Quest for a Desolvated Structure Unveils Breathing Phenomena in a MOF Leading to Greener Catalysis in a Solventless Setup: Insights from Combined Experimental and Computational Studies.

The crystal structure of the metal-organic framework (MOF), {Mn2(1,4-bdc)2(DMF)2}n (1) (1,4-bdcH2, 1,4-benzenedicarboxylic acid; DMF, N,N-dimethylformamide), is known for a long time; however, its desolvated structure, {Mn2(1,4-bdc)2}n (1'), is not yet known. The first-principles-based computational simulation was used to unveil the structure of 1' that shows the expansion in the framework, leading to pore opening after the removal of coordinated DMF molecules. We have used 1' that contains open metal sites (OMSs) in the structure in cyanosilylation and CO2 cycloaddition reactions and recorded complete conversions in a solventless setup. The pore opening in 1' allows the facile diffusion of small aldehyde molecules into the channels, leading to complete conversion. The reactions with larger aldehydes, 2-naphthaldehyde and 9-anthracenecarboxaldehyde, also show 99.9% conversions, which are the highest reported until date in solventless conditions. The in silico simulations illustrate that larger aldehydes interact with Mn(II) OMSs on the surfaces, enabling a closer interaction and facilitating complete conversions. The catalyst shows high recyclability, exhibiting 99.9% conversions in the successive reaction cycles with negligible change in the structure. Our investigations illustrate that the catalyst 1' is economical, efficient, and robust and allows reactions in a solventless greener setup, and therefore the catalysis with 1' can be regarded as "green catalysis".

Pseudo-Gated Adsorption with Negligible Volume Change Evoked by Halogen-Bond Interaction in the Nanospace of MOFs.

The enhancement of gas adsorption utilizing weak interactions in porous compounds is highly demanding for the design of energy-efficient storage materials. Here, we present a rational design for such an adsorption process by using synergistic functions between dynamic motion in a local module and weak but specific host-guest interactions, that is, halogen-bond (XB) interactions in metal-organic frameworks (MOFs). We designed a new porous coordination polymer (PCP), that is, Br-PCP, the pore surfaces of which are decorated with -CH2 Br groups and could be useful for interaction with CO2 molecules. In accordance with our anticipation, in-situ studies suggest that the adsorption step at approximately 54 kPa during CO2 adsorption is indeed facilitated by XB interactions with little change in the structural volume. This approach of integrating flexible XB modules in rigid PCPs is applicable for designing advanced gas storage systems.

Crystal Dynamics in Multi-stimuli-Responsive Entangled Metal-Organic Frameworks.

An understanding of solid-state crystal dynamics or flexibility in metal-organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single-crystallinity. In this article, detailed experimental studies on a twofold entangled MOF (f-MOF-1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f-MOF-1, by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f-MOF-1 a and f-MOF-1 b, could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO2 adsorption profiles. DFT-based calculations provide an insight into the selective and gated adsorption behavior with CO2 of f-MOF-1 b. The gate-opening threshold pressure of CO2 adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (-CH2 -CH2 -) in f-MOF-1 to an azo (-N=N-) functionality in an analogous MOF, f-MOF-2. The modulation of functionality has an indirect influence on the gate-opening pressure owing to the difference in inter-net interaction. The framework of f-MOF-1 is highly responsive toward CO2 gas molecules, and these results are supported by DFT calculations.

A crystalline porous coordination polymer decorated with nitroxyl radicals catalyzes aerobic oxidation of alcohols.

A porous coordination polymer (PCP) has been synthesized employing an organic ligand in which a stable free radical, isoindoline nitroxide, is incorporated. The crystalline PCP possesses one-dimensional channels decorated with the nitroxyl catalytic sites. When O2 gas or air was used as the oxidant, this PCP was verified to be an efficient, recyclable, and widely applicable catalyst for selective oxidation of various alcohols to the corresponding aldehydes or ketones.

MIL-101(Cr)/aminoclay nanocomposites for conversion of CO2 into cyclic carbonates.

We present the use of an amine functionalized two-dimensional clay i.e., aminoclay (AC), in the chemistry of a three-dimensional metal-organic framework (MOF) i.e., MIL-101(Cr), to prepare MIL-101(Cr)/AC composites, which are exploited as catalysts for efficient conversion of CO2 gas into cyclic carbonates under ambient reaction conditions. Three different MOF nanocomposites, denoted as MIL-101(Cr)/AC-1, MIL-101(Cr)/AC-2, and MIL-101(Cr)/AC-3, were synthesized by an in situ process by adding different amounts of AC to the precursor solutions of the MIL-101(Cr). The composites were characterized by various techniques such as FT-IR, PXRD, FESEM, EDX, TGA, N2 adsorption, as well as CO2 and NH3-TPD measurements. The composites were exploited as heterogeneous catalysts for CO2 cycloaddition reactions with different epoxides and the catalytic activity was investigated at atmospheric pressure under solvent-free conditions. Among all the materials, MIL-101(Cr)/AC-2 shows the best catalytic efficiency under the optimized conditions and exhibits enhanced efficacy compared to various MIL-101(Cr)-based MOF catalysts, which typically need either high temperature and pressure or a longer reaction time or a combination of all the parameters. The present protocol using MIL-101(Cr)/AC-2 as the heterogeneous catalyst gives 99.9% conversion for all the substrates into the products at atmospheric pressure.

