Metal-Organic Frameworks of Cu(II) Constructed from Functionalized Ligands for High Capacity H2 and CO2 Gas Adsorption and Catalytic Studies.

Two Cu(II)-based metal-organic frameworks (MOFs) having paddle-wheel secondary building units (SBUs), namely, 1Me and 1ipr, were synthesized solvothermally using two new bent di-isophthalate ligands incorporating different substituents. The MOFs showed high porosity (BET surface area, 2191 m2/g for 1Me and 1402 m2/g for 1ipr). For 1Me, very high CO2 adsorption (98.5 wt % at 195 K, 42.9 wt % at 273 K, 23.3 wt % at 298 K) at 1 bar was found, while for 1ipr, it was significantly less (14.3 wt % at 298 K and 1 bar, 54.4 wt % at 298 K at 50 bar). 1Me exhibited H2 uptake of 3.2 wt % at 77 K and 1 bar of pressure, which compares well with other benchmark MOFs. For 1ipr, the H2 uptake was found to be 2.54 wt % under similar experimental conditions. The significant adsorption of H2 and CO2 for 1Me could be due to the presence of micropores as well as unsaturated metal sites in these MOFs besides the presence of substituents that interact with the gas molecules. The experimental adsorption behavior of the MOFs could be justified by theoretical calculations. Additionally, catalytic conversions of CO2 and CS2 into useful chemicals like cyclic carbonates, cyclic trithiocarbonates, and cyclic dithiocarbonates could be achieved.

Nanosized Bispyrazole-Based Cryptand-Stabilized Palladium(0) Nanoparticles: A Reusable Heterogeneous Catalyst for the Suzuki-Miyaura Coupling Reaction in Water.

A macrobicyclic cryptand with a long rigid cavity incorporating a chelating bispyrazole moiety in each of the three bridges was synthesized. The multiple chelating metal binding sites were utilized for the controlled synthesis and stabilization of ultrafine palladium nanoparticles (Pd NPs) of nearly ∼2 nm size. The as-synthesized Pd NPs were characterized by X-ray photoelectron spectroscopy, transmission electron microscopy, and powder X-ray diffraction. The well-dispersed cryptand-stabilized nanoparticles are found to catalyze the C-C bond-forming Suzuki-Miyaura reaction heterogeneously using water as a green solvent.

Dual Functionalized CuMOF-Based Composite for High-Performance Supercapacitors.

Herein, we utilized our previously reported highly porous CuMOF, {[Cu2(L)(H2O)2]·(5DMF)·(4H2O)}n, decorated with amine and trifluoromethyl functional groups for energy storage application. This robust framework in CuMOF enhances the chemical and thermal stabilities as well as improves the interfacial binding interactions. The poor conductivity of CuMOF usually restricts its practical utility in energy storage systems, due to which rGO was introduced along with CuMOF to form a CuMOF/rGO composite (1) through a facile ultrasonication technique. The synergistic effects between CuMOF and rGO induce a dramatic enhancement in specific capacitance (462 F g-1 at 0.8 A g-1) of 1 with a cycle life of 93.75% up to 1000 cycles. The results highlight 1 as an emerging contestant for next generation supercapacitors.

A Multifunctional Metal-Organic Framework for Oxidative C-O Coupling Involving Direct C-H Activation and Synthesis of Quinolines.

A robust paddle-wheel Cu(II)-based metal-organic framework (MOF) 1, having dual functionalities, namely, Lewis acid and basic sites, has been explored as a heterogeneous catalyst. This MOF, because of its large void volume (10298 Å3, 67.6%), large surface area (1480 m2/g), and high thermal stability, encouraged us to see its applicability in two catalytic reactions, namely, oxidative C-O coupling (cross-dehydrogentaive coupling reaction) involving direct C-H activation and Friedländer reaction under solvent free and ambient conditions. This study demonstrates the green aspect of MOFs in coupling reactions because of the simplified recovery, shorter reaction time, minimum waste, and smooth activation of the C-H bond, which is very challenging in synthetic chemistry.

