Ram K. R. Jetti

Crystal Engineering: Some Further Strategies

Structural studies currently underway in our group are reported in this paper. Molecular symmetry is rarely carried over into the crystal, posing problems when high-symmetry networks are desired. This is illustrated by the low-symmetry structure of 2,4,6-trinitromesitylene. However, the involvement of the Cl3 supramolecular synthon ensures a hexagonal network structure for 2,4,6-tris-(4-chlorophenoxy)-1,3,5-triazine. Arguments following from the equivalence between molecular and supramolecular synthons lead to the tetragonal network structure of the 1:1 complex of tetraphenylmethane and CCl4. With a similar reasoning, 4-(triphenylmethyl)benzoic acid is identified as a precursor of a supramolecular wheel-and-axle host substance. The study of novel and weaker intermolecular interactions is often useful. In N,N′-dibenzyl-1,4-cubanedicarboxamide, the acidity of the cubyl C-H groups leads to the formation of C-H···O hydrogen bonds. Polymorphism is a difficult challenge for the crystal engineer and, in its most intriguing manifestation, two crystalline forms of a substance appear in the same crystallization batch. This is observed for 4,4-diphenyl cyclohexadienone. The ultimate frontier in the subject is an understanding of the phenomenon of crystallization, and the unexpected crystal structure of quinoxaline, with five symmetry-independent molecules, could possibly represent a case of arrested crystallization.

2,4,6-tris-4-(Bromophenoxy)-1,3,5-triazine : A hexagonal host framework assembled with robust Br...Br trimer synthons

Abstract Based on the trigonal symmetry of the title triazine 1 and the Br3 supramolecular synthon, a hexagonal host system has been designed and shown to include hexachlorobenzene, 2,4,6-collidine, 1-methylnaphthalene and mesitylene guests in its porous structure. In the 1.hexachlorobenzene (2:1) inclusion clathrate 2, the guest molecule is ordered and bonded to the host through Cl⋯O interactions. In the channel structures 3, 4 and 5, the solvent molecules are disordered and the host-guest stoichiometry is 1:1. These supramolecular structures highlight the importance of weak hydrogen bonds and heteroatom interactions in the self-assembly of target networks. The generality of the new host system is also demonstrated.

4-(triphenylmethyl)benzoic acid: a supramolecular wheel-and-axle host compound

The title compound crystallises from several aromatic solvents to produce wheel-and-axle host-guest complexes. The crystal structures and inclusion properties of the xylene and chlorobenzene solvates are discussed.

Supramolecular equivalence of halogen, ethynyl and hydroxy groups. A comparison of the crystal structures of some 4-substituted anilines

Chloro, bromo and ethynyl substituents play exactly the same role in the crystal structures of the corresponding 4-substituted anilines and this is related to their similar polarisations. The iodo derivative is, however, distinct and this may be related to its greater size.

Three-Dimensional Quantitative Structural Activity Relationship (3D-QSAR) Studies of Some 1,5-Diarylpyrazoles: Analogue Based Design of Selective Cyclooxygenase-2 Inhibitors

Selective cyclooxygenase inhibitors have attracted much attention in recent times in the design of new non-steroidal anti-inflammatory drugs (NSAID). 3D-QSAR studies have been performed on a series of 1,5-diarylpyrazoles that act as selective cyclooxygenase-2 (COX-2) inhibitors, using three different methods: comparative molecular field analysis (CoMFA) with partial least squares (PLS) fit; molecular field analysis (MFA) and; receptor surface analysis (RSA) with genetic function algorithms (GFA). The analyses were carried out on 30 analogues of which 25 were used in the training set and the rest considered for the test set. These studies produced reasonably good predictive models with high cross-validated and conventional r2 values in all the three cases.

Halogen trimer synthons in crystal engineering: low-temperature X-ray and neutron diffraction study of the 1:1 complex of 2,4,6-tris(4-chloro- phenoxy)-1,3,5-triazine with tribromobenzene

The title complex has been studied using low-temperature X-ray (150 K) and neutron (100 K) diffraction. Molecules of the triazine host form a two-dimensional hexagonal network mediated by trigonally symmetric Cl(3) synthons having Cl.Cl interactions of 3.441 (3) A, a C-Cl.Cl angle of 165 degrees and a Cl.Cl-C angle of 105 degrees, close to the ideal values of 180 and 90 degrees, respectively. The guest molecules are of an appropriate size to fit the hexagonal networks and interact with the host via C-H.pi (phenyl) and C-Br.pi (phenyl) interactions which stabilize the overall structure. Both C-donor bond vectors are directed more closely towards the mid-point (X) of an individual aromatic bond, rather than the ring centroid, with H.X 2.817 (9) A and C-H.X 174.0 (9) degrees, and Br.X 3.353 (4) A and C-Br.X 158.1 (2) degrees.