In situ generation of functionality in a reactive haloalkane-based ligand for the design of new porous coordination polymers.

Herein, we report new porous coordination polymers (PCPs) via a facile synthetic approach called "in situ generation of functionality in the ligand". Upon a synthetic process of PCPs, a neutral (-CH2OH) or a cationic functionality (-CH2-[4,4'-bipyridine](+)) was generated on a isophthalate ligand from a reactive haloalkane (-CH2Br) moiety, affording two new PCPs. The PCPs have two-dimensional layered structures with large potential solvent-accessible voids for CO2 adsorption.

Metal-organic framework based drug delivery systems as smart carriers for release of poorly soluble drugs hydrochlorothiazide and dapsone.

Drug delivery systems (DDSs) that are derived from biocompatible carriers are attractive platforms for sustained release of drugs. In particular, sustained and controlled release of poorly soluble BCS (Biopharmaceutics Classification System) class IV drugs is important and this requires the development of new DDSs. In this work, we exploit two porous metal-organic frameworks (MOFs) MIL-100(Fe) and MIL-53(Fe) as carriers/DDSs for the release of two BCS class IV drugs hydrochlorothiazide (HCT) and dapsone (DAP). The chosen MOFs are known to possess good physicochemical stability and we realized high drug loading capacity that is attributed to the high porosity of the MOFs. The drug-encapsulated MOFs were characterized thoroughly and our results show ∼23.1% loading of HCT in MIL-100(Fe) and ∼27.6% loading of DAP in MIL-Fe(53), respectively. The release study of these drugs was carried out under simulated physiological conditions that shows sustained release of the drug molecules from the MOFs up to 72 h. Cell viability studies through MTT assays show insignificant cytotoxicity signalling biocompatibility of the proposed DDSs. Our investigations suggest MIL-100(Fe) and MIL-53(Fe) are potential DDSs for enhancing the performance of poorly soluble drugs HCT and DAP.

Biocompatible Drug Delivery System Based on a MOF Platform for a Sustained and Controlled Release of the Poorly Soluble Drug Norfloxacin.

Norfloxacin (NFX), an important antibacterial fluoroquinolone, is a class IV drug according to the biopharmaceutics classification system (BCS) and has low solubility and permeability issues. Such poor physicochemical properties of drug molecules lead to poor delivery and are of serious concern to the pharmaceutical industry for clinical development. We present here a conceptually new approach to deliver NFX, by loading the drug molecule on the porous platform of a biocompatible metal-organic framework (MOF), MIL-100(Fe). The loading of the drug on the MOF leading to NFX@MIL-100(Fe) was characterized by Fourier transform infrared (FTIR), UV-visible spectroscopy, thermogravimetric analyses (TGA), and nitrogen adsorption studies. Controlled experiments resulted in the high loading of the drug molecule (∼20 wt %) along with the desired sustained release. We could further control the release of norfloxacin by coating drug-loaded MIL-100(Fe) with PEG, PEG{NFX@MIL-100(Fe)}. Both drug delivery systems (DDSs), NFX@MIL-100(Fe) and PEG{NFX@MIL-100(Fe)}, were tested for their biocompatibility through toxicity studies. The DDSs are biocompatible and show insignificant cytotoxicity, as revealed by cell viability studies through the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

A metal-organic framework with highly polar pore surfaces: selective CO2 adsorption and guest-dependent on/off emission properties.

A 3D porous Zn(II) metal-organic framework {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)(H(2)O)]·4H(2)O} (1; H(2)dht=dihydroxyterphthalate, azpy=4,4'-azobipyridine) has been synthesised by employing 2,5-dihydroxyterephthalic acid (H(4)dht), a multidentate ligand and 4,4'-azobipyridine by solvent-diffusion techniques at room temperature. The as-synthesised framework furnishes two different types of channels: one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The dehydrated framework, {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)]} (1') provides 52.7% void volume to the total unit-cell volume. The pore surfaces of 1' are decorated with unsaturated Zn(II) sites and pendant hydroxyl groups of H(2)dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1' shows highly selective adsorption of CO(2) over other gases, such as N(2), H(2), O(2) and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission (λ(max)≈530 nm) on the basis of the excited-state intramolecular proton-transfer (ESIPT) process of the H(2)dht linker; no emission was observed in dehydrated solid 1'. Such guest-induced on/off emission has been correlated to the structural transformation and concomitant breaking and reforming of the OH···OCO hydrogen-bonding interaction in the H(2)dht linker in 1'/1.

