Raju Mondal

Synthon evolution and unit cell evolution during crystallisation. A study of symmetry-independent molecules (Z′ > 1) in crystals of some hydroxy compounds

A kinetically favoured crystal, with many molecules in the asymmetric unit, may be a fossil relic of the crystal nucleus of a more stable polymorph.

Crystal engineering of zinc(II) metal–organic frameworks: role of steric bulk and angular disposition of coordinating sites of the ligands

Three novel 2D and 3D coordination polymers, [Zn(TPA)(H2MDP)]n (1), [Zn(IPA)(H2MDP)]n (2) and [Zn(HTMA)(H2MDP)]n (3) (H2MDP = methylenebis(3,5-dimethylpyrazole), H2TPA = terephthalic acid, H2IPA = isophthalic acid and H3TMA = trimesic acid), have been synthesized under hydrothermal conditions and structurally characterized by single crystal X-ray diffraction. For all three structures, the zinc atoms show a coordination number of four and adopt tetrahedral geometry, and subsequently form a diamondoid, a (2,4) and a (4,4) network. The employment of sterically hindered H2MDP ligand, on the other hand, successfully prohibits interpenetration among the 2D grids. The complex 1 features a metal–organic framework exhibiting a three-fold interpenetrated diamondoid network with linear dicarboxylate ligand (H2TPA) as an auxiliary ligand. The bent dicarboxylate ligands (H2IPA and H3TMA), on the other hand, lead to 2D non interpenetrative grid networks for 2 and 3. A comparative study of 2 and 3 underpins the importance of the hydrogen bond and π–π interaction in the synthesis of coordination polymers.

Co-MOF as a sacrificial template: manifesting a new Co3O4/TiO2 system with a p–n heterojunction for photocatalytic hydrogen evolution

The present work reports a novel method for preparing an elusive Co3O4/TiO2 p–n heterojunction using Co-based metal organic frameworks (Co-MOFs) as a TiO2-absorbent cum sacrificial template for nanocomposite formation. Four new Co-MOFs based on a bispyrazole ligand and different carboxylic acids, with a wide variety of dimensionality, porosity and surface characteristics, were exploited for this purpose. We detail here the synthesis of cobalt MOFs using the hydro(solvo)thermal method and structural characterization by single crystal X-ray diffraction (XRD). We have also successfully demonstrated our strategy of using MOFs for fabricating superior p–n diode-type Co3O4/TiO2 hetero-nanocomposites for photocatalytic hydrogen production. The characterization results suggested that the nanocomposites consisted of highly crystalline desired anatase TiO2 nanoparticles and spinel Co3O4-like species. The nanocomposite with 2 wt% Co loading exhibited the maximum photoactivity with a hydrogen evolution rate of ∼7 mmol g−1 h−1 under UV-vis light irradiation. The above results indicate that the present preparative strategy is an amenable route for the synthesis of desirable synergistic photocatalysts combining a remarkable reactivity relevant to solar energy conversion.

Construction of Polynuclear Lanthanide (Ln = DyIII, TbIII, and NdIII) Cage Complexes Using Pyridine–Pyrazole-Based Ligands: Versatile Molecular Topologies and SMM Behavior

Employment of two different pyridyl-pyrazolyl-based ligands afforded three octanuclear lanthanide(III) (Ln = Dy, Tb) cage compounds and one hexanuclear neodymium(III) coordination cage, exhibiting versatile molecular architectures including a butterfly core. Relatively less common semirigid pyridyl-pyrazolyl-based asymmetric ligand systems show an interesting trend of forming polynuclear lanthanide cage complexes with different coordination environments around the metal centers. It is noteworthy here that construction of lanthanide complex itself is a challenging task in a ligand system as soft N-donor rich as pyridyl-pyrazol. We report herein some lanthanide complexes using ligand containing only one or two O-donors compare to five N-coordinating sites. The resultant multinuclear lanthanide complexes show interesting magnetic and spectroscopic features originating from different spatial arrangements of the metal ions. Alternating current (ac) susceptibility measurements of the two dysprosium complexes display frequency- and temperature-dependent out-of-phase signals in zero and 0.5 T direct current field, a typical characteristic feature of single-molecule magnet (SMM) behavior, indicating different energy reversal barriers due to different molecular topologies. Another aspect of this work is the occurrence of the not-so-common SMM behavior of the terbium complex, further confirmed by ac susceptibility measurement.

