Synthesis and Characterization of Two Discrete Ln10 Nanoscopic Ladder‐Type Cages: Magnetic Studies Reveal a Significant Cryogenic Magnetocaloric Effect and Slow Magnetic Relaxation

Two unique lanthanide‐based cages [Ln₁₀( L )₅(μ₂‐OH)₆(H₂O)₂₂](Cl)₄?7 H₂O ([Gd₁₀] and [Dy₁₀]) have been synthesized by using a hydrazone‐based ligand H₄ L (H₄ L =2,6‐bis[(3‐methoxysalicylidene)hydrazinecarbonyl]pyridine) and LnCl₃?x H₂O. Structural characterization of [Gd₁₀] reveals an aesthetically pleasing self‐assembly of five L ^4? and ten Gd³⁺ ions forming a 2×[1×5] rectangular array. The ladder‐shaped cage consists of three “rungs” and two “rails” that are occupied by five ligands. Six out of ten gadolinium centers act as rung locks. Further analysis revealed that three chloride ions are encapsulated inside each discrete [Gd₁₀] molecule through hydrogen bonding and other noncovalent interactions. Both the complexes ([Gd₁₀] and [Dy₁₀]) were characterized by powder X‐ray diffraction and thermogravimetric analysis, which shows that they are isostructural in nature. Magnetic investigations reveal that [Gd₁₀] is a good candidate for magnetic refrigeration with a significant entropy change (?ΔS_m) of 37.4 J kg^?1 K^?1 for an applied field of 7 T and at 3 K. Whereas [Dy₁₀] shows single‐molecule‐magnet‐like behavior.     

Dispersion stability of colloids in sub- and supercritical water

Dispersion stability of colloids has been investigated in sub- and supercritical water by measuring the hydrodynamic diffusion coefficients of the particles by means of dynamic light scattering. It is interestingly found that coagulation of the colloids in sub- and supercritical water is a universal phenomenon irrespective of the material of the colloids. Highly charged colloids were found to be more stable in water against high temperature. Numerical analysis reveals that the stability of the colloids at elevated temperature and pressure is primarily governed by the temperature dependence of the dielectric constant of the medium. The effect of the temperature dependence of the ion product of water (pKw) was found to be very little. Surface charge density and Stern potential may change with respect to temperature due to the readjustment of the ion concentration in the diffuse layer through the enhanced ion product and reduced dielectric constant of water. These are the secondary causes of the particle coagulations in sub- and supercritical water.     

A Comparative Study of the Nonlinear Optical Properties of Cd<Subscript>n</Subscript>X<Subscript>n</Subscript> (X: S,Se and Te) Clusters

We report results of hyperpolarizability calculations on Cd_nX_n (X: S, Se and Te; n = 1–10) clusters. Our results show that the geometric configurations of different types of clusters under investigation are quite similar at specific values of n. Both static and frequency dependent components of first and second order hyperpolarizability tensors of Cd_nS_n, Cd_nSe_n and Cd_nTe_n are compared. It is observed that in general nonlinear optical coefficients show identical variation in all the cluster materials. The present investigation also manifests the improvement of these coefficients due to the introduction of asymptotically correct generalized gradient approximation functional over the local density functional and normal gradient corrected functional. Symmetrized fragment orbital analysis has been performed to provide explanation of the observed hyperpolarizability variation. We also analyse how geometries with closely lying energy values influence the hyperpolarizabilities of these cluster materials.     

