Parimal K. Bharadwaj

A coumarin-derived fluorescence probe selective for magnesium.

Two different coumarin derivatives have been connected via an imine linkage to obtain a new fluorescence signaling system. This compound itself does not show any emission due to rapid isomerization around the C[double bond]N bond. However, in the presence of a Mg(II) ion, this isomerization is stopped because of bonding to the metal ion resulting in high-intensity (approximately 550-fold) emission. Other metal ions like Li(I), Ca(II), and Zn(II) show very little emission, while biologically relevant transition-metal ions do not show any emission. In this way, the Mg(II) ion can be detected in the presence of these ions.

High Proton Conductivity by a Metal–Organic Framework Incorporating Zn8O Clusters with Aligned Imidazolium Groups Decorating the Channels

A novel metal-organic framework, [{(Zn(0.25))(8)(O)}Zn(6)(L)(12)(H(2)O)(29)(DMF)(69)(NO(3))(2)](n) (1) {H(2)L = 1,3-bis(4-carboxyphenyl)imidazolium}, has been synthesized under solvothermal conditions in good yield. It shows a Zn(8)O cluster that is coordinated to six ligands and forms an overall three-dimensional structure with channels along the crystallographic a and b axes. The imidazolium groups of the ligand moiety are aligned in the channels. The channels are not empty but are occupied by a large number of DMF and water molecules. Upon heating, these solvent molecules can be removed without breakdown of the overall structure of the framework as shown by variable-temperature powder X-ray diffraction patterns. Of great interest is the fact that the compound exhibits high proton conductivity with a low activation energy that is comparable to those of Nafion presently used in fuel cells.

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.

Synthesis and studies of Cu(II)-thiolato complexes: bioinorganic perspectives

Abstract Ligation of thiolate sulfur to copper at the active sites of quite a number of copper proteins has been established either by X-ray crystallographic and/or by spectroscopic studies. In addition, for Cu(II)-substituted metalloproteins, the presence of Cu(II)-thiolate bonding at the active sites could be established spectroscopically. Cu(II)-thiolate bonding in different enzymes is not always very similar. Obviously, the bioinorganic significance of Cu(II)-thiolate bonding is enormous and has attracted a lot of attention to synthesize model Cu(II)-thiolato complexes as electronic structural analogues of the active sites of these biomolecules. The present review deals with (i) nature of Cu(II)-thiolate bonding present in different metalloproteins, (ii) difficulties involved in the synthesis of Cu(II)-thiolates and ways to surmount them, (iii) characterization of the Cu(II)-thiolate bonding by electronic and EPR spectroscopic techniques and (iv) electron transfer properties of the Cu(II)-thiolato complexes by cyclic voltammetric studies. The properties of the Cu(II)-thiolato complexes have been discussed as possible models for the active site(s) of copper proteins.

Metal–organic framework structures of Cu(II) with pyridine-2,6-dicarboxylate and different spacers: identification of a metal bound acyclic water tetramer

The ligand pyridine-2,6-dicarboxylic acid (pdcH2) reacts under hydrothermal conditions, with Cu(NO3)2·6H2O in the presence of different N-heterocycles used as spacers to form 1D, 2D or 3D metal–organic framework structures depending on the nature of the spacer. These network structures are characterized by X-ray crystallography, variable temperature magnetic measurements, powder X-ray diffraction, infrared and thermal gravimetric analyses. With 4,4′-bipyridine spacer, a 2D network is formed where every alternate Cu(II) ion in the chain is coordinated terminally to an acyclic tetrameric water cluster.

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.

Two-Dimensional Coordination Polymer with a Non-interpenetrated (4,4) Net Showing Anion Exchange and Structural Transformation in Single-Crystal-to-Single-Crystal Fashion

A new non-interpenetrated two-dimensional (2D) rectangular-grid coordination polymer, {[Co(L)(2)(H(2)O)(2)] x (BF(4))(2) x 4 DMF}(n) (1), has been synthesized using a new rod-like ligand, 3,5-bis(4-imidazol-1-ylphenyl)-[1,2,4]triazol-4-ylamine (L). Weakly H-bonded BF(4)(-) anions present within the voids can be exchanged by ClO(4)(-) and NO(3)(-) anions to generate {[Co(L)(2)(H(2)O)(2)] x (ClO(4))(2) x 2 DMF x 2 H(2)O}(n) (2) and {[Co(L)(2)(H(2)O)(2)] x (NO(3))(2) x 2 DMF x 2 H(2)O}(n) (3) in single-crystal-to-single-crystal (SC-SC) manner. In the case of exchange by Cl(-) ion, the crystallinity is not maintained, and so it is proven by IR spectroscopy, PXRD, and elemental analysis. In addition, 3 shows an interesting structural transformation (2D --> 1D) with bond rupture/formation leading to the formation of a new coordination polymer, {[Co(L)(2)(H(2)O)(2)] x (NO(3))(2) x 2 DMF x H(2)O}(n), (5), again in SC-SC fashion.

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.

Selectively sensing first-row transition metal ions through fluorescence enhancement

Transition metal ions, especially the first-row ones are ubiquitous in nature and they perform many biological functions. However, high accumulation of these ions can be quite detrimental to health. Their spatial distribution with high fidelity inside as well as outside cells is, therefore, of paramount importance. In addition to biological sensing, fluorescence enhancement by transition metal ions can be potentially useful in other areas of science. However, most of the transition metal ions are paramagnetic and they very effectively quench fluorescence. Surmounting this problem of fluorescence quenching, a number of systems have been reported which are the topic of the present review.

Template synthesis of a cryptand with hetero-ditopic receptor sites

The cryptand 2 forms exclusively in 70% yield via tripod-tripod Schiff base condensation of tris(2-aminoethyl)amine and the trialdehyde 1 in presence of Cs+ ion followed by reduction with NaBH4.