Identification of a robust and reproducible noncluster-type SBU: effect of coexistent groups on network topologies, helicity, and properties

The present work is part of our ongoing quest for a robust and reproducible noncluster-type secondary building unit (SBU) using pyrazole based ditopic ligands and benzene polycarboxylic acids. We report here the construction of a series of Zn(II) coordination polymers using a tailor-made SBU, designed with the proper utilization of the hydrogen bonds of a pyrazole-based ligand 4,4′-methylene-bispyrazole (H2MBP) and isophthalic acid (H2IPA), and its derivatives (R-H2IPA; R = OH, OCH3, CH3, tBu, Br, I, NH2, N3). The results demonstrate that the substituent effect of R-isophthalates does play a decisive role in directing the structural assemblies of this system. One of the promising features of the 2D networks of this series is the presence of SBU-based helical motifs, which by and large remain elusive despite the plethora of reports on various SBU based networks. Throughout the series, hydrogen bonds, π–π interactions and other intermolecular interactions play a crucial role in stabilizing the resulting networks. Furthermore, some of these complexes exhibit some interesting photoluminescent properties in the solid state.

Construction of helical networks by using multiple V-shaped mixed ligand systems

Helical coordination polymers constructed from multiple ligand systems are not common despite the plethora of recent reports on helical networks. We report here the construction of a series of mixed ligand based helical coordination polymers showing metal dependent architecture. The strategy of using a combination of ligand molecules, each capable of forming helical networks, is novel and should enable us to design helical MOFs of interesting topology.

A novel gel-based approach to wastewater treatment – unique one-shot solution to potentially toxic metal and dye removal problems

This communication reports the development of a unique gel-based adsorbent for wastewater treatment using a new low molecular weight gelator. The most interesting aspect of this technique is that the ligand not only gelled potentially toxic metals, like lead and cadmium, but that it also simultaneously adsorbed toxic dyes, like methyl orange, from an aqueous solution. This offers a one-shot solution to two major water purification problems.

Elusive nanoscale metal-organic-particle-supported metallogel formation using a nonconventional chelating pyridine-pyrazole-based bis-amide ligand.

Support group: A rare case of metallogel formation supported by nanoscale metal-organic particles by using a nonconventional ditopic chelating ligand is reported.

Kinetic and equilibrium studies of σ-adduct formation and nucleophilic substitution in the reactions of 2-phenoxy-3,5-dinitropyridine and 2-ethoxy-3,5-dinitropyridine with aliphatic amines in dipolar aprotic solvents

The reactions of aliphatic amines with 2-phenoxy-3,5-dinitropyridine, 4, and 2-ethoxy-3,5-dinitropyridine, 5, in DMSO result in the rapid reversible formation of anionic sigma-adducts at the 6-position. Kinetic studies show that proton transfer from the initially formed zwitterions to base may be rate-limiting. Slower reactions result, except in the case of 5 and piperidine, in displacement of the 2-substitutent via intermediates which have lower thermodynamic stabilities than their 6-isomers. Base catalysis of the substitution process is attributed in the case of 4 to rate-limiting proton transfer from zwitterionic intermediates, but in 5 to acid catalysis of ethoxide departure (SB-GA mechanism). X-Ray crystallography of 5 shows a planar non-strained structure although the structure of 2-piperidino-3,5-dinitropyridine, 10c, shows distortion resulting from steric interactions of the 2- and 3-substituents. Kinetic and equilibrium results are compared with those for related reactions of the more sterically strained 2,4,6-trinitrobenzene derivatives. Results for the reactions of 4 and 5 with pyrrolidine in three dipolar aprotic solvents are compared. Values of equilibrium constants for sigma-adduct formation decrease in the order DMSO > DMF >> Acetonitrile, while values of rate constants for proton transfer are in the reverse order.

Silver-promoted gelation studies of an unorthodox chelating tripodal pyridine–pyrazole-based ligand: templated growth of catalytic silver nanoparticles, gas and dye adsorption

A novel tripodal pyridine–pyrazole based tris amide ligand, namely, N1,N3,N5-tris(3-(pyridin-2-yl)-1H-pyrazol-5-yl)benzene-1,3,5-tricarboxamide was synthesized. The ligand acts as a promising low molecular weight gelator that gels silver salts in presence of water. The gels formed were found to be highly stable to heat and stress. Moreover, silver ions not only act as a gelling agent but also participate in nanoparticulate formation. The three-dimensional ligand–silver gel matrix serves as an excellent platform for the growth of these silver nanoparticles. The gels were characterized thoroughly using IR spectroscopy, UV-visible spectroscopy, SEM, TEM and rheological studies. TEM pictures revealed the nanoparticles to be attached exclusively onto the gel fibres confirming template formation. The silver nanoparticles not only provide high strength and stability to the gel network but also show excellent catalytic properties in reducing 4-nitrophenol and Methylene Blue dye in presence of sodium borohydride. In addition to the catalytic properties, xerogels displayed gas and dye adsorption properties.