Enantioselective Aldol Reactions in Water by a Proline-Derived Cryptand and Fixation of CO2 by Its Exocyclic Co(II) Complex.

The secondary amine donors present in the bridges of a laterally nonsymmetric oxa-aza cryptand have been derivatized with l-proline to obtain the chiral cryptand L. The cryptand L efficiently catalyzed aldol reactions in water with up to 75% ee. On reacting with Co(II) perchlorate in the presence of KSCN, L readily formed the trinuclear complex {[Co3(L)2(NCS)6]·(15CH3CN)(5acetone)(6H2O)} (1). The complex 1 in combination with the cocatalyst tetrabutylammonium bromide (TBAB) formed an efficient catalytic system in the synthesis of cyclic carbonates from CO2 and epoxides at room temperature and atmospheric pressure.

Chiral Cadmium(II) Metal-Organic Framework from an Achiral Ligand by Spontaneous Resolution: An Efficient Heterogeneous Catalyst for the Strecker Reaction of Ketones.

A thermally stable cadmium-based chiral metal-organic framework (MOF), {[Cd2(L)(H2O)(DMF)]·3DMF·2H2O}n (1; DMF = N,N-dimethylformamide), has been synthesized from an achiral ligand by spontaneous resolution. The MOF features 1D open channels with a large density of active metal sites and has a 3,6-c binodal net with a rare sit 3,6-conn topology. The metal-bound water and DMF solvents could be easily removed along with the guest molecules in the lattice upon activation to afford the desolvated framework 1'. It exhibits microporous nature, as confirmed by the gas-sorption measurements with carbon dioxide uptake of 43.2 cm3 g-1 at 273 K. The open metal sites in the framework make it an outstanding heterogeneous catalyst in the Strecker reaction for the synthesis of α-aminonitriles in a solvent-free state at room temperature with excellent conversion yields.

A 2D Coordination Network That Detects Nitro Explosives in Water, Catalyzes Baylis-Hillman Reactions, and Undergoes Unusual 2D→3D Single-Crystal to Single-Crystal Transformation.

The solvothermal reaction of Zn(NO3)2·6H2O and a linear dicarboxylate ligand H2L, in the presence of urotropine in N,N'-dimethylformamide (DMF), gives rise to a new porous two-dimensional (2D) coordination network, {[Zn3(L)3(urotropine)2]·2DMF·3H2O}n (1), with hxl topology. Interestingly, framework 1 exhibits excellent emission properties owing to the presence of naphthalene moiety in the linker H2L, that can be efficiently suppressed by subtle quantity of nitro explosives in aqueous medium. Furthermore, presence of urotropine molecules bound to the metal centers, 1 is found to be an excellent heterogeneous catalyst meant for atom-economical C-C bond-forming Baylis-Hillman reactions. Additionally, crystals of 1 undergo complete transmetalation with Cu(II) to afford isostructural 1Cu. Moreover, the 2D framework of 1 allows replacement of urotropine molecules by 4,4'-azopyridine (azp) linker resulting in a three-dimensional (3D) metal-organic framework, {[Zn(L)(azp)]·4DMF 2H2O}n (2). The 1→2 transformation takes place in single-crystal-to-single crystal manner supported by powder X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and morphological studies. Remarkably, during this 2D→3D transformation, the original trinuclear [Zn3(COO)6] secondary building unit changes to a mononuclear node, which is unprecedented.

An Amine Functionalized Metal-Organic Framework as an Effective Catalyst for Conversion of CO2 and Biginelli Reactions.

A highly porous and thermally stable anionic Zn(II)-framework, {[(CH3)2NH2+]2[Zn3((µ3-O))(L)2(H2O)]·4DMF·2H2O}n (1), having exposed metal sites and pendant amine groups has been synthesized adopting the solvothermal technique. This anionic 3D framework showed two-fold interpenetration with 45.1% void volume. It has a 3,6-c binodal net with rare sit 3,6-conn topology. The metal bound aqua ligand could be easily removed along with the guest molecules in the lattice upon activation to afford the desolvated framework 1'. This produced exposed metal sites that, along with the pendant amine groups incorporated in the ligand, generated a coordination space in the framework to make it an outstanding heterogeneous catalyst for the chemical fixation of CO2 with various epoxides under atmospheric pressure and in the three-component Biginelli reaction with different aldehydes, ethyl acetoacetate, and urea to afford dihydropyrimidinones.

