Parthasarathi Dastidar

Supramolecular gelling agents: can they be designed?

The last two decades have witnessed an upsurge of research activities in the area of supramolecular gelators, especially low molecular mass organic gelators (LMOGs), not only for academic interests but also for their potential applications in materials science. However, most of the gelators are serendipitously obtained; their rational design and synthesis is still a major challenge. Wide structural diversities of the molecules known to act as LMOGs and a dearth of molecular level understanding of gelation mechanisms make it difficult to pin-point a particular strategy to achieve rational design of gelators. Nevertheless, some efforts are being made to achieve this goal. Once a gelling agent is serendipitously obtained, new gelling agents with novel properties may be prepared by modifying the parent gelator molecule following a molecular engineering rationale; however, such approach is limited to the same class of gelling agent generated from the parent gelating scaffold. A crystal engineering approach wherein the single-crystal structure of a molecule is correlated with its gelling/nongelling behaviour (structure-property correlation) allows molecular level understandings of the self-assembly of the gelator and nongelator molecules and therefore, provides new insights into the design aspects of supramolecular gelling agents. This tutorial review aims at highlighting some of the developments covering both molecular and crystal engineering approaches in designing LMOGs.

An Easy To Prepare Organic Salt as a Low Molecular Mass Organic Gelator Capable of Selective Gelation of Oil from Oil/Water Mixtures

Besides its capability to gelate various organic solvents, dicyclohexyl ammonium 4-chlorocinnamate 1 is shown to perform selective gelation of oil from oil/water mixtures. While the corresponding 3-chloro derivative 2 is an excellent gelator of nonpolar organic solvents, 2-chloro derivative 3 turned out to be a nongelator.

Hydrogen bonded supramolecular network in organic salts: crystal structures of acid–base salts of dicarboxylic acids and amines

Four new organic salts, namely bis(2-aminopyridinium) fumarate–fumaric acid (1∶1) 1, benzylammonium hydrogen fumarate 2, 2-aminopyridinium hydrogen cyclobutane-1,1-dicarboxylate 3 and benzylammonium hydrogen cyclobutane-1,1-dicarboxylate 4 have been synthesized and characterized by FT-IR, 1H NMR and single crystal X-ray crystallography in order to analyse their supramolecular structures. While 1, 2 and 4 form two-dimensional layer structures, 3 displays a one-dimensional network. The role of stronger O–H⋯O and N–H⋯O interactions and weak C–H⋯O interactions on the supramolecular arrangement have been discussed based on the single crystal X-ray structures.

Gel Sculpture: Moldable, Load‐Bearing and Self‐Healing Non‐Polymeric Supramolecular Gel Derived from a Simple Organic Salt

An easy access to a library of simple organic salts derived from tert-butoxycarbonyl (Boc)-protected L-amino acids and two secondary amines (dicyclohexyl- and dibenzyl amine) are synthesized following a supramolecular synthon rationale to generate a new series of low molecular weight gelators (LMWGs). Out of the 12 salts that we prepared, the nitrobenzene gel of dicyclohexylammonium Boc-glycinate (GLY.1) displayed remarkable load-bearing, moldable and self-healing properties. These remarkable properties displayed by GLY.1 and the inability to display such properties by its dibenzylammonium counterpart (GLY.2) were explained using microscopic and rheological data. Single crystal structures of eight salts displayed the presence of a 1D hydrogen-bonded network (HBN) that is believed to be important in gelation. Powder X-ray diffraction in combination with the single crystal X-ray structure of GLY.1 clearly established the presence of a 1D hydrogen-bonded network in the xerogel of the nitrobenzene gel of GLY.1. The fact that such remarkable properties arising from an easily accessible (salt formation) small molecule are due to supramolecular (non-covalent) interactions is quite intriguing and such easily synthesizable materials may be useful in stress-bearing and other applications.

Metal–organic frameworks derived from bis-pyridyl-bis-amide ligands : Effect of positional isomerism of the ligands, hydrogen bonding backbone, counter anions on the supramolecular structures and selective crystallization of the sulfate anion

Three new metal–organic frameworks, namely [Co(μ-L1)2(Cl)2]n1, [Cd(µ-L1)2(NO3)2]n2 and [{Co(μ-L1′)(H2O)4}.SO4·3(H2O)]n3 (L1 = N,N′-bis-(4-pyridyl)isophthalamide, L1′ = N,N′-bis-(3-pyridyl)isophthalamide) have been synthesized and characterized. The single crystal structures of 1–3 and the free ligand L1 are discussed in the context of the effect of positional isomerism of the ligands, hydrogen bonding backbone and counter anions on the supramolecular structural diversities observed in these MOFs. Selective crystallization of the sulfate anion from a mixture of L1′, CoSO4, Co(NO3)2, Co(ClO4)2, Co(OAc)2, Co(BF4)2 was evident from the isolation of 3 which was confirmed by powder X-ray diffraction, elemental analysis and IR data.

