Sandeep K. Gupta

Pentanuclear Lanthanide Mono-organophosphates: Synthesis, Structure, and Magnetism

Research on rare-earth phosphates has recently received substantial interest because of their unique physical and chemical properties. In recent years, because of their low solubility, research interest has been built on developing methodologies to prepare nanostructures and grow single crystals of inorganic rare-earth phosphates. The chemistry of rare-earth organophosphates, however, is still at a latent stage. Contrary to the traditional hydrothermal route, we report rare examples of discrete pentanuclear lanthanide(III) organophosphate clusters assembled from a sterically encumbered monoester of phosphoric acid under mild reaction conditions. Single-crystal X-ray analysis revealed that all of the compounds possess a similar core structure, [Ln5(μ3-OH)(dipp)6(NO3)x(CH3OH)y(H2O)z]2+ [Ln = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), Ho (7), Er (8), Tm (9); dipp = 2,6-diisopropylphenylphosphate], where the anionic charge balance is maintained by the presence of chelating nitrate anions (in the case of 9, x = 0), protonated tmeda, or dipp2- ligands. The vacant coordination sites on the metal ions are satisfied by coordinated methanol or water molecules. The core structure of these clusters is built on a [Ln3(μ3-OH)(dipp)6] triangle where the phosphate ligands bridge to two further Ln(III) ions. The complexes display lanthanide contraction along the series, with Ln(III) ions displaying different coordination environments/geometries as we move along the series. All of the compounds have been characterized by both analytical and spectroscopic techniques. Magnetic studies revealed the presence of weak antiferromagnetic exchange through the bridging μ3-hydroxo moiety and organophosphate groups for the {GdIII5} analogue, with a significant magnetic entropy change (25.8 J kg-1 K-1, ΔH = 7 T). The anisotropic complexes reveal an absence of slow relaxation of magnetization, except for Nd (1), Dy (6), and Er (8), which show slow relaxation in an applied DC field.

Octanuclear Zinc Phosphates with Hitherto Unknown Cluster Architectures: Ancillary Ligand and Solvent Assisted Structural Transformations Thereof

Structural variations in zinc phosphate cluster chemistry have been achieved through a careful selection of phosphate ligand, ancillary ligand, and solvent medium. The use of 4-haloaryl phosphates (X-dippH2) as phosphate source in conjunction with 2-hydroxypyridine (hpy) ancillary ligand in acetonitrile solvent resulted in the isolation of the first examples of octameric zinc phosphates [Zn8(X-dipp)8(hpy)4(CH3CN)2(H2O)2]·4H2O (X = Cl 2, Br 3) and not the expected tetranuclear D4R cubane clusters. Use of 2,3-dihydroxypyridine (dhpy) as ancillary ligand, under otherwise similar reaction conditions with the same set of phosphate ligands and solvent, resulted in isolation of another type of octanuclear zinc phosphate clusters {[(Zn8(X-dipp)4(X-dippH)4(dhpyH)4(dhpyH2)2(H2O)2]·2solvent} (X = Cl, solvent = MeCN 4; Br, solvent = H2O 5), as the only isolated products. X-ray crystal diffraction studies reveal that 2 and 3 are octanuclear clusters that are essentially formed by edge fusion of two D4R zinc phosphates. Although 4 and 5 are also octanuclear clusters, they exhibit a completely different cluster architecture and have been presumably formed by the ability of 2,3-dihydroxypyridine to bridge zinc centers in addition to the X-dipp ligands. Dissolution of both types of octanuclear clusters in DMSO followed by crystallization yields D4R cubanes [Zn(X-dipp)(DMSO)]4 (X = Cl 6, Br 7), in which the ancillary ligands such as hpy, H2O, and CH3CN originally present on the zinc centers of 2-5 have been replaced by DMSO. DFT calculations carried out to understand the preference of Zn8 versus Zn4 clusters in different solvent media reveal that use of CH3CN as solvent favors the formation of fused cubanes of the type 2 and 3, whereas use of DMSO as the solvent medium promotes the formation of D4R structures of the type 6 and 7. The calculations also reveal that the vacant exocluster coordination sites on the zinc centers at the bridgehead positions prefer coordination by water to hpy or CH3CN. Interestingly, the initially inaccessible D4R cubanes [Zn(X-dipp)(hpy)]4·2MeCN (X = Cl 8, Br 9) could be isolated as the sole products from the corresponding DMSO-decorated cubanes 6 and 7 by combining them with hpy in CH3CN.

Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture

Structurally diverse mononuclear, dinuclear, and tetranuclear cobalt organophosphates and a three-dimensional framework based on a D4R cobalt phosphate are reported. The role of auxiliary ligands in determining the nuclearity of the phosphate clusters has further been established. Reaction of cobalt acetate tetrahydrate with 2,6-di-iso-propylphenylphosphate (dippH2) in methanol or DMSO in the presence of ancillary N-donor ligands leads to the formation of mononuclear octahedral cobalt phosphate [Co(dippH)2(py)4] (1), dinuclear octahedral cobalt phosphates [Co(dipp)(NN)(MeOH)2]2·2MeOH (NN = bpy 2; phen 3), tetrahedral cobalt phosphates [Co(dipp)(L)2]2·2(MeOH) (L = imz 4; dmpz 5) and symmetric and asymmetric tetranuclear tetrahedral cobalt phosphates [Co(dipp)(2-apy)]4 (6) and [Co4(dipp)4(2-apy)3(DMSO)]·(DMSO)·(H2O) (7), in nearly quantitative yields. The use of a linear N-donor ditopic linker, 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (dptz), as the ancillary ligand leads to the formation of a robust three dimensional, four-fold interpenetrated network based on the D4R platform, {[Co(dipp)(dptz)0.5]4}n (8), under ambient conditions. The new compounds have been characterized by analytical, thermo-analytical and spectroscopic techniques. Further, the molecular structures of compounds 1-8 have been established using single crystal X-ray diffraction studies. Compound 1 is a mononuclear complex in which the dippH ligands occupy trans-positions around the octahedral cobalt ion. The core structure of compounds 2-5, a Co2P2O4 ring, resembles the S4R (single-4-ring) SBU of zeolites, whereas the Co4P4O12 inorganic core found in compounds 6 and 7 resembles the D4R (double-4-ring) SBU. Cobalt organophosphate framework 8 shows significant CO2 adsorption at 273 K (7.73 wt% at 1 bar and 18.21 wt% at 15.5 bar) with high selectivity to CO2 uptake over N2 and H2 at 273 K. Magnetic studies of these symmetric complexes indicate the presence of weak antiferromagnetic interactions between the metal ions via the phosphate bridging moiety.

Elusive Double-Eight-Ring Zeolitic Secondary Building Unit

The double-eight-ring (D8R), an elusive secondary building unit of zeolites, has been stabilized for the first time, both in solution and solid-state. The present study further establishes that any of the three double-ring building blocks of zeolites, viz. D4R, D6R and D8R ([ArPO3Zn(L)]n (n = 4, 6 or 8)), can be preferentially isolated (over the other two) through a careful choice of metal source, aryl phosphate and ancillary ligand, apart from maintaining a meticulous control on the reaction conditions.

Enriching lanthanide single-ion magnetism through symmetry and axiality

Rapidly growing modern information technology demands energy and cost efficient tools that can efficiently store and process a large amount of data. However, the miniaturization technology that was being used to boost the performance of the electronic devices, keeping up with the pace as estimated by Moores law, is reaching its limit. To overcome these challenges, several alternative routes that can eventually mimic the modern electronics fabrication using silicon have been proposed. Single molecule magnets (SMMs), being considered as one of the potential alternatives, have gone through significant progress and the focus has shifted from the use of polynuclear clusters to mononuclear complexes in the last few years. The recent frenzy in the field of SMMs is driven by a better understanding of the effects of crystal field (CF) and molecular symmetry on the magnetic properties, especially in the case of mononuclear paramagnetic complexes, apart from other controlling factors. This has led to the advent of highly anisotropic single-ion magnets (SIMs) with magnetic blocking temperatures as high as 60 K and anisotropic energy barriers over 1800 K. This article overviews our recent research in the light of the emergence of the importance of CF and symmetry in 4f ion based single-ion magnets (SIMs), especially in the context of SIMs with D5h symmetry, apart from commenting on the synthetic efforts adopted to place these metal ions in unusual coordination geometries.

