Search results for "medium"
showing 10 items of 3746 documents
Superatomic S2 Silver Clusters Stabilized by a Thiolate–Phosphine Monolayer: Insight into Electronic and Optical Properties of Ag14(SC6H3F2)12(PPh3)8…
2014
The electronic structure of two recently crystallographically solved, thiolate–phosphine protected silver clusters Ag14 and Ag16 are analyzed via density functional theory (DFT) and their optical excitations are analyzed from time-dependent DFT perturbation theory. Both clusters can be characterized as having the S2 free-electron configuration in the metal core, which is the first time such a configuration is confirmed for structurally known ligand-protected noble metal clusters. However, their different core shapes and ligand layer induce significantly different optical spectra. Performance of gradient-corrected DFT functionals is discussed and it is shown that the asymptotically correct L…
3D-transition metal mono-substituted Keggin polyoxotungstate with an antenna molecule: synthesis, structure and characterization
2008
Three new organic-inorganic hybrid complexes based on 3d-transition metal monosubstituted Keggin polyoxometalates (POMs) with imidazole (Im) as pendant ligands, formulated as (HIm)(6-)[SiW11O39NiIm]0.8[SiW11O39Ni(H2O)]0.2.7H2O (1), (Im)4Na6[SiW11O39MnIm]0.69[SiW11O39Mn(H2O)]0.31.7.5H2O (2) and (HIm)6[SiW11O39CoIm]0.63[SiW11O39Co(H2O)]0.37.7H2O (3), have been synthesized and characterized by IR spectroscopy, UV-visible spectroscopy, elemental analysis, TG analysis, cyclic voltammetry, magnetic properties, EPR and single-crystal/powder X-ray diffraction. The structural analyses indicate that the 3d metal atoms are incorporated into the vacancy of the alpha-[SiW11O39](8-) (SiW11). Complexes 1-…
Self-Assembly of a Copper(II)-Based Metallosupramolecular Hexagon
2008
The self-assembly of a 1:1 mixture of copper(II) ions and a rigid heteroditopic ligand L containing phen and terpy binding units gives rise in the solid state to green crystals of a hexanuclear metallamacrocycle 1. X-ray crystallography reveals that 1 consists of molecular hexagons of the grid-type family in which each metal ion is bound to two different ligands through the phen and terpy units, plus a weakly coordinated PF6 (-) anion in a highly distorted octahedral geometry. ES-MS studies of acetonitrile solutions of L and copper(II) in a 1:1 ratio show mixtures of polynuclear complexes in which trinuclear L3Cu3 species are predominant.
Ancillary Ligand Effect on Single-Site Styrene Polymerization: Isospecificity of Group 4 Metal Bis(phenolate) Catalysts
2003
Styrene is polymerized isospecifically by group 4 metal complexes that contain a C2-symmetrically coordinated 1,4-dithiabutane-linked bis(phenolato) ligand activated by methylaluminoxane.
Computational Study on Cesium Azide Trapped in a Cyclopeptidic Tubular Structure
2008
The structures and the electronic properties of host-guest complexes formed by a cyclopeptidic tubular aggregate and the species CsN3, CS2(N-3)(2), and CS2N6 have been investigated by means of density functional theory. Taking advantage of the azide property to act as a bridge ligand between two or more metal cations, it may be possible to trap N-3(-) ions inside a confined space. This could be important for the preparation of polynitrogen molecules N-n. Results show that there are significant attractive interactions between the azide ion and the cavity walls, which make the ion stay inside the inner empty space of the cyclopeptidic aggregate. The confinement of the species CS2(N-3)(2) forc…
The first member of a second generation family of ligands derived from metal-ion assisted reactivity of di-2,6-(2-pyridylcarbonyl)pyridine: Synthesis…
2012
Abstract Nucleophilic attack by the carbanion −CH2COCH3 at the carbonyl carbon atoms of 2,6-di-(2-pyridylcarbonyl)pyridine, pyCOpyCOpy, in the presence of Mnn+ ions under basic conditions has yielded the cationic cluster [Mn4(OH)2(L)2(H2O)2](ClO4)4 (1), where L2− is the (py)C(CH2COCH3)(O−)(py)C(CH2COCH3)(O−)(py) dianion. The cluster cation possesses a planar {MnII2MnIII2(μ3-OH)2(μ-OR)4}4+ rhombus core, resulting from two μ3-OH− ions and two η1:η2:η1:η1:η1:η2:η1:μ3 bridging L2− groups from the in situ formed ligand. Complex 1 is antiferromagnetically coupled with an unusual S = 2 ground state resulting from spin frustration effects within the triangular Mn3 subunits of the cluster.
