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Switching from positive (PTE) to negative thermal expansion (NTE) in a molecular crystal is a rare phenomenon. Here we report the switching of PTE to NTE in 1,1′-(octa-1,3,5,7-tetrayne-1,8-diyl)dicyclopentanol (1) along the crystallographic a-axis. Small PTE has been observed from 100 K to 200 K due to zigzag packing arrangement and then the large transverse vibration overpowers the PTE resulting in NTE after 200 K. [ABSTRACT FROM AUTHOR]

Mechanical properties of multicomponent compounds are largely dependent upon the intermolecular interactions between the components and obviously on the overall crystal packing. Herein, we have shown the modulation of mechanical properties of two cocrystals between pyrogallol (PG) and isonicotinamide (INMI) by altering the stoichiometry of the components. One of the cocrystals shows elasticity and the other shows brittleness. Analysis of the crystal structures of the cocrystals revealed the difference in the intermolecular interactions and crystal packing, leading to different mechanical properties. [ABSTRACT FROM AUTHOR]

A dearomative [3+2]-cycloaddition reaction of oxindole-embedded azaoxyallyl cations with indoles at the C3 position has been developed. The use of this new class of azaoxyallyl cation species in the reaction permits access to more-elaborate hexahydropyrrolo[2,3- b ]indole moieties that contain a spiro-oxindole ring. The transformation displays a broad substrate scope and good regio- and stereoselectivity for the cycloaddition step. Several observations suggested that this class of azaoxyallyl cations can display a different reactivity pattern from those of commonly employed azaoxyallyl cation systems. [ABSTRACT FROM AUTHOR]

Variable temperature single crystal X-ray diffraction (VT-SCXRD) study reveals an unusual thermal expansion behaviour of two new co-crystals prepared by solvent drop grinding using a flexible ligand [1,4-bis[(2-methylimidazol-1-yl)methyl]benzene] (BIMB) and aromatic carboxylic acids, viz. terephthalic acid (TA) and 4,4′-biphenyl dicarboxylic acid (BPDCA). The analysis of the crystal structures of the co-crystals using VT-SCXRD shows that the tiltation of the carboxylic acid molecules along a certain crystallographic axis induces uniaxial negative thermal expansion (NTE). In this study, supramolecular interactions, such as O–H⋯N, C–H⋯O, C–H⋯π, π⋯π, and van der Waals interactions, play an important role in the biaxial positive thermal expansion (PTE) in both the co-crystals. [ABSTRACT FROM AUTHOR]

The separation of xylene isomers from their mixture is a challenging task. The present study describes the highly selective inclusion of p-xylene (p-XY) from a mixture of xylene isomers by three novel organic bis-hydrazone compounds, ethanedial-1,2-bis(4,4′-dichlorobenzophenone hydrazone) (1), ethanedial-1,2-bis(4,4′-dibromobenzophenone hydrazone) (2) and ethanedial-1,2-bis(4,4′-diiodobenzophenone hydrazone) (3). These van der Waals host compounds specifically enclathrate p-XY from a mixture of xylene isomers. Crystallization of the host compounds in all possible mixtures of xylene isomers, X-ray diffraction (both single-crystal and powder) and thermogravimetric analysis (TGA) confirm the selective inclusion property of the bis-hydrazone compounds. Compounds 1, 2 and 3 might be suitable candidates for the separation of p-XY from a mixture of xylene isomers. [ABSTRACT FROM AUTHOR]

Design of material showing contraction upon heating is highly challenging due to varying mechanism. However, imidazole is found to be a potential molecule that may provide low CTE materials when incorporated in the matrix. Here we have reported thermal expansion property of imidazolium salts of five aliphatic α, ω‐alkane dicarboxylic acids and three aromatic acids. Either uniaxial or biaxial negative thermal expansion (NTE) has been observed in most of the salts. In some cases, axial zero thermal expansion (ZTE) has been observed. The role of imidazolium moiety for the anomalous thermal expansion behaviour of the salts has been analyzed in this study. The controlled TE behaviour of the salts is attributed to the hydrogen bonding and transverse vibration in all imidazolium salts. Owing to the high transverse vibration observed in imidazolium ion as well as the heavier oxygen atoms of acids in each case, the distance between hydrogen bonded atoms decreases–which provides either low expansion or contraction along one of the principal axes. [ABSTRACT FROM AUTHOR]

Structures and properties of host-guest complexes of hexa-substituted benzene derivatives are studied. The preparation of four polymorphs of hexa-host by melt crystallization is reported.