D. Krishna Kumar

Rugby-Ball-Shaped Sulfate−Water−Sulfate Adduct Encapsulated in a Neutral Molecular Receptor Capsule

We report a tren-based tris(urea) receptor molecule that shows preferential binding with sulfate/phosphate anions. The receptor acts as a neutral molecular capsule, within which a unique sulfate-(H(2)O)(3)-sulfate adduct is encapsulated.

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.

Fluorescent Acridine-Based Receptors for H2PO4–

Two new pseudopeptidic molecules (one macrocyclic and one open chain) containing an acridine unit have been prepared. The fluorescence response of these receptors to a series of acids was measured in CHCl(3). Receptors are selective to H(2)PO(4)(-) versus HSO(4)(-), and an even higher selectivity is found over other anions such as Cl(-), Br(-), CH(3)COO(-), and CF(3)COO(-). We show that the macrocyclic receptor is more selective for H(2)PO(4)(-) than the related open chain receptor. The supramolecular interactions of triprotonated receptors with different anions have been modeled in silico and have been studied by different experimental techniques. Optimized geometries obtained by computational calculations agree well with experimental data, in particular fluorescence experiments, suggesting that the selective supramolecular interaction takes places through coordination of the anions to the triprotonated form of the receptor.

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 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.

Exploring conformationally flexible hydrogen-bond-functionalized ligand and counter anions in metal–organic frameworks of Cu(II)

Single crystal structures of six metal–organic frameworks (MOFs) derived from N,N′-bis(3-pyridyl)urea L1 and Cu(II) salts having different counter ions have been investigated to study the plausible role of conformationally flexible hydrogen-bond-functionalized ligand L1 and counter ions on the resultant topologies of the MOFs. The ligand adopts the energetically least stable syn–syn conformation in most of the MOFs. The 1 : 2 metal–ligand coordination polymers display looped chain topologies with square-pyramidal metal center whereas 1 : 1 metal–ligand coordination polymers show 1D zigzag infinite chain. Although the urea functionality of the ligand does recognize the anions via various N–H⋯O/F interactions, the counter anions do not influence the primary framework structures.

Conformation dependent network structures in the coordination polymers derived from pyridylisonicotinamides, carboxylates and Co(II): Entrapment of (H2O)14 water cluster of an unprecedented topology

Four coordination polymers, namely [{(H2O)4Co(μ-L1)2}·fumarate·2H2O]n1, [{(H2O)3(μ-fumarate)Co(μ-L1)2}·fumarate·2H2O]n2, [{(H2O)4Co(μ-L2)2}·terephthalate·3H2O]n3 and [{(H2O)4Co(μ-L2)2}·terephthalate·10H2O]n4 (L1 = N-(3-pyridyl)nicotinamide, L2 = N-(4-pyridyl)isonicotinamide) have been synthesized and structurally characterized mainly by single crystal X-ray diffraction techniques in order to explore the effect of hydrogen bond functionalized backbone and ligating topologies on the resultant supramolecular architectures in mixed ligand systems containing dicarboxylates as counter bridging anions. Interestingly, the counter bridging anions fumarate and terephthalate failed to coordinate to the metal centers in 1 and 3–4, respectively; fumarate, however, terminally connects to the metal centers in 2 resulting in the ladder topology. A (H2O)14 water cluster, having unprecedented topology, has been found entrapped in the lattice of 4.

First snapshot of a nonpolymeric hydrogelator interacting with its gelling solvents

A pyridyl urea based low molecular weight supramolecular hydrogelator has been synthesized; crystallized from its gelling solvents, the single crystal structure of the gelator molecule interacting with its gelling solvents reported herein is the first example in the literature.

Supramolecular structural diversities in the metal–organic frameworks derived from pyridylamide ligands: studying the effects of ligating topologies, hydrogen bonding backbone of the ligands and counter anions{

Six metal–organic frameworks (MOFs) namely [{(CH3COO)(μ–OOCH3)Zn}2(μ–L1)2]n1, [{(H2O)2Zn}(μ–L1)4}BF4]n2, [{(Cl)2Zn}(μ–L1)]n3, [{(NO3)2Cd(μ–L1)2}·nitrobenzene]n 4, [{Cl2Zn(μ–L2)}]n5 and [{(NO3)2Cd}(μ–L2)4]n6 (where L1 = N-(3-pyridyl)nicotinamide and L2 = N-(3-pyridyl)isonicotinamide) have been synthesized and crystallographically characterized. The main aim of this work is to study the effects of ligating topologies and hydrogen bonding backbone of the ligands and counter anions on the supramolecular structural diversities in the resulting MOFs. The structures of these MOFs are discussed in this context.