From Zn(II)-Carboxylate to Double-Walled Zn(II)-Carboxylato Phosphate MOF: Change in the Framework Topology, Capture and Conversion of CO2, and Catalysis of Strecker Reaction.

The ligand H2L has been built by linking an imidazole moiety to the 5-position of isophthalic acid. It forms two types of porous frameworks, {[Zn(L)]·2DMF·2H2O}n (1) and {[(CH3)2NH2][Zn2(L)(H2O)PO4]·2DMF}n (2). 1 is a porous neutral framework and has rtl rutile 3,6-conn topology, while 2 is an organo-metallophosphate anionic porous framework with double-walled hexagonal channels. Framework 1' (desolvated) exhibits moderate CO2 adsorption (58 cc g-1 at 273 K, 1 bar), whereas 2' (desolvated) shows a microporous nature with a high adsorption of CO2 (111.7 cc g-1 or 22 wt % at 273 K, 1 bar). Interestingly, this adsorbed CO2 could be converted very efficiently to cyclic carbonates under mild conditions using 2' as the catalyst in the presence of tetrabutylammonium bromide as the cocatalyst. The presence of open metal sites in 2' makes it an efficient heterogeneous catalyst for solvent-free three-component Strecker reaction using various aldehydes/ketones together with amines and trimethylsilyl cyanide in high yields at room temperature. The straightforward experimental and product isolation procedure along with easy recovery and reusability of the catalyst provided an attractive route for the synthesis of α-amino nitriles.

Synthesis of a NbO Type Homochiral Cu(II) Metal-Organic Framework: Ferroelectric Behavior and Heterogeneous Catalysis of Three-Component Coupling and Pechmann Reactions.

A chiral tetracarboxylic acid ligand, H4L, incorporating the (S)-(+)-2-methylpiperazine moiety in its middle, solvothermally forms a homochiral Cu(II) framework, {[Cu2(L)(H2O)2]·(4DMF)(4H2O)}n (LCu). It forms a non-interpenetrated structure consisting of [Cu2(COO)4] paddle-wheel secondary bonding units (SBUs) with NbO topology. Interestingly, the framework LCu exhibits excellent ferroelectric properties. It shows a remnant polarization (Pr) of ∼3.5 µC cm-2 and a coercive field (Ec) of ∼12 kV cm-1 with a distinct electric hysteresis loop. Dielectric studies of LCu reveal almost frequency-independent behavior with a dielectric constant (εr) of ∼42 and a low dielectric loss (tan δ) of ∼0.04 up to 106 Hz, for potential use in high-frequency applications. In addition, activated framework LCu' having uncoordinated metal sites acts as an efficient heterogeneous catalyst in the three-component coupling of amines, aldehydes, and alkynes, as well as in Pechmann reactions of phenols with ß-ketoesters.

A Versatile CuII Metal-Organic Framework Exhibiting High Gas Storage Capacity with Selectivity for CO2 : Conversion of CO2 to Cyclic Carbonate and Other Catalytic Abilities.