Polar host–guest assembly mediated by halogen⋯π interaction: inclusion complexes of 2,4,6-tris(4-halophenoxy)-1,3,5-triazine (halo = chloro, bromo) with trihalobenzene (halo = bromo, iodo)

Crystallization of four isomorphous host–guest complexes in the polar space group P63 is ascribed to the recurring halogen(guest)⋯π(host) interaction.

Proton transfer and N(+)–H⋯S(−) hydrogen bonds in the crystal structure of 4-aminothiophenol

4-Aminothiophenol exists as 4-ammonio-1-benzenethiolate in the solid and liquid state. The crystal structure is characterised by a tetrahedral beta-As type network which is the driving force for the proton transfer.

Hexagonal Host Framework of sym-Aryloxytriazines Stabilised by Weak Intermolecular Interactions

ABSTRACT 2,4,6-Tris(4-halophenoxy)-1,3,5-triazine 1 is a convenient C3 starting material for the self-assembly of hexagonal open frameworks mediated via the halogen…halogen trimer synthon and the π-stacked Piedfort Unit (PU). We examine in this paper crystal structures of 2,4,6-tris(2-iodo-3-pyridyloxy)-1,3,5-triazine 2 , 2,4,6-tris(3-iodophenoxy)-1,3,5-triazine 3 , 2,4,6-tris(6-methyl-3-pyridyloxy)-1,3,5-triazine 4 , and 2,4,6-tris[4-(4′-bromophenyl) phenoxy)]-1,3,5-triazine 5 . Triazine 2 forms isostructural 2:1 host⋅guest adducts (guest = mesitylene, collidine) in the rhombohedral space group R3¯ such that the host architecture is stabilised by the C3i-PU and a helix of C─H…N interactions. The crystal structure of 3 is different from its chloro/bromo derivatives signifying the importance of the more polarisable I atom compared to Cl, Br. Pairs of C─H…O and C─H…N hydrogen bonds and C3i-PU sustain the columnar structure of 3 (space group R3¯). The PU has pseudo trigonal symmetry in picolinoxy triazine 4 (space group P21/n). In contrast to the phenyl derivatives, the extended aryl arms in biphenyl 5 do not adopt a trigonal conformation: two biphenyl groups are oriented parallel that participate in Br…Br and Br…π interactions. We note that 1 and 2 readily form hexagonal host lattices for guest inclusion, while 3 , 4 , and 5 crystallise in solvent-free form. Thermal measurements (TGA, DSC) indicate that guest release occurs at a higher temperature in the cage type host⋅guest clathrates compared to the channel inclusion compounds for the same solvent. Statistics from the Cambridge Structural Database using CSD Symmetry show that the phenoxytriazine scaffold is unique among the trigonal molecules for the carry-over of symmetry relation from molecule to crystal. The ease of predicting crystal packing and space group in this family of compounds ( 1 , 2 ) makes them good candidates for the crystal engineering of host frameworks.

Crystal Structures and Packing of 2,4,6-tris(4-Fluorophenoxy)-1,3,5-triazine and 2,4,6-tris(3,4-Dimethylphenoxy)-1,3,5-triazine. New Materials for Octupolar Nonlinear Optics

The crystal structures and packing of 2,4,6-tris(4-fluorophenoxy)-1,3,5-triazine and 2,4,6-tris(3,4-dimethylphenoxy)-1,3,5-triazine are discussed. These structures have been determined as a continuation of a series of octupolar NLO materials we have been investigating. The crystal structures are characterized by C–H...F and C–H...π hydrogen bonds, respectively. A characteristic of these triazine structures is the presence of dimeric Piedfort Units (PU) that are extended into more elaborate two-dimensional (2-D) networks. The structure of the fluoro derivative is compared with that of the corresponding unsubstituted and chloro/bromo-substituted derivatives. The structure of the dimethyl triazine is compared with that of the corresponding 4-methyl derivative. The noncentrosymmetric nature of the dimethyl derivative was confirmed by a powder SHG signal at 1.064 μm of the order of ∼0.5 × KDP. Interestingly, the dimethyl derivative studied here is isostructural with the corresponding 4-methyl triazine. This H/Me isostructurality is shown to be an uncommon phenomenon by an analysis with the CSD.