Crystal structure and characterization of a new one-dimensional copper(II) coordination polymer containing a 4-amino-benzoic acid ligand.

A CuII coordination polymer, catena-poly[[[aqua-copper(II)]-bis-(µ-4-amino-benz-o-ato)-κ2 N:O;κ2 O:N] monohydrate], {[Cu(pABA)2(H2O)]·H2O}n (pABA = p-amino-benzoate, C7H4NO2 -), was synthesized and characterized. It exhibits a one-dimensional chain structure extended into a three-dimensional supra-molecular assembly through hydrogen bonds and π-π inter-actions. While the twinned crystal shows a metrically ortho-rhom-bic lattice and an apparent space group Pbcm, the true symmetry is monoclinic (space group P2/c), with disordered Cu atoms and mixed roles of water mol-ecules (aqua ligand/crystallization water). The luminescence spectrum of the complex shows an emission at 345 nm, cf. 349 nm for pABAH.

Exploiting a Multi-Responsive Oxadiazole Moiety in One Three-Dimensional Metal-Organic Framework for Remedies to Three Environmental Issues.

Multifunctional metal-organic frameworks (MOFs) rely on the properties of metal centers (nodes) and/or linkers (struts) for their diverse applications in the emerging field of research. Currently, there is a huge demand for MOF materials in the field of capture/fixation/sensing of air pollutants, harmful chemical effluents, and nuclear waste. However, it is a challenging task to utilize one MOF for providing remedies to all these issues. On the basis of our current research activities, we have identified that an oxadiazole moiety-a five-membered ring with two different heteroatoms (O and N)-in a carboxylate linker can be the key to generating such MOF materials for its (a) inherent polarizable nature and molecular docking ability and (b) photoluminescence properties. In this work, we report a 3D MOF {[Co2(oxdz)2(tpbn)(H2O)2]·4H2O}n (1), self-assembled at room temperature from a three-component reaction, with an oxadiazole moiety (where H2oxdz = 4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoic acid and tpbn = N,N',N,"N″'-tetrakis(2-pyridylmethyl)-1,4-diaminobutane). The inherent polarizable nature of the oxadiazole moiety in 1 has been efficiently exploited for (i) multimedia iodine capture and (ii) fixation of CO2 under solvent-free and ambient conditions. On the other hand, the luminescent nature of the framework is found to be an efficient, highly preferred turn-on sensor for the ultra-fast detection of ketones with a limit as low as parts-per-trillion (mesitylene oxide: 447 ppt; cycloheptanone: 4.7 ppb; cyclohexanone: 17.2 ppb; acetylacetone: 18 ppb).

Topologically Driven Pore/Surface Engineering in a Recyclable Microporous Metal-Organic Vessel Decorated with Hydrogen-Bond Acceptors for Solvent-Free Heterogeneous Catalysis.

The use of metal-organic frameworks (MOFs) comprising custom-designed linkers/ligands as efficient and recyclable heterogeneous catalysts is on the rise. However, the topologically driven bifunctional porous MOFs for showcasing a synergistic effect of two distinct activation pathways of substrates (e.g., involving hydrogen bonding and a Lewis acid) in multicomponent organic transformations are very challenging. In particular, the novelty of such studies lies in the proper pore and/or surface engineering in MOFs for bringing the substrates in close proximity to understand the mechanistic aspects at the molecular level. This work represents the topological design, solid-state structural characterization, and catalytic behavior of an oxadiazole tetracarboxylate-based microporous three-dimensional (3D) metal-organic framework (MOF), {[Zn2(oxdia)(4,4'-bpy)2]·8.5H2O}n (1), where the tetrapodal (4-connected) 5,5'-(1,3,4-oxadiazole-2,5-diyl)diisophthalate (oxdia4-), the tetrahedral metal vertex (Zn(II)), and a 2-connected pillar linker 4,4'-bipyridine (4,4'-bpy) are unique in their roles for the formation, stability, and function. As a proof of concept, the efficient utilization of both the oxadiazole moiety with an ability to provide H-bond acceptors and the coordinatively unsaturated Zn(II) centers in 1 is demonstrated for the catalytic process of the one-pot multicomponent Biginelli reaction under mild conditions and without a solvent. The key steps of substrate binding with the oxadiazole moiety are ascertained by a fluorescence experiment, demonstrating a decrease or increase in the emission intensity upon interaction with the substrates. Furthermore, the inherent polarizability of the oxadiazole moiety is exploited for CO2 capture and its size-selective chemical fixation to cyclic carbonates at room temperature and under solvent-free conditions. For both catalytic processes, the chemical stability, structural integrity, heterogeneity, versatility in terms of substrate scope, and mechanistic insights are discussed. Interestingly, the first catalytic process occurs on the surface, while the second reaction occurs inside the pore. This study opens new ways to catalyze different organic transformation reactions by utilizing this docking strategy to bring the multiple components close together by a microporous MOF.