A linear tetracarboxylic acid ligand, H4 L, with a pendent amine moiety solvothermally forms two isostructural metal-organic frameworks (MOFs) LM (M=ZnII , CuII ). Framework LCu can also be obtained from LZn by post- synthetic metathesis without losing crystallinity. Compared with LZn , the LCu framework exhibits high thermal stability and allows removal of guest solvent and metal-bound water molecules to afford the highly porous, LCu '. At 77 K, LCu ' absorbs 2.57 wt % of H2 at 1 bar, which increases significantly to 4.67 wt % at 36 bar. The framework absorbs substantially high amounts of methane (238.38 cm3 g-1 , 17.03 wt %) at 303 K and 60 bar. The CH4 absorption at 303 K gives a total volumetric capacity of 166 cm3 (STP) cm-3 at 35 bar (223.25 cm3 g-1 , 15.95 wt %). Interestingly, the NH2 groups in the linker, which decorate the channel surface, allow a remarkable 39.0 wt % of CO2 to be absorbed at 1 bar and 273 K, which comes within the dominion of the most famous MOFs for CO2 absorption. Also, LCu ' shows pronounced selectivity for CO2 absorption over CH4 , N2 , and H2 at 273 K. The absorbed CO2 can be converted to value-added cyclic carbonates under relatively mild reaction conditions (20 bar, 120 °C). Finally, LCu ' is found to be an excellent heterogeneous catalyst in regioselective 1,3-dipolar cycloaddition reactions ("click" reactions) and provides an efficient, economic route for the one-pot synthesis of structurally divergent propargylamines through three-component coupling of alkynes, amines, and aldehydes.

Linker-Induced Structural Diversity and Photophysical Property of MOFs for Selective and Sensitive Detection of Nitroaromatics.

The linker [1,1':3',1″-terphenyl]-4,4',4″,6'-tetracarboxylic acid (H4L) was used to construct two three-dimensional (3D) metal-organic frameworks (MOFs), namely, {[Cd2(L)(L1)(DMF)(H2O)](2DMF)(3H2O)}n (1) and {[Cd4(L)2(L2)3(H2O)2](8DMF)(8H2O)}n (2) (DMF = N,N'-dimethylformamide) in the presence of colinkers 4,4'-bipyridine (L1) and 2-amino-4,4'-bipyridine (L2), respectively, under solvothermal condition. A small change in the colinker leads to significant differences in the overall structure of the MOFs. Topological analysis reveals that the framework 1 exhibits 6,4-connected forbidden sub-configuration (FSC) topology, while the framework 2 exhibits twofold interpenetrated and (3,4,4)-connected new network topology with Schläfli point symbol {4.6(2)}{4.6(4).8}{4(2).6(2).8(2)}. The crystallographic investigation reveals the framework 2 having single helix structure, which is further coiled through noncovalent interaction, afforded a double-helix structure similar to DNA. These double helices are further connected through the colinker L2 to form an overall 3D structure. Besides framework 2 exhibits remarkable fluorescence intensity compared to 1. Framework 2 displayed a strong emission at 457 nm when a sample of 2 was dispersed in ethanol and excited at 334 nm. This emission is selectively and completely quenched in the presence of 2,4,6-trinitrophenol (TNP) allowing its detection in the presence of other nitroaromatic compounds. The quenching constant for TNP was found to be 3.89 × 10(4) M(-1), which is 26 times higher than that of TNT demonstrating greater and selective quenching ability. The emission is restored to its original value when the sample after collected by filtration is dispersed in fresh ethanol for 1 d. Interestingly, when solid 2 is exposed to different nitroaromatic compounds, its emission is quenched selectively in the presence of nitrobenzene. In this case, the emission is restored upon heating the sample to 150 °C for 2 h.

A Porous Cu(II)-MOF with Proline Embellished Cavity: Cooperative Catalysis for the Baylis-Hillman Reaction.

l-Proline has been covalently attached in a rigid linear ligand, H4L, having an isophthalate moiety at each terminal to form the chiral ligand, H4LPRO. This linker has been used for the construction of a porous MOF, LCuPRO. The free l-proline moiety in the cavity of the framework in the presence of imidazole as a cocatalyst functions synergistically to catalyze the Baylis-Hillman reaction between α,ß-unsaturated carbonyl compounds and aromatic aldehydes. High porosity of the framework is proven by the nitrogen adsorption isotherm.

A Partially Fluorinated, Water-Stable Cu(II)-MOF Derived via Transmetalation: Significant Gas Adsorption with High CO2 Selectivity and Catalysis of Biginelli Reactions.