Synthesis, structural analysis, and magnetic behaviour of three fumarate bridged coordination polymers: five-fold interpenetrated diamond-like net of NiII, sheets of NiII and CoII

The hydrothermal reactions of Ni(NO(3))(2).6H(2)O, disodium fumarate (fum) and 1,2-bis(4-pyridyl)ethane (bpe)/1,3-bis(4-pyridyl)propane (bpp) in aqueous-methanol medium yield one 3-D and one 2-D metal-organic hybrid material, [Ni(fum)(bpe)] (1) and [Ni(fum)(bpp)(H(2)O)] (2), respectively. Complex possesses a novel unprecedented structure, the first example of an "unusual mode" of a five-fold distorted interpenetrated network with metal-ligand linkages where the four six-membered windows in each adamantane-type cage are different. The structural characterization of complex 2 evidences a buckled sheet where nickel ions are in a distorted octahedral geometry, with two carboxylic groups, one acting as a bis-chelate, the other as a bis-monodentate ligand. The metal ion completes the coordination sphere through one water molecule and two bpp nitrogens in cis position. Variable-temperature magnetic measurements of complexes 1 and 2 reveal the existence of very weak antiferromagnetic intramolecular interactions and/or the presence of single-ion zero field splitting (D) of isolated Ni(II) ions in both the compounds. Experimentally, both the J parameters are close, comparable and very small. Considering zero-field splitting of Ni(II), the calculated D values are in agreement with values reported in the literature for Ni(II) ions. Complex 3, [[Co(phen)]](2)(fum)(2)](phen = 1,10-phenanthroline) is obtained by diffusing methanolic solution of 1,10-phenanthroline on an aqueous layer of disodium fumarate and Co(NO(3))(2).6H(2)O. It consists of dimeric Co(II)(phen) units, doubly bridged by carboxylate groups in a distorted syn-syn fashion. These fumarate anions act as bis-chelates to form corrugated sheets. The 2D layer has a (4,4) topology, with the nodes represented by the centres of the dimers. The magnetic data were fitted ignoring the very weak coupling through the fumarate pathway and using a dimer model.     

Licamichauxiioic-A and -B acids--two ent-kaurene diterpenoids from Licania michauxii

Bioassay-guided fractionation allowed the isolation of two new cytotoxic ent-kaurene diterpenoids, licamichauxiioic-A and -B acids (1 and 2) from the root extract of Licania michauxii Prance (Chrysobalanaceae). They were characterized as ent-15-oxo-9(11),16-kauradien-19-oic acid (1) and ent-15-oxo-13(14),16-kauradien-19-oic acid (2) by various spectroscopic methods, in particular, 1D and 2D NMR spectra, and chemical evidence.     

Alkali metal ion mediated self-assembly of vanadium(V) ions and pyrazole based polydentate ligands leading to the formation of helix, double helix and supramolecular cage-like structures

The anionic cis-dioxovanadium(V) complex species LVO₂⁻ and L₁VO₂⁻ of two tridentate ONO ligands (H₂L and H₂L₁) can bind alkali metal ions in a bis-monodentate fashion like a bridging carboxylate group. Here H₂L (N′-[(E)-(2-hydroxyphenyl)methylidene]-3-methyl-1H-pyrazole-5-carbohydrazide) and H₂L₁ (N′-[(1E)-1-(2-hydroxyphenyl) ethylidene]-3-methyl-1H-pyrazole-5-carbohydrazide) are Schiff base ligands generated by the condensation of 5-methyl 3-pyrazole carbohydrazide with salicylaldehyde and o-hydroxy acetophenone. The six products thus obtained are water soluble polymeric compounds in which the complementary units are held together by the simultaneous use of hydrogen bonding and coulombic interactions. [L₁VO₂Li(H₂O)₂]_∞ (2), [LVO₂Na(H₂O)₂]_∞ (3) and [LVO₂K(H₂O)₂]_∞ (5) have been crystallographically characterized. Crystallographic characterization reveals that 2 and 3 are polymeric helicates and 5 is a polymeric cluster with L₁VO₂⁻ or LVO₂⁻ units bridging the labile alkali metal ions which occupy positions on the axis of the polymeric chain.     