Subtle Ligand Spacer Change in 2D Metal-Organic Framework Sheets for Dual Turn-On/Turn-Off Sensing of Acetylacetone and Turn-On Sensing of Water in Organic Solvents.

Metal-organic framework (MOF)-based sensors for the detection of various analyte molecules has been a subject of absolute importance. However, most of these sensors rely on the turn-off (quenching) transduction response, while those reporting turn-on response are very rare. In this article, we have synthesized two new MOF-based sensors, {[Zn2(oxdz)2(tpbn)]·14H2O}n (1) and {[Zn2(oxdz)2(tpxn)]·10H2O·2C2H5OH}n (2), via the self-assembly of Zn(II) metal ions, a fluorogenic oxdz2- linker, and bis(tridentate) ligands (tpbn and tpxn) under ambient conditions. Their formation from such a self-assembly process has been evaluated on the basis of the geometry around the five-coordinated Zn(II), preferential meridional binding of the bis(tridentate) ligands, and diverse binding of the carboxylate groups in oxdz2-. Although 1 and 2 are isostructural, a difference in the transduction mechanism for the sensing of acetylacetone in organic solvents (turn-on for 1 and turn-off for 2) is observed and can be attributed to the spacer in the bis(tridentate) ligands. We have demonstrated the competing effect of the nonradiative interactions and photoinduced electron transfer toward the sensing mechanism. The results are well-supported by the Fourier transform infrared spectroscopy study, intensity versus concentration plots, spectral overlap measurements, time-resolved fluorescence studies, and MM2 and density functional theory calculations. Furthermore, we have showcased the utilization of 1 for the sensing of trace amounts of water in organic solvents.

Effect of the flexible chain length of a dimetal subunit on the formation of 1D coordination polymers to molecular rectangles.

Utilizing the angular and rigid furan dicarboxylate (fdc2-) ion, a new series of four (1-4) metal-organic coordination networks (MOCNs) is synthesized in good yields through a one-pot self-assembly reaction in methanol under ambient conditions to demonstrate the effect of Cu2 dimetal subunits, connected by flexible polypyridyl bis(tridentate) ancillary ligands, tpxn, where x refers to the number of methylene groups connecting the alkyl nitrogen atoms in the ancillary ligands and is equal to 2, 4, 6, and 7. The solid-state molecular structures of 1-4 are determined by single-crystal X-ray diffraction. A change in the dimensionality of the resultant MOCN is observed from a 1D coordination polymer (CP) for 1, 2, and 3 to a molecular rectangle for 4. Furthermore, each unit of 4 contains one NaClO4. Using electrospray ionization (ESI) mass spectrometry, their structural integrity in solution and their purity of existence as a single product are confirmed. Further characterization of 1-4 by FTIR and UV-vis (in solution and solid-state) spectroscopy, and FESEM and TEM is also reported. The presence of unsaturated metal centers in 1-4 provided an opportunity to compare their Lewis acid catalytic activities for the Knoevenagel condensation reaction of malononitrile with various aldehydes.

Tunable Strategies Involving Flexibility and Angularity of Dual Linkers for a 3D Metal-Organic Framework Capable of Multimedia Iodine Capture.

The widespread use of nuclear power poses severe health and environmental risks owing to the nonregulated release and disposal of radioactive wastes in the environment. Among these wastes, the capture and removal of radioactive iodine poses a big challenge. To develop a novel material for capturing molecular iodine, we have strategically synthesized a nitrogen-rich three-dimensional (3D) metal-organic framework (MOF), {[Mn2(oxdz)2(tpbn)(H2O)2]·2C2H5OH}n (1), utilizing a bent heterocyclic dicarboxylate linker (H2oxdz: (4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoic acid)) and a flexible bis(tridentate) ligand (tpbn: N, N', N″, N‴-tetrakis(2-pyridylmethyl)-1,4-diaminobutane). Based on its single-crystal structure, 1 is an eightfold interpenetrated 3D framework, consisting of a unique 4-connected {Mn2(tpbn)} subunit, in which the pores line up with the nitrogen atoms of the oxadiazole moiety. This can be considered as a big leap for the development of 3D MOFs using flexible bis(tridentate) ligands. To emphasize the role of the flexible methylene chain length in such ligand in the dimensionality of the resultant framework, the tphn (N, N', N″, N‴-tetrakis(2-pyridylmethyl)-1,6-diaminohexane) ligand with two additional methylene groups provides a one-dimensional (1D) CP {[Mn2(oxdz)2(tphn)(H2O)]·CH3OH}n (2). This spacer chain lengthening has a profound effect on the coordination of such ligand with Mn(II), further affecting the binding of oxdz. The inherent polarizable nature of the oxadiazole moiety and the presence of permanent pore of dimensions (19.122 × 19.253 Å2) in 1 have been exploited for the capture/removal of iodine not only from vapor and an organic solution but also from an aqueous media. It exhibits competent 100% reversible sorption of iodine with an uptake capacity of (1.1 ± 0.05) g/g of 1. The uptake value has been corroborated by both gravimetric and titrimetric analyses. The interaction of iodine with 1 has been notably studied with molecular simulations, kinetic models of sorption, field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDX) analysis. Moreover, 1 is highly stable and is recyclable without much loss of sorption capability up to five cycles.