A partially fluorinated, angular tetracarboxylic acid linker (H4L) incorporating a pendant amine moiety forms a three-dimensional Zn(II) framework, 1. The structure consists of paddle-wheel Zn2(CO2)4 secondary building units (SBUs) and Zn12(CO2)24 supramolecular building blocks (SBBs). Thermal stability of 1 is found to be low. However, it undergoes transmetalation reaction with Cu(II) at room temperature without losing crystallinity affording an isostructural framework, 1Cu. Framework 1Cu is thermally robust and allows generation of the solvent-free porous framework 1Cu' upon activation with coordinatively unsaturated metal centers. Framework 1Cu' exhibits water stability and at 77 K, adsorbs 2.56 wt % of H2 up to 1 bar that significantly increases to 4.01 wt % at 13 bar. Also, this framework gives a high adsorption of 164.70 cc/g of CH4 (11.7 wt %) at 303 K and 60 bar. The channel surfaces decorated with -NH2 group and unsaturated metal centers in 1Cu' allow a promising 36.4 wt % of CO2 adsorption at 1 bar and 273 K. Moreover, it exhibits pronounced selectivity of CO2 adsorption over N2 and H2 at 273 K. Finally, the versatility of 1Cu' is shown by its excellent heterogeneous catalytic activity in the Biginelli coupling reactions involving an aldehyde, urea, and ethylacetoacetate to afford dihydroprimidinones.

Single-Crystal to Single-Crystal Linker Substitution, Linker Place Exchange, and Transmetalation Reactions in Interpenetrated Pillared-Bilayer Zinc(II) Metal-Organic Frameworks.

A twofold interpenetrated pillared-bilayer framework, {[Zn3 (L)2 (L2 )(DMF)]⋅(18DMF)(6H2 O)}n (1), has been synthesized from the ligands tris(4'-carboxybiphenyl)amine (H3 L) and 1,2-bis(4-pyridyl)ethylene (L2 ). The structure contains [Zn3 (COO)6 ] secondary building units (SBUs), in which three Zn(II) ions are almost linear with carboxylate bridging. This framework undergoes reversible pillar linker substitution reactions at the terminal Zn(II) centers with three different dipyridyl linkers of different lengths to afford three daughter frameworks, 2-4. Frameworks 2-4 are interconvertible through reversible linker substitution reactions. Also, competitive linker-exchange experiments show preferential incorporation of linker L3 in the parent framework 1. The larger linker L5 does not undergo such substitution reactions and framework 5, which contains this linker, can be synthesized solvothermally as a twofold interpenetrated structure. Interestingly, when framework 5 is dipped in a solution of L3 in DMF, linker substitution takes place as before, but linker L5 now moves and diagonally binds two Zn(II) centers to afford 6 as a nonpenetrated single framework. This linker place exchange reaction is unprecedented. All of these reactions take place in a single-crystal to single-crystal (SC-SC) manner, and have been observed directly through X-ray crystallography. In addition, each 3D framework undergoes complete copper(II) transmetalation.

Versatile Tailoring of Paddle-Wheel Zn(II) Metal-Organic Frameworks through Single-Crystal-to-Single-Crystal Transformations.

A new tetracarboxylate ligand having short and long arms formed 2D layer Zn(II) coordination polymer 1 with paddle-wheel secondary building units under solvothermal conditions. The framework undergoes solvent-specific single crystal-to-single crystal (SC-SC) transmetalation to produce 1Cu . With a sterically encumbered dipyridyl linker, the same ligand forms non-interpenetrated, 3D, pillared-layer Zn(II) metal-organic framework (MOF) 2, which takes part in SC-SC linker-exchange reactions to produce three daughter frameworks. The parent MOF 2 shows preferential incorporation of the longest linker in competitive linker-exchange experiments. All the 3D MOFs undergo complete SC-SC transmetalation with Cu(II) , whereby metal exchange in different solvents and monitoring of X-ray structures revealed that bulky solvated metal ions lead to ordering of the shortest linker in the framework, which confirms that the solvated metal ions enter through the pores along the linker axis.

