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Two MOFs are showing a low-energy solid-state transformation and a dissolution–recrystallization structural transformation respectively. The hydrogen uptake was found to be influenced by the polycatenation present in one of the frameworks.

We synthesized a two-fold interpenetrated 3D MOF with two crystallographically distinct C[double bond, length as m-dash]C bonds, which undergoes [2+2] photo-cycloaddition and thermal reversible reaction, in a single-crystal-to-single-crystal (SCSC) manner. The softer nature and comparable mechanical properties of the crystals of the parent and cyclized MOFs revealed by nanoindentation allowed rationalizing their structural softness and SCSC transformation behaviour.

Two metal–organic frameworks of Cd(II) from 3,4-pyridinedicarboxylate (3,4-pyrdc) and 1,2-bis(4-pyridyl)ethane (bpe) ligands, {[Cd(3,4-pyrdc)(bpe)(CH3OH)]·(H2O)}n (1) and {[Cd(3,4-pyrdc)(bpe)0.5(H2O)]·(H2O)}n (2) have been synthesized by changing the reaction medium. Compound 1 exhibited a 2D structure while compound 2 showed a 3D structure, which have both been revealed by single crystal X-ray studies. Both of the compounds showed interesting solvent mediated reversible structural transformations which have been established by exhaustive X-ray powder diffraction studies, elemental analysis, IR spectroscopy and thermogravimetric analysis. The desolvated form of the 2D framework (1) undergoes an irreversible structural transformation to form the 3D framework (2) upon soaking with water. The 2D to 3D transformation occurs through precise bond rotation and bond breaking which has been established by a GC–MS study. Sorption experiments with different gases and volatiles were performed for both the metal–organic frameworks and clearly indicated the differences in their inherent flexibility and transformability.

Two new mixed ligand metal–organic frameworks of Zn(II) with disodium 5-hydroxyisophthalate and 4,4′-azobispyridine (azbpy) ligands, {[Zn(azbpy)(HO-1,3-bdc)(H2O)]·(azbpy)}n (1) and {[Zn(azbpy)0.5(HO-1,3-bdc)(C2H5OH)]·(H2O)}n (2) have been synthesized by changing the reaction medium (methanol to ethanol) and structurally characterized by elemental analysis, IR, PXRD, TG and single crystal X-ray diffraction. Compound 1 exhibits a 2D sheet network structure with free azbpy ligands in its void space, and is stabilized by π–π and C–H⋯π interactions, whereas 2 has a 2D layered architecture with lattice water molecules in its void space. Compound 2 has a flexible structure and shows gated adsorption (gas and solvent) behavior, while framework 1 is nonporous. These two MOFs exhibit remarkable electrical conductivity values at room temperature and their comparison is discussed carefully. Theoretical calculations suggest that both the compounds are p-type semiconductors and correlate the structure–property relationship. Schottky barrier diode electronic devices have been fabricated by using these two semiconductor materials with aluminium (Al) and indium tin oxide (ITO) in sandwich configuration, ITO/MOF-1 or 2/Al, and both the devices exhibit sound rectification behavior. The photoluminescent properties of both the compounds in the solid state are also investigated in detail.

Porous coordination polymers (PCPs), also referred to as metal–organic frameworks (MOFs), have firmly established themselves as a class of excellent solid-state sorbents for carbon dioxide (CO2) along with their other several exciting properties. The mixed-ligand PCPs, constructed with polycarboxylates and N,N′-donor ligands, have been adopted for the fabrication of novel functional PCPs/MOFs, as the combination of different ligands with metal ions offer a better control over the structural variation of the frameworks compared to a single ligand. In this highlight, we have emphasized some of such important mixed linker-based MOFs with different carboxylate ligands and N,N′-donor linkers that act as excellent materials for CO2 adsorption and separation. The prospect of such mixed-ligand MOFs for the effective separation and sequestration of CO2 is also addressed by means of discussing different strategies for designing mixed-ligand MOFs that not only can potentially improve the amount of CO2 adsorption but also can increase the selectivity of CO2 uptake over other gases and volatiles.

