Madhab C. Das

A Zn4O-containing doubly interpenetrated porous metal–organic framework for photocatalytic decomposition of methyl orange

A doubly interpenetrated semiconducting MOF Zn(4)O(2,6-NDC)(3)(DMF)(1.5)(H(2)O)(0.5)·4DMF·7.5H(2)O (UTSA-38) of a cubic net has been constructed, which exhibits photocatalytic activity for the degradation of methyl orange in aqueous solution.

A porous coordination polymer exhibiting reversible single-crystal to single-crystal substitution reactions at Mn(II) centers by nitrile guest molecules.

The porous coordination polymer {[Mn(L)(H(2)O)](H(2)O)(1.5)(DMF)}(n) (1) containing a water molecule coordinated at the apical position of each distorted octahedral Mn(II) center has been synthesized using the solvothermal technique by reacting Mn(NO(3))(2) x 4 H(2)O with a new flexible ligand (LH(2)) having isophthalic fragment and pyridine donors at the two ends. The coordinated water molecule could be substituted by nitrile guest molecules such as acetonitrile, acrylonitrile, allylnitrile, and crotononitrile (affording compounds 2-5, respectively) without loss of crystallinity. Interestingly, compound 1 selectively captures cis-crotononitrile into its cavity from a mixture of cis and trans isomers. Hence, the cis isomer can be separated from the trans isomer. In each case, 1.5 lattice water molecules and a dimethylformamide (DMF) molecule are also simultaneously replaced by certain numbers of these guest molecules. When these first-generation compounds 2-5 are dipped in DMF at room temperature with the lid of the vial open to the atmosphere, the mother crystal 1 is regenerated in each case. Thus, all of these substitution reactions are completely reversible. Also, the first-generation compounds 2-5 can be interconverted among one another by dipping them in appropriate nitrile guests. All of these phenomena could be observed in single-crystal to single-crystal fashion.

A new approach to construct a doubly interpenetrated microporous metal-organic framework of primitive cubic net for highly selective sorption of small hydrocarbon molecules

A new approach has been realized to construct a three-dimensional doubly interpenetrated cubic metal-organic framework Zn(2)(PBA)(2)(BDC)·(DMF)(3)(H(2)O)(4) (UTSA-36, HPBA=4-(4-pyridyl) benzoic acid, H(2)BDC=1,4-benzenedicarboxylic acid) through the self-assembly of the pyridylcarboxylate linker 4-(4-pyridyl) benzoate and bicarboxylate linker 1,4-benzenedicarxylate with paddle-wheel [Zn(2)(COO)(4)]. The activated UTSA-36a exhibits highly selective gas sorption of C(2)H(6), C(2)H(4) and C(2)H(2) over CH(4) with the Henry laws selectivities of 11 to 25 in the temperature range of 273 to 296 K attributed to the unique 3D intersected pore structure of about 3.1 to 4.8 Å within the framework, indicating that UTSA-36a is a potentially very useful and promising microporous material for such industrially important separation of C(2) hydrocarbons over methane.

Coordination polymers with pyridine-2,4,6-tricarboxylic acid and alkaline-earth/lanthanide/transition metals: synthesis and X-ray structures