Speciation of uranyl ions in fulvic acid and humic acid: a DFT exploration

The speciation of uranyl ions in fulvic acid (FA) and humic acid (HA), based on models of larger sizes, is systematically studied using density functional theory (DFT). Four uranyl binding sites are suggested for FA and based on their energetics, the preferential binding sites are proposed. The computed binding sites include two chelating types, one through the carboxylate group and one via the hydroxo group. A systematic way to attain the possible structure for Stevenson's HA model is carried out using a combined molecular dynamics (MD) and quantum chemical approach. Calculated structures and energetics reveal many interesting features such as conformational flexibility of HA and binding of hydrophobic molecules in agreement with the experimental suggestions. Five potential binding sites are proposed for uranyl binding to HA and the calculated geometries correlate nicely with the experimental observations. Our binding energy calculations reveal that apart from uranyl binding at the carboxylate functional group, binding at other functional groups such as those involving quinone and hydroxo sites are also possible. Finally, based on our cluster calculations the strength of uranyl binding to HAs and FAs is largely influenced by neighbouring groups via hydrogen bonding interactions.     

One-dimensional description of multidimensional electron transfer reactions in condensed phase

We derive a one-dimensional energy diffusion equation for describing the dynamics of multidimensional electron transfer reactions in condensed phase, which is conceptually simpler and computationally more economic than the conventional approaches. We also obtain an analytical expression for the rate of electron transfer reactions for a general one-dimensional effective potential as well as an energy dependent diffusitivity. As an illustrative example, we consider application to electron transfer in a contact ion pair system modeled through harmonic potentials consisting of two slow classical modes and a high frequency vibrational mode for which the numerical results calculated using the proposed one-dimensional approach are shown to be in good agreement with experimental results. The energy diffusion equation and the rate expression for electron transfer obtained from the present theory, therefore, open up the possibility of describing the dynamics of electron transfer in complex systems, through a simpler approach.     

Bursts in single-file motion mediated conduction

We present a cellular automaton (CA) model of particles in a single-file motion with free particle exchange at the boundaries of a one-dimensional channel connected to two infinite reservoirs in order to study the self-transmission of particles with excluded mutual passage. The parallel, local and homogeneous rule sets of the CA algorithm consider two different interactions of varying strength between particles, without any specific particle-channel interaction. CA model results suggest that one hallmark of single-file motion is the conduction bursts at a particular time scale, which have thus far only been discovered for hydrogen bond networked water translocation. The cumulative transport probabilities of particles through single-file channels of different length follow a single profile, which can be obtained through proper scaling of time. The universal features of our results suggest new experiments in single-file channel with fluids other than water.     

A versatile series of nickel(II) complexes derived from tetradentate imine/pyridyl ligands and various pseudohalides: azide and cyanate compared

The chemical reactions of a family of tetradentate pyridyl/imine ligands, L1, L2, and L3 (L1=[ N, N'-bis(2-pyridinylmethylene)]ethane-1,2-diamine; L2=[ N, N'-bis(pyridin-2-yl)benzylidene]ethane-1,2-diamine; L3=[ N, N'-bis(2-pyridinylmethylene)]propane-1,3-diamine), with Ni (II) in the presence of various pseudohalides (N3(-), SCN(-), and NCO(-)) have served to prepare six different complexes, [Ni 2(L1)2(N3)2](ClO4)2.H2O (1), [Ni 2(L2)2(N3)2](ClO4)2 (2), [Ni2(L2)2(NCS)4] (3), [Ni2(L2)2(NCO) 2](ClO4)2 (4), [Ni2(L3)2(NCO)2](ClO4)2 (5), and [Ni(L3)(N 3)2] (6), which have been characterized by X-ray crystallography. Interestingly, four of these complexes are dinuclear and exhibit end-on (EO) pseudohalide bridges (1, 2, 4, and 5), one is dinuclear and bridged exclusively by the tetradentate ligand (3), and one is mononuclear (6). The bulk magnetization of the complexes bridged by EO pseudohalides has been studied, revealing these ligands to mediate ferromagnetic coupling between the Ni(II) ions, with modeled coupling constants, J, of +31.62 (1), +28.42 (2), +2.81 (4), and +1.72 (5) cm(-1) (where the convention H=-2JS1S2 was used). The striking difference in the coupling intensity between N3(-) and NCO(-) has prompted an investigation by means of density functional theory calculations, which has confirmed the experimental results and provided insight into the reasons for this observation.