A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions.

Reaction of Zn(OAc)2 ⋅2 H2 O with 2,6-diisopropylphenyl phosphate (dippH2 ) in the presence of pyridine-4-carboxaldehyde (Py-4-CHO) in methanol resulted in the isolation of a tetrameric zinc phosphate cluster [Zn(dipp)(Py-4-CH(OH)(OMe))]4 ⋅4 MeOH (1) with four hemiacetal moieties stabilized on the double-4-ring inorganic cubane cluster. The change of solvent from methanol to acetonitrile leads to the formation of [Zn(dipp)(Py-4-CHO)]4 (2), in which the coordinated Py-4-CHO retains its aldehydic form. Dissolution of 1 in CD3 CN readily converts it to the aldehydic form and yields 2. Similarly 2, which exists in the aldehyde form in CD3 CN, readily converts to the hemiacetal form in CD3 OD/CH3 OH. Compound 1 is an unprecedented example in which four hemiacetals have been stabilized on a single molecule in the solid state retaining its stability in solution as revealed by its (1) H NMR spectrum in CD3 OD. The solution stability of 1 and 2 has further been confirmed by ESI-MS studies. To generalize the stabilization of multiple hemiacetals on a single double-four-ring platform, pyridine-2-carboxaldehyde (Py-2-CHO) was used as the auxiliary ligand in the reaction between zinc acetate and dippH2 , leading to isolation of [Zn(dipp)(Py-2-CH(OH)(OMe))]4 (3). Understandably, recrystallization of 3 from acetonitrile yields the parent aldehydic form, [Zn(dipp)(Py-2-CHO)]4 (4). Single-crystal X-ray diffraction studies reveal that supramolecular bonding, aided by hydrogen-bonding interactions involving the hemiacetal functionalities (C-OH, C-OMe, and C-H), are responsible for the observed stabilization. The hemiacetal/aldehyde groups in 1 and 2 readily react with p-toluidine, 2,6-dimethylaniline, and 4-bromoaniline to yield the corresponding tetra-Schiff base ligands, [Zn(dipp)(L)]4 (L=4-methyl-N-(pyridin-4-ylmethylidene)aniline (5), 2,6-dimethyl-N-(pyridin-4-ylmethylene)-aniline (6), and 4-bromo-N-(pyridin-4-ylmethylene)aniline (7)). Isolation of 5-7 opens up further possibilities of using 1 and 2 as new supramolecular synthons and ligands.

Lanthanide Organophosphate Spiro Polymers: Synthesis, Structure, and Magnetocaloric Effect in the Gadolinium Polymer

Spirocyclic lanthanide organophosphate polymers, {[Ln(dipp)(dippH)(CH3OH)(H2O)2](CH3OH)2}n [Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11)], have been prepared from the reaction of Ln(NO3)3·xH2O with sterically hindered 2,6-diisopropylphenyl phosphate (dippH2) using aqueous NaOH as the base. The one-dimensional chainlike lanthanide (III) organophosphate coordination polymers have been characterized with the aid of analytical and spectroscopic methods. The single crystal structure determination of polymers (2-5 and 7-11) reveals that in these compounds the hydrophobic organic groups of the phosphate provide a protective coating for the inorganic lanthanide phosphate polymeric chain. The encapsulation of inorganic lanthanide phosphate core, which has very low solubility product, within the organic groups assists in the facile crystallization of the polymers. The di- and monoanionic organophosphate ligands dipp2- and dippH- display [2.111] and [2.110] binding modes, respectively, in 2-5 and 7. However, they exhibit only [2.110] binding mode in the case of 8-11. This results in the formation of two different types of polymers. While the lighter rare-earth metal ions in 2-5 and 7 display eight coordinate biaugmented trigonal prismatic geometry, the heavier rare-earth metal ions in 9-11 exhibit a seven coordinate capped trigonal prismatic environment. The Tb(III) ion in 8 displays distorted pentagonal bipyramidal geometry. Magnetic studies reveal the presence of weak antiferromagnetic interactions between the Ln(III) ions through the organophosphate ligand. The isotropic Gd(III) polymer 7 exhibits a maximum entropy change of 17.83 J kg-1 K-1 for a field change of 7.0 T at 2.5 K, which is significant considering the high molecular weight of the organophosphate ligand. These polymers represent the first family of any structurally characterized rare-earth organophosphate polymers derived from monoesters of phosphoric acid.