A 4-tert-butylcalix[4]arene tetrahydroxamate podand based on the 1-oxypiperidine-2-one (1,2-PIPO−) chelate. Self-assembly into a supramolecular ionop…
2014
An octadentate tetrahydroxamic calix[4]arene podand incorporating 1-hydroxypiperidine-2-one (1,2-PIPOH) binding units has been designed as a specific chelator for tetravalent metal cations like Zr4+ or Hf4+. This receptor, which can be considered as the first ever abiotic ligand possessing only cyclic six-membered hydroxamate groups, has been synthesized and characterized in its tetraprotonated form (1H4). Contrary to expectation, however, this new chelator did not form a 1 : 1 complex upon reaction with M(acac)4 (M = Zr and Hf; acac = acetylacetonate), but rather self-assembled into a dimeric species of 2 : 2 stoichiometry. The latter could be characterized in solution by mass spectrometry…
Postsynthetic Improvement of the Physical Properties in a Metal-Organic Framework through a Single Crystal to Single Crystal Transmetallation
2015
As ingle crystal to single crystal transmetallation process takes place in the three-dimensional (3D) metal- organic framework (MOF) of formula Mg II 2{Mg II 4(Cu II 2- (Me3mpba)2)3}·45 H2 O( 1 ;M e 3mpba 4¢ = N,N'-2,4,6-trimethyl- 1,3-phenylenebis(oxamate)). After complete replacement of the Mg II ions within the coordination network and those hosted in the channels by either Co II or Ni II ions, 1 is transmetallated to yield two novel MOFs of formulae Co2 II {Co II 4(Cu II 2(Me3- mpba)2)3}·56 H2 O( 2 )a nd Ni2 II {Ni II 4(Cu II 2(Me3mpba)2)3}· 54 H2 O( 3). This unique postsynthetic metal substitution affords materials with higher structural stability leading to enhanced gas sorption and m…
A Bis(mu-phenoxo)-Bridged Dizinc Complex with Hydrolytic Activity
2013
The dinuclear complex [Zn2(papy)2]·2CH3OH [H2papy = N- (2-hydroxybenzyl)-N-(2-picolyl)glycine] was synthesized and characterized. The crystal structure of the complex reveals that both ZnII ions are pentacoordinate with distorted pentagonal bipyramidal coordination arrangements. The phenoxyl groups of each ligand bridge the two metal atoms, whereas each carboxylate of the ligand is terminally bound to one ZnII ion. Potentiometric studies of the ZnII:H2papy system in a methanol/water mixture show the existence of a mononuclear species at lower pH; but at a pH above 5, a dimeric species starts to dominate and transforms further into a bis(μ-phenoxo) bridged dizinc complex by deprotonation of …
Electron paramagnetic resonance and magnetic circular dichroism of Gd3+ ions in oxyfluoride glass–ceramics containing CaF2 nanocrystals
2015
Abstract Magnetic resonance investigations of the Gd3 + centre structure in the oxyfluoride glass and glass–ceramics (GCs) have been carried out. The electron paramagnetic resonance (EPR) measurements showed the presence of low symmetry Gd3 + centres in the glass sample. The cubic Gd3 + centre appeared in the glass–ceramics after heat treatment of the samples at temperatures above 650 °C. The magnetic circular dichroism (MCD) measurements of both glass and glass–ceramics showed wide absorption band around 310 nm. In the GC samples MCD–EPR of the cubic Gd3 + centre has been detected.