Redox chemistry of tantalum clusters on silica characterized by X-ray absorption spectroscopy

SiO(2)-supported clusters of tantalum were synthesized from adsorbed Ta(CH(2)Ph)(5) by treatment in H(2) at 523 K. The surface species were characterized by X-ray absorption spectroscopy (extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES)) and ultraviolet-visible spectroscopy. The EXAFS data show that SiO(2)-supported tantalum clusters were characterized by a Ta-Ta coordination number of approximately 2, consistent with the presence of tritantalum clusters, on average. When these were reduced in H(2) and reoxidized in O(2), the cluster nuclearity remained essentially unchanged, although reduction and oxidation occurred, respectively, as shown by XANES and UV-vis spectra; in the reoxidation, the tantalum oxidation state change was approximately two electronic charges per tritantalum cluster. The data demonstrate an analogy between the chemistry of group 5 metals on the SiO(2) support and their chemistry in solution, as determined by the group of Cotton.     

Synthesis and electrochemical and spectroscopic characterization of biicosahedral Au25 clusters

The synthesis and electrochemical and spectroscopic characterization of biicosahedral Au(25) clusters with a composition of [Au(25)(PPh(3))(10)(thiolate)(5)Cl(2)](2+) are described. The biicosahedral Au(25) clusters protected with various types of thiol ligands including alkanethiols, 2-phenylethanethiol, 11-mercaptoundecanoic acid, and 11-mercapto-1-undecanol were synthesized in high yields using a one-step, one-phase procedure in which Au(PPh(3))Cl is reduced with tert-butylamine-borane in the presence of the thiol ligand in a 3:1 v/v chloroform/ethanol solution. All biicosahedral Au(25) clusters prepared exhibit characteristic optical absorption and photoluminescence properties. The emission energy is found to be substantially smaller than the optical absorption energy gap of 1.82 eV, indicating a subgap energy luminescence. The electrochemical HOMO-LUMO gap (~1.54 eV) of the clusters is also substantially smaller than the optical absorption energy gap but rather similar to the emission energy. These electrochemical and optical properties of the biicosahedral Au(25) clusters are distinctly different from those of the Au(25)(thiolate)(18) clusters.     

Electrochemical and spectroscopic characterization of organic compound uptake in silica core-shell nanocapsules

Oil-filled silica nanocapsules consisting of a hydrophobic liquid core and a silicate shell have been shown to efficiently extract hydrophobic compounds from aqueous media. With a view toward quantifying the selectivity of these systems, a series of electrochemical and spectroscopic measurements was performed. Uptake and kinetics experiments were carried out through electrochemical measurements by using solutions of lipophilic electroactive molecules of different sizes and with different affinities for silica. Other solutions with fluorescent probes were used for spectrophotometry measurements. In this work we report the environment where the lipophilic compounds studied end up after absorption and the kinetics of their uptake by the oil-filled silica nanocapsules with different shell thicknesses.     

Coexistence of ferroelectricity and ferromagnetism in tantalum clusters

The atomic and electronic structures of Ta(N) (N=2-23) clusters have been determined in the framework of pseudopotential density-functional calculations, based upon an unbiased global search with guided simulated annealing to an empirical potential. It is found that the ground-state structures of Ta(N) are very similar to those of Nb(N), showing no preference for the icosahedral growth. Also, a size- and structure-dependent ferroelectricity is found in these tantalum clusters. More importantly, it is found that the ferroelectricity and ferromagnetism can coexist in the homogeneous transition-metal cluster, offering a possibility to obtain a new type of "multiferroic" materials composed of the clusters. Finally, the far-infrared spectroscopy is suggested to be an efficient tool to distinguish the ferroelectric clusters.     

Mono(cycloheptatrienyl) tantalum chemistry: synthesis and characterization of new tantalum halide, hydride, and alkyl species