2,6-Dimethylphenol derived H-phosphonate and α-hydroxyphosphonate: facile synthesis, crystal chemistry, supramolecular association and metal complexation

Facile synthesis of a H-phosphonate [(ArO)P(O)H(OH)] (2) and an α-hydroxyarylphosphonate [(ArO)P(O)(OH)(CMe2OH)] (3) has been achieved through the hydrolysis of [(ArO)PCl2] (1) with toluene and acetone as the solvent, respectively. The dichloride (1) itself was synthesized from the reaction of ArOH (Ar = 2,6-Me2C6H3O) with phosphorous trichloride. The unstable H-phosphonate (2) and the three different crystalline modifications of the α-hydroxyarylphosphonate (3) (viz., ansolvate, solvate and hydrate) have been crystallized under different crystallization conditions and their molecular structures have been determined by single crystal X-ray diffraction studies. These four crystalline forms exhibit either two-dimensional sheet-like polymeric structures or tubular structures with water channels, assisted by supramolecular interactions such as H-bonding, CH–π, etc. Crystallisation of the α-hydroxyarylphosphonate (3) from a warm dimethylformamide solution leads to the ready decomposition of the solvent to the dimethylammonium cation, giving the salt of 3, [Me2NH2][(ArO)(CMe2OH)POO] (6). The reactivity of the α-hydroxyarylphosphonate 3 with metal acetates has been investigated and its copper and zinc complexes [M(HL)2(2,2′-bpy)(H2O)](H2O) (M = Cu (7); Zn (8) (HL = [(ArO)P(O)(O)(CMe2OH)]−) have been isolated and structurally characterized. Mononuclear 7 and 8 form linear polymeric chains assisted by intermolecular H-bonding interactions involving a large number of proton donor and acceptor sites present in these complexes.

Bulky Isopropyl Group Loaded Tetraaryl Pyrene Based Azo-Linked Covalent Organic Polymer for Nitroaromatics Sensing and CO2 Adsorption

An azo-linked covalent organic polymer, Py-azo-COP, was synthesized by employing a highly blue-fluorescent pyrene derivative that is multiply substituted with bulky isopropyl groups. Py-azo-COP was investigated for its sensing and gas adsorption properties. Py-azo-COP shows selective sensing toward the electron-deficient polynitroaromatic compound picric acid among the many other competing analogs that were investigated. Apart from its chemosensing ability, Py-azo-COP (surface area 700 m2 g–1) exhibits moderate selectivity toward adsorption of CO2 and stores up to 8.5 wt % of CO2 at 1 bar and 18.2 wt % at 15.5 bar at 273 K, although this is limited due to the electron-rich −N=N– linkages being flanked by isopropyl groups. Furthermore, the presence of a large number of isopropyl groups imparts hydrophobicity to Py-azo-COP, as confirmed by the increased adsorption of toluene compared to that of water in the pores of the COP.

Generation of multi-block topology for discretisation of three-dimensional domains

Abstract Simulations of field equations in science and engineering require tessellation of space. In the parlance of computational mechanics, this is called mesh generation or grid generation. It refers to the creation of discrete points and their connectivity (neighbourhood information) so as to input the geometry of the domain to the simulation codes. Different methods require different kinds of neighbourhood information. If a simulation method accepts triangulated surface patches in two dimensions and tetrahedra in three dimensions a great amount of automation is possible. But if a method demands a set of curvilinear coordinates typified by a two or three-dimensional array of points, a lot of human interaction is needed. This interaction happens to be mostly in the creation of sub-domains called blocks, in which these points are generated. The present paper proposes to make the blocks with a view to minimise user interaction.