Significant Gas Adsorption and Catalytic Performance by a Robust Cu(II) -MOF Derived through Single-Crystal to Single-Crystal Transmetalation of a Thermally Less-Stable Zn(II) -MOF.

By using a bent tetracarboxylic acid ligand that incorporates a pendent-NH2 functional group, a 3D Zn(II)-framework (1) based on Zn2 (CO2)4 secondary building units and Zn12 (CO2)24 supramolecular building blocks has been synthesized. Framework 1 is thermally less stable, which precludes its application as a porous framework for gas adsorption or catalytic studies. This framework undergoes single-crystal to single-crystal transmetalation to give isostructural 1Cu. Unlike 1, the Cu(II) analogue is very stable and can be activated by removing metal-bound lattice solvent molecules by heating to afford 1Cu'. The activated 1Cu' exhibits excellent H2 storage (2.29 wt%) at 77 K and a high 32.1 wt% CO2 uptake at 273 K. Additionally, it displays significant selectivity for CO2 adsorption over N2 and H2 and can catalyse size-selective Knoevenagel condensation reactions.

A fluorescent chemosensor for Hg(2+) and Cd(2+) ions in aqueous medium under physiological pH and its applications in imaging living cells.

A new BODIPY derivative with 2,2'-(ethane-1,2-diylbis(oxy))bis(N,N-bis(pyridine-2-ylmethyl)aniline unit as the metal receptor has been designed and synthesized. The dye selectively detects either Cd(2+) or Hg(2+) ions in the presence of hosts of other biologically important and environmentally relevant metal ions in aqueous medium at physiological pH. Binding of metal ions causes a change in the emission behavior of the dye from weakly fluorescent to highly fluorescent. Confocal microscopic experiments validate that the dye can be used to identify changes in either Hg(2+) or Cd(2+) levels in living cells.

Construction of non-interpenetrated charged metal-organic frameworks with doubly pillared layers: pore modification and selective gas adsorption.

The rigid and angular tetracarboxylic acid 1,3-bis(3,5-dicarboxyphenyl)imidazolium (H4L(+)), incorporating an imidazolium group, has been used with different pyridine-based linkers to construct a series of non-interpenetrated cationic frameworks, {[Zn2(L)(bpy)2]·(NO3)·(DMF)6·(H2O)9}n (1), {[Zn2(L)(dpe)2]·(NO3)·(DMF)3·(H2O)2}n (2), and {[Zn2(L)(bpb)2]·(NO3)·(DMF)3·(H2O)4}n (3) [L = L(3-), DMF = N,N'-dimethylformamide, bpy = 4,4'-bipyridine, dpe = 1,2-di(4-pyridyl) ethylene, bpb = 1,4-bis(4-pyridyl)benzene]. The frameworks consist of {[Zn2(L)](+)}n two-dimensional layers that are further pillared by the linker ligands to form three-dimensional bipillared-layer porous structures. While the choice of the bent carboxylic acid ligand and formation of double pillars are major factors in achieving charged non-interpenetrated frameworks, lengths of the pillar linkers direct the pore modulation. Accordingly, the N2 gas adsorption capacity of the activated frameworks (1a-3a) increases with increasing pillar length. Moreover, variation in the electronic environment and marked difference in the pore sizes of frameworks permit selective CO2 adsorption over N2, where 3a exhibits the highest selectivity. In contrast, the selectivity of CO2 over CH4 is reversed and follows the order 1a > 2a > 3a. These results demonstrate that even though the pore sizes of the frameworks are large enough compared to the kinetic diameters of the excluded gas molecules, the electronic environment is crucial for the selective sorption of CO2.

A chemosensor built with rhodamine derivatives appended to an aromatic platform via 1,2,3-triazoles: dual detection of aluminum(III) and fluoride/acetate ions.

A triazole-ring-appended rhodamine dye (L) has been synthesized that serves as a chromogenic and fluorogenic sensor for dual sensing of aluminum(III) and fluoride or acetate ions specifically.