The new tantalum(II) complex (eta (6)-C 7H 8)TaCl 2(PMe 3) 2 ( 1) was synthesized by the reduction of TaCl 5 with n-butyllithium in the presence of PMe 3 and cycloheptatriene. Compound 1 adopts a four-legged piano stool structure in which the tantalum center is bound to a eta (6)-cycloheptatriene ring in addition to two chlorides and two phosphine ligands in a transoid arrangement. Treatment of 1 with methyllithium results in a loss of the equivalents of HCl and formation of the eta (7)-cycloheptatrienyl complex (eta (7)-C 7H 7)TaCl(PMe 3) 2 ( 2), whereas treatment of 1 with sodium or sodium borohydride affords small amounts of the eta (5)-cycloheptadienyl complex (eta (5)-C 7H 9)TaCl 2(PMe 3) 2 ( 3). Compound 2 adopts a three-legged piano stool structure; the eta (7)-C 7H 7 ring is fully aromatic and planar. The molecular structure of 3 is similar to that of 1, except for the eta (5) binding mode of the seven-membered ring. Treatment of the previously described sandwich compound (C 5Me 5)Ta(C 7H 7) with allyl bromide affords the tantalum(V) product (C 5Me 5)Ta(C 7H 7)Br ( 4), which reacts with LiAlH 4 to give the tantalum(V) hydride (C 5Me 5)Ta(C 7H 7)H ( 5). Compound 4 also reacts with alkylating agents to generate the methyl, allyl, and cyclopropyl complexes (C 5Me 5)Ta(C 7H 7)Me ( 6), (C 5Me 5)Ta(C 7H 7)(eta (1)-CH 2CHCH 2) ( 7), and (C 5Me 5)Ta(C 7H 7)(c-C 3H 5) ( 8). Compounds 4- 8 all adopt bent sandwich structures in which the dihedral angle between the two carbocyclic rings is 34.9 degrees for the bromo compound 4, 26.6 degrees for the hydride 5, 33.1 degrees for the methyl compound 6, 34.2 degrees for the allyl compound 7, and 37.5 degrees for the cyclopropyl compound 8. (1)H and (13)C NMR data are reported for the diamagnetic compounds.     

Noncovalent synthesis in aqueous solution and spectroscopic characterization of multi-porphyrin complexes

The interactions of the tetracationic meso-tetrakis(N-methyl-4-pyridyl)porphyrin (H(2)TMPyP) and its metallo derivatives (MTMPyP) (where M=copper(II), zinc(II), and gold(III) with the octa-anionic form (at neutral pH) of 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis(hydroxycarbonylmethoxy)calix[4]arene (C(4)TsTc) lead to a series of complex species whose stoichiometry and porphyrin sequence can be easily tuned. Crystallographic, spectroscopic, and diffusion NMR studies converge towards a common picture in which a central 1:4 porphyrin/calixarene unit serves as a template for the formation of more complex species. These species arise by successive, stepwise addition of single porphyrin molecules above and below the plane of the 1:4 central core to ultimately give a 7:4 complex. Noticeably, the stoichiometry of the various complex species corresponds to the actual concentration ratio of porphyrins and calixarenes in solution allowing the stoichiometry of these species to be easily tuned. This behavior and the remarkable stability of these species allow homo-porphyrin and hetero-(metallo)porphyrin species to be formed with control of not only the stoichiometry but also the sequence of the porphyrin array. The flexibility and ease of this approach permit, in principle, the design and synthesis of porphyrin arrays for predetermined purposes. For example, we have shown that it is very easy to design and obtain mixed porphyrin species in which a foreseen photoinduced electron-transfer is indeed observed.     

Solvothermal synthesis and characterization of two high-nuclearity mixed-valent manganese phosphonate clusters

Two novel mixed-valent manganese (Mn(II)/Mn(III)) cluster compounds were synthesized in solvothermal reactions and characterized by single-crystal X-ray diffraction, bond valence sum calculations, IR spectra, elemental analysis, and magnetic measurements. Compound 1 is a Mn 19 cluster with a cylindrical core structure. Compound 2 possess a Mn 16 core where all of the manganese sites have unique ligation environments. The magnetic measurements on both compounds indicate dominant antiferromagnetic interactions between the metal centers.     

Synthesis and spectroscopic characterization of a new family of Ni(4) spin clusters

A new family of tetranuclear Ni complexes [Ni(4)(ROH)(4)L(4)] (H(2)L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni(4)O(4)] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni(II) centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximately -0.93 cm(-)(1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.     

Strained-cyclophane-induced beta-turn template: design, synthesis, and spectroscopic characterization

[structure: see text] Three tetrapeptides incorporating a 14-membered (R(i+1), S(i+2)) cycloisodityrosine at the i + 1 and i + 2 positions were designed and synthesized. Conformational analysis by (1)H NMR and CD spectra as well as molecular modeling indicated that they all adopt a beta-turn conformation. While the CD spectrum of compound 2 is characteristic of the typical type-II beta-turn (maximum at approximately 200 nm and a minimum at approximately 220 nm), that of 1a (atropisomer of 2) is opposite in sign to the expected spectrum of the type-II beta-turn.     

