An exceptional 5:4 enantiomeric structure

Abstract

The only crystals that could be grown from racemic solutions of the PF6(-) salt of the resolvable cation [Ru(2,9-dimethyl-1,10-phenanthroline)2(dipyrido[3,2-d:2',3'-f]quinoxaline)](2+) have translational symmetry only (space group P1), contain nine independent sets of ions, and include numerous independent solvent molecules (11 acetone, one diethyl ether and possibly several water molecules). Layers of hydrophobic cations alternate with layers containing most of the anions and solvent molecules. All nine cations have the same basic conformation, which is distorted by the presence of the methyl substituents on the two 1,10-phenanthroline ligands. Four pairs of enantiomeric cations within a layer are related by approximate inversion centers; the ninth cation, which shows no sign of disorder, makes the layer chiral. Within the cation layers stripes parallel to [110] of six cations alternate with stripes of three; the local symmetry and the cation orientations are different in the two stripes. These stripes are reflected in the organization of the anion/solvent layer. The ca 80:20 inversion twinning found indicates that enantiomeric preference is transmitted less perfectly across the anion/solvent layer than within the cation layer. The structure is exceptional in having nine independent formula units and an unbalanced set (ratio 4:5) of resolvable enantiomers. The difficulty in growing crystals of this material is consistent with its structural complexity.

Non-equimolar discrete compounds in binary chiral systems of organic substances

“…That is why it does not encompass compounds containing solvent molecules. For example, E. Wachter et al 96 recently described the crystal structure of a PF 6…”

“…According to statistical data, 96 the occurrence of nonequimolar discrete compounds among substances with chiral molecules is negligible, being 1 : 100 000. If so, is their study practical?…”

Disclosing the Rich Crystal Chemistry of Lesinurad by ab Initio Laboratory X-ray Powder Diffraction Methods

“…As these tablets are intended for oral dosage, the solubility of the active pharmaceutical ingredient in the gastric environment (in both thermodymamic and kinetic aspects) may then be affected by a number of physicochemical parameters, including crystal form, morphology, crystal size, degree of crystallinity, and intergrain interactions with excipients and the environment. This is particularly relevant for molecules showing non-interconverting enantiomers (here, of the atropoisomeric type), which, though being processed as 1:1 racemic forms, may further crystallize as a conglomerate of enantiomorphic crystals or even as unbalanced racemates . Thus, starting from material preparation, isolation, processing, and formulation, up to chemical and thermal stability, bioavailability, and pharmacokinetic issues, the strict control of the solid state molecular aggregation type becomes a fundamental question to be answered, particularly when the number of crystal forms is large, and the pertinent energy landscape is highly corrugated.…”

High-Z′ structures of organic molecules: their diversity and organizing principles

“…Based on previous experience in this laboratory (e.g. Koutentis et al, 2001;Bakus et al, 2013;Hao et al, 2005;Wachter et al, 2016) it seemed likely that both frustration and nucleation can be important factors but that the diversity of high-Z 0 structures is too great to allow for any broad generalizations.…”