Hydrogen-bonded microporous network, helix and 1-D zigzag chains in MOFs of Zn(II): studying the effects of ligating topologies, hydrogen bonding backbone and counter-anions

Effects of hydrogen bonding backbone, ligating topology and counter-anions on the resultant framework topologies of a series of metal–organic frameworks (MOFs) derived from two hydrogen bond functionalized ligands, namely N,N′-bis(4-pyridyl)urea L1 and N,N′-bis(3-pyridyl)urea L2 (which are positional isomers) and Zn(II) salts having different counter-anions such as chloride, acetate and sulfate have been studied using single crystal X-ray diffraction techniques. The counter-anions seem to play major role in controlling the network topologies in the MOFs derived from L1 having linear ligating topology whereas the flexible ligating topology of the ligand appears to have much influence on the resultant networks of the MOFs derived from L2. The complementary hydrogen bonding interactions of the urea backbone is not observed in any of these MOFs. However, urea functionality efficiently binds the counter-anions/solvents in these structures and contributes significantly to shaping the hierarchical supramolecular structure formation. The structures of [{Zn(L1)(SO4)}·EG·3H2O]n (3) and [{Zn(L2)(Cl)2]n (4) are especially interesting. While 3 showed robust microporous architecture even after removal of guest molecules as revealed by XRPD data, 4 showed 2-D network of left-handed helix sustained by N–H⋯Cl interactions. The microporosity of 3 has been studied by N2 gas sorption

Zn(II) metal–organic frameworks (MOFs) derived from a bis-pyridyl-bis-urea ligand: effects of crystallization solvents on the structures and anion binding properties

The Reaction of N,N′-bis-(3-pyridyl)ethylene-bis-urea (BPEBU) with ZnSO4 in 2:1 ligand:metal ratio in various crystallization solvents afforded four metal–organic frameworks (MOFs), namely [{Zn(H2O)3(SO4)(µ-BPEBU)}·EG·2H2O]n2, [{Zn(µ-BPEBU)2(H2O)2}.SO4·2H2O.2THF}n3, [{Zn(µ-BPEBU)2(H2O)2}·SO4·2H2O.2acetone}n4 and [{Zn(µ-BPEBU)2(H2O)2}·SO4·2H2O·2(1,4-dioxane)}n5, which were mainly characterized by single cyrstal X-ray diffraction. MOFs 3–5 are found to be isomorphous. The study revealed that the crystallization solvents affected the structures; the single crystal structure of the free BPEBU was also discussed in the context of the conformation and ligating topology of the ligand found in the reported MOFs. When the MOFs syntheses were performed in a competitive environment wherein other oxo-counter anions such as NO3−, ClO4− and CF3SO3− were present, the isolated MOFs turned out to be the corresponding sulfate MOFs 2–4 indicating interesting sulfate selectivity.

Supramolecular Synthons in Noncovalent Synthesis of a Class of Gelators Derived from Simple Organic Salts: Instant Gelation of Organic Fluids at Room Temperature via in Situ Synthesis of the Gelators

The supramolecular synthon approach has been employed to synthesize noncovalently a series of low molecular mass organic gelators (LMOGs) derived from benzylammonium salts of variously substituted benzoic acids. The majority of the salts (75%) prepared showed interesting gelation properties. Instant gelation of an organic fluid, namely methyl salicylate, was achieved at room temperature by using most of the gelator salts by in situ synthesis of the gelators. Table top rheology and scanning electron microscopy (SEM) were used to characterize the gels. Single crystal X-ray diffraction studies revealed the presence of both 1D and 2D supramolecular synthons. X-ray powder diffraction (XRPD) studies indicated the presence of various crystalline phases in the fibers of the xerogels. By using these data, a structure-property correlation has been attempted and the working hypothesis for designing the gelator has been reinforced.

Supramolecular Synthons in Designing Low Molecular Mass Gelling Agents: l-Amino Acid Methyl Ester Cinnamate Salts and their Anti-Solvent-Induced Instant Gelation

Easy access to a class of chiral gelators has been achieved by exploiting primary ammonium monocarboxylate (PAM), a supramolecular synthon. A combinatorial library comprising of 16 salts, derived from 5 L-amino acid methyl esters and 4 cinnamic acid derivatives, has been prepared and scanned for gelation. Remarkably, 14 out of 16 salts prepared (87.5 % of the salts) show moderate to good gelation abilities with various solvents, including commercial fuels, such as petrol. Anti-solvent induced instant gelation at room temperature has been achieved in all the gelator salts, indicating that the gelation process is indeed an aborted crystallization phenomenon. Rheology, optical and scanning electron microscopy, small angle neutron scattering, and X-ray powder diffraction have been used to characterize the gels. A structure-property correlation has been attempted, based on these data, in addition to the single-crystal structures of 5 gelator salts. Analysis of the FT-IR and (1)H NMR spectroscopy data reveals that some of these salts can be used as supramolecular containers for the slow release of certain pest sex pheromones. The present study clearly demonstrates the merit of crystal engineering and the supramolecular synthon approach in designing new materials with multiple properties.

Supramolecular Assembly of Functionalized Metalloporphyrins. Porous Crystalline Networks of Zinc-Tetra(4-Carboxyphenyl)Porphyrin

Abstract Two crystalline compounds based on the Zn-tetra (4-carboxyphenyl)porphyrin building block have been prepared and structurally analysed by x-ray diffraction in order to ellucidate the characteristic supramolecular aggregation modes of this functionalized metalloporphyrin moiety. Crystals of the 1:1 complex with sec-phenethyl alcohol (1) are monoclinic, space group I2/a (No. 15), a = 14.420(1), b = 24.678(2), c = 33.688(2) A, β = 90.03(2)°, Dc = 1.082 g/cm3, Z = 8, R = 0.080 for 3053 data. Those of the 1:3 complex with dimethylsulphoxide (2) are monoclinic, space group P21/n (No. 14), a = 14.881(1), b = 8.986(2), c = 37.550(3) A, β = 94.21(1)°, Dc = 1.444 g/cm3, Z = 4, R = 0.093 for 4200 data. The two compounds consist of nearly square-pyramidal five-coordinate metalloporphyrin species, molecules of the sec-phenethyl alcohol (in 1) and dimethylsulphoxide (in 2) linking to the metal center at the axial site. In 2, two additional molecules of dimethylsulphoxide associate through H-bonds to the termin...