Effecting structural diversity in a series of Co(II)-organic frameworks by the interplay between rigidity of a dicarboxylate and flexibility of bis(tridentate) spanning ligands.

In a one-pot self-assembly reaction of Co(OAc)2·4H2O, thiophene-2,5-dicarboxylic acid (H2tdc) and four different bis(tridentate) polypyridyl spanning ligands under ambient conditions, a series of structurally diverse metal-organic frameworks has been synthesised and characterized by single crystal X-ray diffraction: {[Co2(tdc)2(tpbn)(H2O)2]·solvent}n (solvent = 2H2O, 1; solvent = 2CH3OH, 2H2O, 1a), {[Co2(tdc)2(tphn)]·solvent}n (solvent = H2O, 2; solvent = CH3OH, 2.5H2O, 2a), {[Co2(tdc)2(tpchn)(H2O)2]·solvent}n (solvent = 5H2O, 3; solvent = C2H5OH, 2H2O, 3a), and {[Co2(tdc)2(tpxn)]·solvent}n (solvent = 6H2O, 4; when no solvent, 4a), where tpbn (N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,4-diaminobutane), tphn = N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, tpchn = N,N'-(cyclohexane-1,4-diylbis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) and tpxn = N,N'-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine). There is a profound effect of the nature of spacer between the alkyl nitrogens in the spanning ligands (flexible vs. semiflexible) on the molecular structures of 1a-4a. The notable differences are (a) the binding mode of the tridentate part of polypyridyl ligands to the Co(ii) center is facial in 1a, 3a and 4a but meridional in 2a, (b) the Co(ii) centers in 1a-3a are hexacoordinated (with a coordinated water in 1a and 3a) but are pentacoordinated in 4a, and (c) the binding mode of tdc linker is bis(monodentate) in 1a, 3a and 4a but chelated in one end and monodentate in the other end in 2a. Thus, the overall framework structure of 1a, 2a, 3a and 4a is cis-decalin type 2D polymer, ladder-shaped 1D polymer, hexagonal 2D net and cis-decalin type 2D polymer, respectively. Their thermal stabilities have been established by thermogravimetric analysis (TGA). The presence of an unsaturated metal center in 4 has provided us an opportunity for its use as an efficient Lewis acid catalyst for the Knoevenagel condensation reaction of malononitrile with various aldehydes (100% conversion in 60 minutes with 2 mol% catalyst in methanol).

A rational design and green synthesis of 3D metal organic frameworks containing a rigid heterocyclic nitrogen-rich dicarboxylate: structural diversity, CO2 sorption and selective sensing of 2,4,6-TNP in water.

A heterocyclic fluorogenic linker, thiadiazole dicarboxylate (H2tdz), offering diverse coordination modes has been utilized to synthesize two three-dimensional (3D) metal organic frameworks {[Cd2(tdz)2(4,4'-bpy)2]·6.5H2O}n (1) and [Cu(tdz)(4,4'-bpy)]n (2) from a one-pot self-assembly reaction under ambient conditions in high yields. MOFs 1 and 2 are structurally characterized by elemental analysis, FTIR and UV-Vis spectroscopy, single crystal and powder X-ray diffraction, and thermogravimetric analysis. Their single crystal molecular structures reveal that 1 contains a permanent pore along the b-axis with dimensions of 14.576 × 17.567 Å2 while 2 forms a 3D two-fold interpenetrated structure with an extensively reduced pore size. The effect of interpenetration has been further elaborated by a comparison of their CO2 uptake capacities at 263 K, 273 K and 298 K. The uptake value of CO2 for non-interpenetrated 1 (24.8 cm3 g-1) is much higher compared to that of interpenetrated 2 (5 cm3 g-1) at 298 K. Furthermore, to obtain an insight into the adsorbate-adsorbent interaction owing to the presence of the polar thiadiazole moiety and polarizable CO2 molecules, the isosteric heat of adsorption (Qst) was calculated for 1 (29.1 kJ mol-1) and 2 (23.4 kJ mol-1). Thermal and chemical stabilities of 1 in water were ascertained by variable temperature powder X-ray diffraction (VT-PXRD) analysis. MOF 1 having a d10 metal center was utilized to harness the availability of nitrogen atoms in the tdz2- linker for the selective sensing of nitrophenols in water with a detection limit for TNP as low as 1.4 ppm. Moreover, 1 is found to be stable and recyclable up to four cycles.