Two new isostructural porous coordination polymers of Zn(II) {[Zn2(NH2-bdc)2(4-bpdb)]·(H2O)4}n (1) and {[Zn2(NH2-bdc)2(4-bpdh)]·(H2O)4}n (2) [4-bpdb = 1,4-bis-(4-pyridyl)-2,3-diaza-1,3-butadiene, 4-bpdh = 2,5-bis-(4-pyridyl)-3,4-diaza-2,4-hexadiene and NH2-bdc = 5-amino-1,3-benzenedicarboxylate] have been synthesized using a mixed ligand system by solvent diffusion and structurally characterized through single crystal X-ray diffraction, variable temperature powder X-ray diffraction and thermogravimetric analysis. Both the coordination polymers are constructed using linear Schiff base linkers of similar length having N–N base functionalities but the only difference is the presence of methyl groups in adjacent carbon atoms of the N–N group in the 4-bpdh ligand. Single-crystal structure analysis revealed that both compounds 1 and 2 have two-dimensional (2D) pillared-bilayer framework structures containing 1D channels (8.3 × 3.8 A2 for 1 and 8.0 × 1.6 A2 for 2) filled with lattice water molecules. Channel dimensions in 2 decrease due to the presence of methyl groups. The desolvated frameworks of 1 and 2 are rigid which is evidenced by variable temperature PXRD. Both the compounds show type-I CO2 uptake profiles and the differences in CO2 adsorption uptakes have been corroborated to their void space (27.1% for 1 and 17.1% for 2). Desolvated forms of compound 1 exhibit remarkably high water adsorption capacity even at low vapor pressure whereas desolvated forms of compound 2 show very low water vapor uptake, which could be ascribed to the hydrophobic nature of the pore surface of 2.

Two new Cd(II) coordination polymers, namely [Cd(3-bpdh)2(ClO4)2]n (1) and {[Cd(3-bpdh)(suc)(H2O)]·3(H2O)}n (2), have been synthesized using a substituted bent N,N'-donor ligand 2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene (3-bpdh) and aliphatic dicarboxylate disodium succinate (suc) with Cd(II) perchlorate salts at room temperature by a slow diffusion technique for the exploration of our previous reported work. Both the structures were determined by single-crystal X-ray diffraction analysis and also by other physicochemical methods. Structure analysis revealed that complex 1 is a 1D chain structure containing coordinated perchlorate with a metal centre, and complex 2 shows a porous 3D framework with encapsulation of lattice water molecules into the void along the crystallographic a-axis. The PXRD study shows the bulk purity of both the complexes and TGA analysis of 2 exhibits that the structure is thermally stable up to 250 °C. Complex 1 shows a nice anion exchange property with replacement of weakly coordinated perchlorate with the inclusion of new anions; and the anion exchanged solids were characterised by FT-IR, PXRD and photoluminescence properties. The desolvated framework of 2 exhibits sorption of CO2 and water vapor and surface adsorption of N2 corroborating with the nature of the pore environment present in 2. The photoluminescence study has been also done for both complexes in the solid state which exhibits ligand based emissions at room temperature.

Two isostructural coordination polymers based on Co(II) and Ni(II), {[M(azpy)3(p-bdc)2(H2O)4]·(CH3OH)2(H2O)3}n [where M = Co (1) and Ni (2); azpy = N,N′-bis-pyridin-4-ylmethylene-hydrazine and p-bdc = 1,4-benzenedicarboxylate], have been synthesized using mixed ligand systems at room temperature and characterized by single-crystal X-ray diffraction and other physicochemical methods. Structure determination reveals that both the complexes crystallize in the monoclinic space group C2/c and exhibit one-dimensional (1D) ladder like structures constructed by p-bdc and azpy ligands in which Schiff base (azpy) linkers serve as a bridging as well as pendent ligands. These pendent ligands are involved in H-bonding and π–π interactions with lattice water, methanol molecules and bridging azpy ligands, to form the 3D supramolecular structure. Notably, both the frameworks efficiently catalyze the Knoevenagel condensation reactions of a wide range of substituted benzaldehydes with active methylene compounds in heterogeneous medium under environmentally friendly conditions and the products were obtained in excellent yields. These catalysts were also found to exhibit excellent recyclability and re-usability without any significant loss of activity. A variable temperature magnetic study of both compounds was carried out and their magnetic properties justified on the basis of their crystal structure.

