Quaternary and quinary molecular solids based on structural inequivalence and combinatorial approaches: 2-nitroresorcinol and 4,6-dichlororesorcinol

• Main Authors: Madhu Rajkumar, Gautam R. Desiraju
• Format: Article
• Language: English
• Published: International Union of Crystallography 2021-03-01
• Series: IUCrJ
• Subjects: cocrystal; crystal engineering; supramolecular synthon; intermolecular interaction; multicomponent solid
• Online Access: http://scripts.iucr.org/cgi-bin/paper?S2052252520016589

A synthetic strategy for the formation of stoichiometric quaternary and nonstoichiometric quinary solids is outlined. A series of 2-nitroresorcinol-based quaternary cocrystals were developed from binary precursors in two conceptual stages. In the first stage, ternary solids are synthesized based on the structural inequivalence at two recognition sites in the binary. In the second stage, the ternary is homologated into a stoichiometric quaternary based on the same concept. Any cocrystal without an inequivalence becomes a synthetic dead end. The combinatorial approach involves lower cocrystal systems with different structural environments and preferred synthon selection from a synthon library in solution. Such are the stepping stones for the isolation of higher cocrystals. In addition, a quaternary cocrystal of 4,6-dichlororesorcinol is described wherein an unusual synthon is observed with two resorcinol molecules in a closed loop with two different ditopic bases. The concept of the virtual synthon in binaries with respect to isolated ternaries is validated for the 4,6-dichlororesorcinol system. It is possible that only some binary systems are amenable to homologation into higher cocrystals. The reasons for this could have to do with the existence of preferred synthon modules, in other words, the critical components of the putative higher assembly that cannot be altered. Addition of the third and fourth component might be more flexible, and the choices of these components, possible from a larger pool of chemically related molecules.