Pyridine-2,4,6-tricarboxylic acid (ptcH(3)) reacts with Cd(II), Mn(II), Ni(II), Mg(II), Ca(II), Sr(II), Ba(II), Dy(III) salts forming different products depending on the reaction conditions. In the presence of pyridine at room temperature the acetate, chloride or nitrate salt of Cd(II) breaks the ligand to form an open framework structure with the empirical formula, {[Cd(Ox)(H(2)O)(2)]H(2)O}(n) (Ox = oxalate), 1. In the absence of pyridine, no crystalline compound could be isolated at room temperature (RT). However, under hydrothermal conditions and in the absence of pyridine, a discrete tetrameric complex with the formula, {[Cd(2)(cda)(2)(H(2)O)(4)](H(2)O)(3)}(2) (cdaH(2) = 4-hydroxypyridine-2,6-dicarboxylic acid), 2, is formed where the carboxylate group at the 4-position of the ligand is reduced to a hydroxyl group. When Ni(II), Mn(II), Mg(II), Ca(II), Sr(II), Ba(II), Dy(III) salts are used in place of Cd(II), no crystalline product could be isolated at RT. But under hydrothermal conditions, coordination polymers ({[Ni(1.5)(ptc)(pip)(0.5)(H(2)O)(4)].H(2)O}(n), (pip = piperazine), 3; {Mn(1.5)(ptc).2H(2)O}(n), 4; {Mg(3)(ptc)(2).8H(2)O}(n), 5; {[Mg(ptc)(H(2)O)(2)].1/2[Mg(H(2)O)(6)].H(2)O}(n), 6; {Ca(1.5)(ptc).2H(2)O}(n), 7; {Sr(1.5)(ptc).5H(2)O}(n), 8; {[Ba(ptc)(H(2)O)][Ba(ptcH(2))H(2)O]}(n), 9; {[Dy(ptc).3H(2)O].H(2)O}(n), 10) are formed. The structures exhibit different dimensionality depending on the nature of the metal ions. In 1 a discrete acyclic water hexamer is also identified. All the compounds are characterized in the solid state by X-ray crystallography, IR and elemental analysis.

Effect of Bulkiness on Reversible Substitution Reaction at MnII Center with Concomitant Movement of the Lattice DMF: Observation through Single-Crystal to Single-Crystal Fashion

The porous coordination polymer ({[Mn(L)H(2)O](H(2)O)(1.5)(dmf)}(n), 1) (DMF=N,N-dimethylformamide) exhibits variety of substitution reactions along with movement of lattice DMF molecule depending upon bulkiness of the external guest molecules. If pyridine or 4-picoline is used as a guest, both lattice and coordinated solvent molecules are simultaneously substituted (complexes 6 and 7, respectively). If a bulky guest like aniline is used, a partial substitution at the metal centers and full substitution at the channels takes place (complex 8). If the guest is 2-picoline (by varying the position of bulky methyl group with respect to donor N atom), one Mn(II) center is substituted by 2-picoline, whereas the remaining center is substituted by a DMF molecule that migrates from the channel to the metal center (complex 9). Here, the lattice solvent molecules are substituted by 2-picoline molecules. For the case of other bulky guests like benzonitrile or 2,6-lutidine, both the metal centers are substituted by two DMF molecules, again migrating from the channel, and the lattice solvent molecules are substituted by these guest molecules (complex 10 and 11, respectively). A preferential substitution of pyridine over benzonitrile (complex 12) at the metal centers is observed only when the molar ratio of PhCN:Py is 95:5 or less. For the case of an aliphatic dimethylaminoacetonitrile guest, the metal centers remain unsubstituted (complex 13); rather substitutions of the lattice solvents by the guest molecules take place. All these phenomena are observed through single crystal to single crystal (SC-SC) phenomena.

Structural variation of transition metal coordination polymers based on bent carboxylate and flexible spacer ligand: polymorphism, gas adsorption and SC-SC transmetallation

Reaction of the bent dicarboxylate ligand H2OBA (H2OBA = 4,4′-oxybisbenzoic acid) and the flexible linker 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L1), under diverse reaction conditions, forms two polymorphic Co(II) coordination polymers (CPs): {[Co(OBA)(L1)]·DMF}n (1), as a three dimensional (3D) framework with a pcu alpha-Po primitive cubic topology, and {[Co(OBA)(L1)]·DMF}n, (2), as a two dimensional (2D) structure with a 6-c uninodal net topology. Gas adsorption measurements of the desolvated Co(II) CPs show negligible uptake of all gases in 1, while 2 exhibits moderate uptake of CO2, with good selectivity over N2 and CH4. With Zn(II), reaction of H2OBA and L1 produces a different 2D CP, {[Zn0.5(OBA)0.5(L1)0.5]}n (3). Finally, three isostructural Cd(II) CPs, {[Cd(OBA)(L1)]·DMF}n (4), {[Cd(OBA)(L1)]·DEF}n (5), and {[Cd(OBA)(L1)]·DMA}n (6) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide), that differ only in the lattice solvent molecules and show 2D structural arrangements are prepared. Interestingly, CP 4 undergoes single-crystal to single-crystal (SC-SC) transmetallation reaction at room temperature, yielding isostructural {[Cu(OBA)(L1)]·DMF}n (7) that cannot be synthesized independently. Moreover, the luminescence properties of compounds 1, 2, 3, and 4 have been studied in the solid state at room temperature. All the complexes are characterized by elemental analysis, IR, TGA, PXRD and single crystal X-ray diffraction.