Topological difference in 2D layers steers the formation of rigid and flexible 3D supramolecular isomers: impact on the adsorption properties.

Starting with the same precursors, pyridine-2,3-dicarboxylate (pyrdc) and 4,4'-bipyridyl (bipy), two 3D porous coordination polymers, {[Cu(bipy)(0.5)(pyrdc)]·3H(2)O} (1) and {[Cu(bipy)(0.5)(pyrdc)]·0.5bipy·3H(2)O} (2), have been synthesized by changing the solvent system from MeOH/H(2)O to EtOH/H(2)O. Single-crystal structure analysis revealed that 1 and 2 are supramolecular isomers with 3D pillared-layer structures having 1D channel systems. Isomer 1 has a flexible structure and shows gated adsorption behavior, while framework 2 has a rigid backbone and exhibits the adsorption properties of typical microporous materials.

Open metal site (OMS)-inspired investigation of adsorption and catalytic functions in a porous metal-organic framework (MOF).

In this article, we report the adsorption and catalytic study of the three-dimensional (3D) metal-organic framework (MOF) {Mn2(1,4-bdc)2(DMF)2} (1) (1,4-bdcH2, 1,4-benzene dicarboxylic acid; DMF, N,N-dimethylformamide) together with the synthesis and structure of two new Mn(II)-MOFs {Mn3(Br-bdc)3(DMF)4} (2) and {Mn3(NO2-bdc)3(DMF)4} (3) (Br-bdcH2, 2-bromo-1,4-benzene dicarboxylic acid; NO2-bdcH2, 2-nitro-1,4-benzene dicarboxylic acid) under solvothermal conditions. Compounds 2 and 3 have two-dimensional (2D) extended structures and feature trimeric {Mn3(CO2)6} units that serve as secondary building units for the frameworks. The desolvated compound of 1, denoted as 1', having potential Mn(II) open metal sites (OMSs) lined in a one-dimensional (1D) Mn-chain interconnected by carboxylate groups, exhibits guest-selective adsorption of solvent vapours wherein the compound shows a stepwise profile with H2O vapour, while a gated isotherm was recorded with MeOH. After realizing the favourable interaction of 1' with polar solvent molecules, we have used Mn(II) OMSs in 1' for efficient cyanosilylation reactions of aromatic aldehydes. We have recorded 100% conversion for eight aromatic aldehydes, while several other aldehydes showed appreciable conversion. Notably, the recorded conversions in the case of many substrates are higher than those for many other reported MOF catalysts.

Guest-specific double- or single-step adsorption in a flexible porous framework based on a mixed-ligand system.

A 2D flexible metal-organic porous solid, {[Ni(1,3-adc)(bpp)(H2O)2](H2O)(EtOH)}n (1), has been synthesized using flexible organic linkers. The desolvated framework, {[Ni(1,3-adc)(bpp)]}n (1'), undergoes structural contraction and exhibits double-step hysteretic adsorption for CO2, H2O, and MeOH and single-step gate-opening behavior with EtOH. These observations are correlated with the effect of the polarity and window dimension of the pore to the corresponding adsorbate molecules.

A vanadium (VO2+) metal-organic framework: selective vapor adsorption, magnetic properties, and use as a precursor for a polyoxovanadate.

The reaction of VOSO(4) with 2-carboxyethylphosphonic acid (H(2)-CEP) in presence of piperazine (PIP) produces a 3D inorganic-organic hybrid framework, {(H(2)PIP)(0.5)[VO(CEP)]·H(2)O} (1) with bidirectional channels occupied by the H(2)PIP cations and H(2)O molecules. The PO(3)(2-) unit of CEP connects three V(IV) centers to generate a 1D ladder, which is further linked to four such ladders by the CEP linkers to form a 3D hybrid framework. The dehydrated framework, {(H(2)PIP)(0.5)[VO(CEP)]} (1') shows selective and gated adsorption behavior with H(2)O but not with methanol and ethanol. Very interestingly, when 1 is treated with an aqueous solution of LiNO(3)/NaNO(3), the framework breaks down and results in a new polyoxovanadate (POV) cluster, [H(5)(H(2)PIP)(3)][V(V)(12)V(IV)(2)O(38)(PO(4))]·8H(2)O (2) at pH ≈ 2.1. The cluster has been characterized by single-crystal X-ray diffraction, (31)P NMR, EPR, and magnetic studies. The temperature-dependent magnetic susceptibility measurement suggests antiferromagnetic ordering in 1 with T(N) ≈ 3.8 K.

