid-state landscape of highly polymorphic molecules, through an in-depth study of 5-methyl-2-((2-nitrophenyl)amino)thiophene-3-carbonitrile (ROY), the most polymorphic small molecule known. An ENaCt screen encompassing 1536 individual crystallization experiments, spanning 320 unique conditions, resulted in direct access to single crystals, suitable for X-ray diffraction analysis, for all six of the known polymorphs accessible from solution (Y, R, YN, ON, ORP and R18). In addition, two polymorphs (Y04 and Y19) previously accessed only via melt and heteroseeded melt experiments, and a new polymorph of ROY (O22) were obtained. Furthermore, ENaCt screening resulted in the identification of the first ROY solvate (ROY· methyl anthranilate) and the first example of a ROY dimer, formed via in situ oxidation. ENaCt is thus shown to be an impactful tool for the experimental mapping of the solid-state landscape of highly polymorphic molecules and, through the discovery of a new polymorph O22, has ensured that tetradecamorphic ROY retains the record for the most polymorphic small molecule.

Figure 1

Molecular structure of 5-methyl-2-((2-nitrophenyl)amino)-3-thiophenecabonitrile (ROY). Key descriptors that relate to intramolecular conjugation in ROY highlighted, the phenyl-thiophene rotatable bonds and their associated torsion angles (τ_SCNC and τ_CNCC) and the phenyl-thiophene mean plane angle (θ, illustrated with the Y polymorph).

Figure 2

Optical microscopy of ROY crystals from ENaCt polymorph screen (top left to bottom right: Y, ON, R, ORP, YN, R18, Y04, Y19).

Figure 3

Optical microscopy of O22 crystals observed from ENaCt polymorph screen (50 mg/mL ROY in DMSO/mineral oil), crystal structure of O22 (anisotropic displacement parameters shown at 50%), and selected crystal structure parameters.

Figure 4

Optical microscopy of ROY dimer crystal observed from ENaCt polymorph screen (14.3 mg/L ROY in 2-nitroethanol/mineral oil), crystal structure of ROY dimer (anisotropic displacement parameters shown at 50%, disordered nitro groups displayed as the major disorder component for clarity), molecular structure and selected crystal structure parameters.

Figure 5

Optical microscopy of ROY/methyl anthranilate observed from seeded ENaCt experiments [50 mg/mL ROY in methyl anthranilate/mineral oil(seeded)], crystal structure of ROY/methyl anthranilate (anisotropic displacement parameters shown at 50%, disordered solvent modeled over two positions as shown), and selected crystal structure parameters.

Figure 6

Optical microscopy of concomitant crystallization of ROY polymorphs, from ENaCt screen. (a) Y and R, (b) Y and ON, (c) R and ON, (d) R and YN.