Diversity of binding of sulfate and nitrate anions with laterally asymmetric aza cryptands

An X-ray crystallographic study of binding of nitrate and sulfate anions with two laterally asymmetric aza cryptands (Lo and Lm) having different cavity dimensions is reported. A variety of binding modes for these anions have been observed. Cryptand Lm having a larger cavity than that of Lo, prefers to encapsulate a planar nitrate over Td sulfate in its cavity when both anions are available for complexation. A discrete (H2O)29 cluster can be identified as surrounded by six sulfate ions attached to two Lm units. 1H NMR titration results indicate that either [H3Lo]3+ or [H3Lm]3+ binds strongly with sulfate rather than nitrate in solution.

Binding of various anions in laterally non-symmetric aza-oxa cryptands through H-bonds: characterization of water clusters of different nuclearity

The X-ray crystallographic study of various anionic complexes have been performed on two laterally non-symmetric aza-oxa cryptands Lo and Lm having different cavity dimension. The anions are bound through various N/C/O–H⋯X (X = anion) bonding interactions with the cryptand receptors. For the chloride complex of Lo (complex 1), the encapsulated chloride resides perfectly on C3 axis and prefers to stay at the ‘oxa’ end despite H- bonding interactions with protonated ‘aza’ N atoms. In case of the perchlorate (complex 2), the anion remains outside the cavity. The anions occupy the cavity created by four arms of two cryptand molecules and also the space provided by the two arms of a single cryptand molecule. Although, Lm has a larger cavity, perchlorate still remains outside (complex 3). To probe preferences for encapsulation, cryptand Lm is allowed to react with three different binary mixtures of acids. For the case of HCl–H2SO4 and HCl-HNO3 binary systems (complex 4 and 5, respectively), Cl− is incorporated in the cavity leaving SO4− or NO3− outside. In case of HBr-HNO3 mixture, NO3− is preferred over Br− in the cavity (complex 6). In each case for Lm, the encapsulated anion is displaced away from ‘oxa’ end to the ‘aza’ end of the cavity. Interestingly, in 2–6 the secondary amino N in the bridges and the bridgehead N at the ‘aza’ end gets protonated. In these compounds, water clusters of various nuclearity have been identified.

Two azo-functionalized luminescent 3D Cd(II) MOFs for highly selective detection of Fe3+ and Al3+

Two cadmium-based 3D luminescent MOFs {[Cd2(SA)2(L)2]·H2O}n (Cd-MOF-1) and [Cd(CDC)(L)]n (Cd-MOF-2) (H2SA = succinic acid, H2CDC = 1,4-cyclohexanedicarboxylic acid, L = [3,3′-azobis(pyridine)]) have been assembled by employing organic dicarboxylic acid linkers with an unexploited azo-functionalized N,N′ spacer via a room temperature slow evaporation process, and they are characterized by single crystal X-ray analysis, TGA, FT-IR, PXRD and elemental analysis. The topological analysis reveals that Cd-MOF-1 features a 6c-uninodal rare ‘rob’ topology with the point symbol {48·66·8}, whereas Cd-MOF-2 shows a ‘pcu’ alpha-Po primitive cubic topology with the point symbol {412·63}. These MOFs are highly emissive at 382 nm and 398 nm when excited at 305 nm and 312 nm, respectively. The exposed azo groups are presumed to act as functional sites for the recognition of metal ions through quenching of fluorescence intensity. The fluorescence measurements show that these MOFs can selectively and sensitively detect Fe3+ as well as Al3+ and thus, they demonstrate potential as dual-responsive luminescent probes for metal-ion sensing. EDS elemental mapping, PXRD of loaded MOF materials, and a plausible quenching mechanism have been discussed. More importantly, both the MOFs exhibit rapid response times toward sensing of Fe3+ and Al3+.