Probing time dependent phase transformation in a flexible metal-organic framework with nanoindentation.

Phase transformation in a flexible metal-organic framework, {[Zn4(1,4-NDC)4(1,2-BPE)2]·xSolvent}n, which loses guest molecules rapidly at room temperature, leading to several phase transitions, is examined using the nanoindentation technique. Nanoindentation results revealed that the time dependent transformation of an open to a closed phase happens gradually, through multiple intermediate phases, with the mechanical properties (elastic modulus and hardness) increasing as the transformation progresses from an open to a closed phase.

Metal-Organic Frameworks as Platform for Lewis-Acid-Catalyzed Organic Transformations.

Metal-organic frameworks (MOFs) are highly promising Lewis acid catalysts; they either inherently possess Lewis acid sites (LASs) on it or the LASs can be generated through various post-synthetic methods, the later can be performed in MOFs in a trivial fashion. MOFs are suitable platform for catalysis because of its highly crystalline and porous nature. Moreover, with recent advancements, thermal and chemical stability is not a problem with many MOFs. In this Minireview, an enormous versatility of MOFs, in terms of their microporosity/mesoporosity, size/shape selectivity, chirality, pore size, etc., has been highlighted. These are advantageous for designing and performing various targeted organic transformations. Although, many organic transformations catalyzed by MOFs with LASs have been reported in the recent past. In this Minireview, we have restricted ourselves to four important organic reactions: (i) cyanosilylation, (ii) Diels-Alder reaction, (iii) C-H activation, and (iv) CO2 -addition. The discussion focuses mostly on the recent reports (42 examples).

The densely fluorinated nanospace of a porous coordination polymer composed of perfluorobutyl-functionalized ligands.

A perfluorobutyl-functionalized two-dimensional porous coordination polymer (PCP), {[Cu(bpbtp)(L)(DMF)]·(DMF)}n (H2bpbtp = 2,5-bis(perfluorobutyl)terephthalic acid, L = 2,5-bis(perfluorobutyl)-1,4-bis(4-pyridyl)benzene, DMF = N,N-dimethylformamide) has been synthesized and structurally characterized. The pore surface of the PCP is decorated with pendant perfluorobutyl groups which fabricate a densely fluorinated nanospace resulting in unique gas sorption properties.

Unusual room temperature CO2 uptake in a fluoro-functionalized MOF: insight from Raman spectroscopy and theoretical studies.

We herein report an unusual CO(2) adsorption behavior in a fluoro-functionalized MOF {[Zn(SiF(6))(pyz)(2)]·2MeOH}(n) (1) with a 1D channel system, which is made up of pyrazine and SiF(6)(2-) moieties. Surprisingly, desolvated 1 (1') adsorbs higher amounts of CO(2) at 298 K than at 195 K, which is in contrast to the usual trend. Combined Raman spectroscopic and theoretical studies reveal that slanted pyrazine rings in 1' with an angle of 17.2° with respect to the (200) Zn(II)-Si plane at low temperature block the channel windows and thus reduce the uptake amount.

High heat of hydrogen adsorption and guest-responsive magnetic modulation in a 3D porous pillared-layer coordination framework.

A bimetallic pillared-layer coordination framework {[Mn(3)(bipy)(3)(H(2)O)(4)][Cr(CN)(6)](2)·2(bipy)·4(H(2)O)}(n) has been constructed using a cyanometallate anion ([Cr(CN)(6)](3-)) and an organic linker (4,4'-bipyridyl) that provides high heat of hydrogen adsorption (∼11.5 kJ mol(-1)) and shows guest dependent magnetic modulation.

A pillared-bilayer porous coordination polymer with a 1D channel and a 2D interlayer space, showing unique gas and vapor sorption.

A new 2D pillared-bilayer porous coordination polymer (PCP) has been synthesized and structurally characterized that shows selective adsorption of CO(2) over other gases (N(2), O(2), Ar, H(2), CH(4)) and guest selective single/double-step adsorption of vapor correlated to the successive confinement of adsorbates in a 1D channel and a 2D interlayer space.

Coordination driven axial chirality in a microporous solid assembled from an achiral linker via in situ C-N coupling.

Metal mediated in situ C-N coupling between 4,4'-azobipyridine and disodium-trans-glutaconate at room temperature has formed a new multifunctional linker Z-dhpe which subsequently self-assembles with Zn(II) or Cd(II) resulting in a chiral or an achiral metal-organic framework, respectively, depending on its different coordination modes.