Five new mixed ligand coordination polymers, {[Co0.5(H2O)(1,4-bdc)(3-bpdb)Co0.5(H2O)2]·(H2O)3}n (1), [Co(3-bpdh)(1,4-bdc)(H2O)2]n (2), [Zn(3-bpdb)(1,4-bdc)]n (3), {[Zn(3-bpdh)(1,4-bdc)]·(3-bpdh)0.5}n (4) and [Cd(3-bpdb)(1,4-bdc)(H2O)]n (5) [where 3-bpdb = 1,4-bis-(3-pyridyl)-2,3-diaza-1,3-butadiene; 3-bpdh = 2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene and 1,4-bdc = benzene-1,4-dicarboxylate] were synthesized at room temperature. The structures of 1–5 were determined by single crystal X-ray diffraction analysis and were further characterized by elemental analysis, infrared spectroscopy (IR) and powder X-ray diffraction (PXRD). Compound 1 exhibits a one-dimensional (1D) chain structure with monodentate pendant 1,4-bdc ligands which further extend to a three-dimensional (3D) supramolecular structure by H-bonding and π⋯π interactions. Compound 2 displays a 4-connected two-dimensional (2D) framework with the point symbol {44·62}. Moreover, the 2D structure of 2 is also ultimately packed into 3D supramolecular frameworks through H-bonding and π–π stacking interactions. Compound 3 shows a 5-fold interpenetrated diamondoid net with 66 topology. Compound 4 is a 2D 3-connected net having a point symbol {63} and is extended to a supramolecular 3D structure through C–H⋯π and π⋯π interactions with lattice 3-bpdh ligands. Compound 5 features a 3-fold interpenetrated diamondoid net with 66 topology. The thermal stabilities and luminescent properties of 3–5 were also studied in detail. The complexes exhibit ligands based photoluminescence properties at room temperature.

We report the synthesis, structural characterization and adsorption properties of three new porous coordination polymers {[Cu(Meazpy)0.5(glut)](H2O)}n (2), {[Zn(azpy)0.5(terep)](H2O)}n (3), and {[Zn(Meazpy)0.5(terep)]}n (4) [glut = glutarate, terep = terephthalate, azpy = N,N'-bis-(pyridin-4-ylmethylene)hydrazine and Meazpy = N,N'-bis-(1-pyridin-4-ylethylidene)hydrazine] composed of mixed linkers systems. Structure determination reveals that all three compounds have three-dimensional (3D) coordination frameworks bridged by dicarboxylates and Schiff base linkers. In all cases 2D dicarboxylate layers are supported by paddle-wheel M2(CO2)4 SBUs extended in three dimensions by designed Schiff base linkers. Compound 1, which has been reported in a paper earlier by our group, is a robust porous three-dimensional (3D) framework whose pore surface was found to be decorated with the -CH=N- groups of a linear Schiff base (azpy) and it showed reversible single-crystal-to-single-crystal transformation and selective CO2 uptake. By using another linear Schiff base linker Meazpy, we have synthesized compound 2 which is isostructural with 1, having an additional methyl group pointing towards the pore. Like 1 it also shows a reversible single-crystal-to-single-crystal transformation upon dehydration and rehydration. The dehydrated framework of 2 exhibits 50% enhanced CO2 uptake compared to 1. This has been achieved by the pore surface modification effected upon changing the pillar backbone from a -CH=N- to -CMe=N- group. It also adsorbs water vapour at 298 K. In the case of the two isostructural 3D MOFs 3and 4, the use of a rigid carboxylate (terephthalate) linker arrested porosity by three-fold interpenetration. We showed that the use of aliphatic dicarboxylate (glutarate) results in a non-interpenetrated framework rather than the common interpenetrated framework with aromatic dicarboxylates in mixed ligand systems.

