Morphological evolution of two-dimensional porous hexagonal trimesic acid framework

Hexagonal single crystal structure (Form II) of trimesic acid (TMA) has been isolated by dissolving the interpenetrated Form I of TMA in tetrahydrofuran. Form II (hexagonal) was converted to Form I (interpenetrated) at room temperature through some intermediate structures. A detailed time-dependent FESEM study shows that the external morphology of Form II (hexagonal) is a hollow hexagonal tube that mimics its crystal structure. The block-shaped (morphology) of Form I (interpenetrated) was converted to the hollow hexagonal tube through some intermediate morphologies which are corresponding to particular crystal structures. Here, we have established a strong correlation between crystal structures with the morphology. These hollow tubes have been employed for Rhodamine B dye adsorption studies and showed an uptake of 82%, much more significant than Form I (interpenetrated) (39%) due to the presence of a pore channel in the crystal structure

Thermal Behavior of Benzoic Acid/Isonicotinamide Binary Cocrystals

YesA comprehensive study of the thermal behavior of the 1:1 and 2:1 benzoic acid/isonicotinamide cocrystals is reported. The 1:1 material shows a simple unit cell expansion followed by melting upon heating. The 2:1 crystal exhibits more complex behavior. Its unit cell first expands upon heating, as a result of C–H···π interactions being lengthened. It then is converted into the 1:1 crystal, as demonstrated by significant changes in its X-ray diffraction pattern. The loss of 1 equiv of benzoic acid is confirmed by thermogravimetric analysis–mass spectrometry. Hot stage microscopy confirms that, as intuitively expected, the transformation begins at the crystal surface. The temperature at which conversion occurs is highly dependent on the sample mass and geometry, being reduced when the sample is under a gas flow or has a greater exposed surface area but increased when the heating rate is elevated

Trapping virtual pores by crystal retro-engineering

Stable guest-free porous molecular crystals are uncommon. By contrast, organic molecular crystals with guest-occupied cavities are frequently observed, but these cavities tend to be unstable and collapse on removal of the guests—this feature has been referred to as ‘virtual porosity’. Here, we show how we have trapped the virtual porosity in an unstable low-density organic molecular crystal by introducing a second molecule that matches the size and shape of the unstable voids. We call this strategy ‘retro-engineering’ because it parallels organic retrosynthetic analysis, and it allows the metastable two-dimensional hexagonal pore structure in an organic solvate to be trapped in a binary cocrystal. Unlike the crystal with virtual porosity, the cocrystal material remains single crystalline and porous after removal of guests by heating

Nucleophilic Substitution at a Coordinatively Saturated Five-Membered NHC∙Haloborane Centre

In this paper, we have used a saturated five-membered N-Heterocyclic carbene (5SIDipp = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidine) for the synthesis of SNHC-haloboranes adducts and their further nucleophilic substitutions to put unusual functional groups at the central boron atom. The reaction of 5-SIDipp with RBCl2 yields Lewis-base adducts, 5-SIDipp·RBCl2 [R = H (1), Ph (2)]. The hydrolysis of 1 gives the NHC stabilized boric acid, 5-SIDipp·B(OH)3 (3), selectively. Replacement of chlorine atoms from 1 and 2 with one equivalent of AgOTf led to the formation of 5-SIDipp·HBCl(OTf) (4) and 5-SIDipp·PhBCl(OTf) (5a), where all the substituents on the boron atoms are different. The addition of two equivalents of AgNO3 to 2 leads to the formation of rare di-nitro substituted 5-SIDipp·BPh(NO3)2 (6). Further, the reaction of 5-SIDipp with B(C6F5)3 in tetrahydrofuran and diethyl ether shows a frustrated Lewis pair type small molecule activated products, 7 and 8

N-H center dot center dot center dot O and C-H center dot center dotcenter dot O interaction mimicry in the 1:1 molecular complexes of 5,5-diethylbarbituric acid with urea and acetamide

The analogy between N-H center dot center dot center dot O and C-H center dot center dot center dot O intermolecular interactions is studied with variable temperature (180-100 K) single crystal X-ray diffraction analysis.5,5-Diethylbarbituric acid (barbital) forms isostructural molecular complexes (co-crystals) with urea (1) and acetamide (2) that respectively contain these analogous interactions.The behaviour of these two interactions as a function of temperature is very similar. This indicates that the C-H center dot center dot center dot O bond in barbital acetamide plays a similar chemical and structural role as does the N-H center dot center dot center dot O bond in barbital urea. The close relationship between these interactions and their comparable nature is further adduced from the formation of a ternary solid solution (3) of barbital, urea and acetamide. The fact that the C-H center dot center dot center dot O interaction in barbital acetamide is weaker than the N-H center dot center dot center dot O interaction in barbital urea is shown by the fact that acetamide is under expressed and urea is over expressed with respect to the quantities of these substances present in solution prior to crystallization of these ternary crystals

Triclabendazole: An Intriguing Case of Co-existence of Conformational and Tautomeric Polymorphism

The crystal polymorphism of the anthelmintic drug, triclabendazole (TCB), is described. Two anhydrates (Forms I and II), three solvates, and an amorphous form have been previously mentioned. This study reports the crystal structures of Forms I (1) and II (2). These structures illustrate the uncommon phenomenon of tautomeric polymorphism. TCB exists as two tautomers A and B. Form I (Z'=2) is composed of two molecules of tautomer A while Form II (Z'=1) contains a 1:1 mixture of A and B. The polymorphs are also characterized by using other solid-state techniques (differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), PXRD, FT-IR, and NMR spectroscopy). Form I is the higher melting form (m.p.: 177 degrees C, Delta Hf=approximate to 105 +/- 4 Jg-1) and is the more stable form at room temperature. Form II is the lower melting polymorph (m.p.: 166 degrees C, Delta Hf=approximate to 86 +/- 3 Jg-1) and shows high kinetic stability on storage in comparison to the amorphous form but it transforms readily into Form I in a solution-mediated process. Crystal structure analysis of co-crystals 3-11 further confirms the existence of tautomeric polymorphism in TCB. In 3 and 11, tautomer A is present whereas in 4-10 the TCB molecule exists wholly as tautomer B. The DFT calculations suggest that the optimized tautomers A and B have nearly the same energies. Single point energy calculations reveal that tautomer A (in Form I) exists in two low-energy conformations, whereas in Form II both tautomers A and B exist in an unfavorable high-energy conformation, stabilized by a five-point dimer synthon. The structural and thermodynamic features of 1-11 are discussed in detail. Triclabendazole is an intriguing case in which tautomeric and conformational variations co-exist in the polymorphs

Crystal Engineering in the Desiraju Research Group in Bangalore

Over the years, crystal engineering has transformed into a mature and multidisciplinary subject. New understanding, challenges, and opportunities have emerged in the design of complex structures and structure-property evaluation. Revolutionary pathways adopted by many leaders have shaped and directed this subject. In this short essay to celebrate the 60th birthday of Prof. Gautam R. Desiraju, we, his current research group members, contemplate the development of some of the topics explored by our group in the context of the overall subject. These topics, though not entirely new, are of significant interest to the crystal engineering community