ROY revisited, again: the eighth solved structure

Abstract

X-ray powder diffraction and crystal structure prediction (CSP) algorithms were used in synergy to establish the crystal structure of the eighth polymorph of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), form R05. R05 crystallizes in the monoclinic space group P21 with lattice parameters a = 11.479(4) Å, b = 11.030(1) Å, c = 10.840(6) Å, β = 118.23(1)°. This is both the first acentric ROY polymorph, and the first with Z' > 1. The torsion angles defined by the S-C-N-C atom sequence of each molecule in the asymmetric unit (R05-1 and R05-2) are 44.9° and -34.0°. These values are between those previously determined for the red and orange forms of ROY. The crystal packing and intermolecular interactions in R05 are explained herein through Hirshfeld surface analysis and an updated energy stability ranking is determined using computational methods. Although the application of CSP was critical to the structure solution of R05, energy stability rankings determined using a series of DFT van der Waals (vdW)-inclusive models substantially differ from experiment, indicating that ROY polymorphism continues to be a challenge for CSP.

Overcoming the difficulties of predicting conformational polymorph energetics in molecular crystals via correlated wavefunction methods

“…42 However, as multiple recent studies have noted, predicting the energetics of these polymorphs has proved challenging. 41,42,133 Many well-regarded van der Waals-inclusive GGA and hybrid density functionals predict highly incorrect polymorph orderings, including PBE-D3, PBE-NP, optPBE-vdW, PBE + MBD, and PBE0 + MBD (Fig. 5).…”

“…5). 41,42 Most strikingly, the DFT calculations frequently suggest that the Y polymorph is one of the least stable forms, when it is actually the most stable polymorph experimentally. Inclusion of zero-point energies or thermal contributions does not correct the rankings, either.…”

“…40 The prolic polymorph-former 5-methyl-2-[(2nitrophenyl)amino]-3-thiophenecarbonitrile, nicknamed "ROY" aer its colorful red-orange-yellow crystals, is another example. State-of-the-art density functional models predict the Y polymorph to be one of the least stable forms, 41,42 when it is actually the most stable one. These backwards stability rankings reect errors approaching 10 kJ mol À1 .…”

“…46 Many functionals erroneously predict the exothermic anthracene photodimerization reaction to be strongly endothermic, 47,48 which is problematic 49 when studying a class of interesting anthracene-based photomechanical materials. 50,51 Switching to a hybrid density functional can address some of the limitations of GGA-type functionals that cause incorrect polymorph rankings and other problems, 20,28,45,46,52,53 but it does not rectify the incorrect ROY polymorph rankings, 41 for example. Approaches based on periodic second-order Møller-Plesset perturbation theory (MP2), 54-68 the random phase approximation (RPA), [68][69][70][71] and quantum Monte Carlo 72-75 are also being developed that can improve the reliability of polymorph stability rankings.…”

Color polymorphism in organic crystals

“…In this paper, one will focus on the six most representative families of compounds displaying color polymorphism, whose most famous members are represented in Fig. 2: N-(4-methyl-2-nitrophenyl) acetamide [20][21][22][23][24][25][26] , dimethyl 2,5-dichloro-3,6-dihydroxyterephthalate [27][28][29][30][31][32][33][34][35] , 5-methyl-2-((2-nitrophenyl)amino)thiophene-3-carbonitrile (ROY) [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] , 2,4,6-trinitro-N-(p-tolyl)aniline [54][55][56][57][58][59][60][61] , dimethyl-5-benzoyl-3-phenylindolizine-1,2-dicarboxylate 14 , and 1,4,7,10-tetrabutyltetracene [62][63][64] .…”