Ten different metal-organic hybrids of divalent metal ions have been synthesized using the diphenyl phosphate ligand and three different N,N donor spacers. All the complexes have been characterized by single crystal X-ray crystallography and other physico-chemical methods. For six complexes, the diphenyl phosphate (dpp) ligand acts as a bridge between the metal ions, whereas in four complexes dpp acts as a monodentate ligand. All nine complexes (1-9) show two dimensional coordination structures created by the N,N donor spacers, and, the remaining one (10) has a one dimensional ribbon-like structure. Here the aromatic rings present in the compounds viz. phenyl rings of dpp and the pyridyl rings of N,N donor spacers, have a key role to extend the dimensionality in their solid-state structures. In each instance, the aromatic rings of dpp and N,N donor spacers are nicely involved in the supramolecular structure generation by the help of π-π interactions. In all the cases, the role of weak interactions on the crystal packing and their effect in the generation of different supramolecular architectures has been thoroughly investigated. Variable temperature magnetic measurements of three relevant complexes have been carried out. Compounds 1 and 3 clearly indicate the existence of a weak antiferromagnetic interaction between the metal ions through the bridging dpp ligand. In 5, where dpp acts as a monodentate ligand; the metal ion may be considered as an isolated monomer.

Three cadmium complexes, [Cd(H2biim)2(SCN)2] (1), [Cd(H2biim)2(SeCN)2] (2) and [Cd(H2biim)2(dca)2] (3) [H2biim = 2,2′-biimidazole; dca = dicyanamide] have been synthesized and characterized by X-ray single crystal structure analysis. In complexes 1 and 2 monomeric units assemble into supramolecular helix whereas in 3 monomeric units assemble into supramolecular 2-D grid with (4,4) net topology. The appearance of helical topology in complexes 1 and 2 is due to the linear nature of SCN−/SeCN− whereas (4,4) net topology in complex 3 is due to the bent nature of dca−. In each case R12(7) hydrogen bonding synthon is attained facilitating the helical and (4,4) net topology.

Four succinato-bridged complexes of copper(II) have been synthesized. Complex 1, [Cu(2)(mu-OH(2))(2)L(bpy)(2)(NO(3))(2)](n) and 2, [Cu(2)(mu-OH(2))(2)L(phen)(2)(NO(3))(2)](n)(bpy = 2,2[prime or minute]-bipyridine; phen = 1,10-phenanthroline and LH(2)= succinic acid) exhibit 1D coordination polymer structures where both the nitrate ions are directly linked to the copper(ii) producing synthons in a 2D sheet. A novel 2D grid-like network, ([Cu(4)L(2)(bpy)(4)(H(2)O)(2)](ClO(4))(4)(H(2)O))n3, is obtained upon changing the nitrate by perchlorate anion in complex 1, where the channels are occupied by the anions. On changing the nitrate by tetrafluoroborate anion in complex 2, a novel octanuclear complex, [Cu(8)L(4)(phen)(12)](BF(4))(8).8H(2)O 4, is isolated. The coligand bpy and phen in these complexes show face-to-face (in 1,2,3,4) or edge-to-face (in 4 )pi-pi interactions forming the multidimensional supramolecular architectures. Interestingly, the appearance of edge-to-face pi-pi interactions in complex facilitates the formation of discrete octanuclear entities. Variable-temperature (300-2 K) magnetic measurements of complexes have been done. Complexes 1 and 2 show very weak antiferromagnetic (OOC-CH(2)-CH(2)-COO) and ferromagnetic coupling (mu-H(2)O). Complex 3 also shows antiferromagnetic (syn-syn mu-OCO), and ferromagnetic coupling (mu-O of the -COO group). Complex 4 with two types (syn-syn and syn-anti) of binding modes of the carboxylate group shows strong antiferromagnetic interaction.

A metal-organic framework (MOF) of cadmium(ii) is reported here which is the first example of an experimentally achieved MOF based electronic device, and in the present case it is a Schottky diode.

Dicarboxylate supported metal organic hybrids of Co(II), Zn(II) and Cd(II) have been synthesized using two different pyridyl based N,N′ linkers having Schiff base functionalized site. The use of flexible dicarboxylate glutarate in designing such frameworks has created a marked diversity in topology. The different N,N′ donor linkers also played an active part in the channel modification in the synthesized MOFs. The structural and topological diversity has been analyzed from the single crystal X-ray structure. Five compounds, {[Co(azpy)(glut)]·(CH3OH)}n (1), {[Co(meazpy)(glut)(H2O)2]·(H2O)3}n (2), {[Zn(azpy)0.5(glut)(H2O)]·(azpy)0.5}n (3), {[Zn(meazpy)0.5(glut)(H2O)]·(H2O)2}n (4) and {[Cd(azpy)(glut)]·(CH3OH)}n (5), show porous structures with solvent accessible voids. The nature of the pores as well as the existence of lattice solvent molecules in 1 and 2 are different due to the use of a different pillar ligand in their fabrication. In case of 3 and 4 there are some nice effects of non-covalent interaction in the construction of their solid state structure, which has also originated by the change of pillar N,N′ donor linkers. Complex 5 is topologically as well as structurally similar to 1 forming a 2D-grid like structure. In {[Cd2(meazpy)2(glut)(NO3)]2}n (6) there is a formation of 2D sheets with the coordinated counter anion. Interestingly, here the sheets are disposed in a perpendicular fashion to each other and do not contain any solvent accessible void. Upon removal of the solvent molecules, the frameworks 1–5 show moderate CO2 and H2 uptake at 273 K and 77 K, respectively. The desolvated frameworks show different quantities of CO2 and H2 uptake which has been corroborated to their structures.

Three new metal–organic hybrids have been synthesized in a reaction of Cd(II) with 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (3-bpdb) and disodium succinate (Na2suc). All three compounds, i.e., {[Cd(3-bpdb)(Cl)]·ClO4}n (1), {[Cd(3-bpdb)3(H2O)2]·(3-bpdb)(ClO4)2}n (2) and {[Cd(3-bpdb)(suc)(H2O)2]·(H2O)2}n (3) are characterized by single-crystal X-ray diffraction and other physicochemical methods. Structure determination reveals that 1 shows an α-polonium type 3D coordination network created by an exactly perpendicular Cl–Cd–Cl linkage. The framework of 1 contains 1D channels, which are filled with ClO4−. Compound 2 shows a 1D coordination structure with two bridging and two pendent 3-bpdb ligands. These pendent ligands are involved in H-bonding, π–π and C–H⋯π interactions with its coordinated water molecules and lattice 3-bpdb ligands, to form the 3D supramolecular structure. Compound 3 is a 2D 4-connected net with succinate and the 3-bpdb ligand and extended to 3D supramolecular architecture by H-bonding and π–π interactions. During the syntheses, an in situ chemical transformation of perchlorate to chloride has occurred along with the oxidation of imine to amide and the chlorides so produced are found integrated in compound 1, which facilitates the oxidation of imine in a very unprecedented way.

A 2-D croconato bridged complex of Cu(II), [{Cu(C5O5)(bpee)}·0.5H2O]n (1) [bpee, trans-1,2-bis(4-pyridyl)ethylene] has been synthesized by in situ hydrothermal reaction and characterized by X-ray diffraction analysis, variable temperature magnetic susceptibility measurement and EPR study. The structure determination reveals that the adjacent 2-D sheets are extended to 3-D supramolecular architecture through π–π and trifurcated C–H⋯O interactions, which inhibit the oxygen atom of croconate from forming the usual bridging mode.

A novel 2D rectangular grid coordination polymer with a mixed valence localized copper(I/II), [Cu2ICuII(pyz)2(C5O5)2 (H2O)2]n (pyz, pyrazine; C5O52−, croconate dianion) has been hydrothermally synthesized viain situ redox reaction of Cu(II), and characterized by X-ray diffraction analysis, magnetic and EPR study. The structure extends to a 3D supramolecular network through H-bonding.