Chromium(V) peptide complexes: synthesis and spectroscopic characterization

A series of stable Cr(V) model complexes that mimic the binding of Cr(V) to peptide backbones at the C-terminus of proteins have been prepared for N,N-dimethylurea derivatives of the tripeptides Aib3-DMF, AibLAlaAib-DMF, and AibDAlaAib-DMF (Aib = 2-amino-2-methylpropanoic acid, DMF = N,N-dimethylformamide). The Cr(ll) precursor complexes were synthesized by the initial deprotonation of the amide and acid groups of the peptide ligands in DMF with potassium tert-butoxide in the presence of CrCl2. The Cr(II) intermediates thus formed were then immediately oxidized to Cr(V) using tert-butyl hydroperoxide. Spectroscopic and mass-spectrometric analyses of the Cr(V) complexes showed that a new metal-directed organic transformation of the ligand had occurred. This involved a DMF solvent molecule becoming covalently bound to the amine group of the peptide ligand, yielding a urea group, and a third coordinated deprotonated urea nitrogen donor. A metal-directed oxidative coupling has been proposed as a possible mechanism for the organic transformation. The Cr(V/IV) reduction potential was determined for the three Cr(V) complexes using cyclic voltammetry, and in all cases it was quasi-reversible. These are the first isolated and fully characterized Cr(V) complexes with non-sulfur-containing peptide ligands.     

The synthesis and spectroscopic characterization of nano calcium fluorapatite using tetra-butylammonium fluoride

Pure homogeneous nano sized biocompatible fluorapatite (FAp) particles were synthesized by a wet chemical procedure using water soluble tetra-butylammonium fluoride (TBAF) without using high temperatures and any purification processes. Combination of the Bragg's law and the plane-spacing equation for the two high intensity lines, namely, (002) and (300), gives a=9.3531 ?, c=6.8841 ?, confirms the identity of the highly crystalline synthetic material as well as its purity. The effect of various pH's in crystal formation and on their size was also evaluated. The calculated crystallinities were excellent with a rate around 5.0. The synthesized nano FAp was fully characterized by spectroscopic techniques (XRD, SEM, EDS, BET, FT-IR and ICP-AES). The nitrogen adsorption-desorption isotherm showed a type IV diagram and calculation of the surface area was investigated as well.     

Fluoroalkyl-functionalized silica particles: synthesis, characterization, and wetting characteristics

Fluoroalkyl-functionalized silica particles for use in nonwetting surfaces were prepared by treatment of silica particles with fluoroalkyl-functional chlorosilanes. Both fumed and precipitated silica were studied, as well as the efficiency of surface coverage using mono-, di-, and trifunctional chlorosilanes. The most effective surface treatment was accomplished via the surface grafting of monofunctional chlorosilanes in the presence of preadsorbed dimethylamine under anhydrous conditions at room temperature. Confirmation of covalent attachment was accomplished via Fourier transform infrared (FT-IR) spectroscopy, while elemental analysis, thermogravimetric analysis, and nitrogen adsorption isotherms were used to determine grafting densities and additional key geometric characteristics of the grafted layer. The effect of residual silanol content on the moisture uptake properties of the modified silica particles was determined by measuring the water uptake of unbound particles, while liquid wetting properties were determined by dynamic contact angle analysis of elastomeric composites. Although residual silanol content was shown to effect wetting properties, results suggest that surface geometry dominates the performance of liquid-repellent surfaces. The potential use of fluoroalkyl-functionalized silica particles for hydrophobic and oleophobic applications is discussed.     

Rapid one-step synthesis, characterization and functionalization of silica coated gold nanoparticles

This paper describes a rapid, simple and one-step method for preparing silica coated gold (Au@SiO₂) nanoparticles with fine tunable silica shell thickness and surface functionalization of the prepared particles with different groups. Monodispersed Au nanoparticles with a mean particle size of 16 nm were prepared by citrate reduction method. Silica coating was carried out by mixing the as prepared Au solution, tetraethoxysilane (TEOS) and ammonia followed by microwave (MW) irradiation. Although there are several ways of coating Au nanoparticles with silica in the literature, each of these needs pre-coating step as well as long reaction duration. The present method is especially useful for giving the opportunity to cover the colloidal Au particles with uniform silica shell within very short time and forgoes the use of a silane coupling agent or pre-coating step before silica coating. Au@SiO₂ nanoparticles with wide range of silica shell thickness (5-105 nm) were prepared within 5 min of MW irradiation by changing the concentration of TEOS only. The size uniformity and monodispersity were found to be better compared to the particles prepared by conventional methods, which were confirmed by dynamic light scattering and transmission electron microscopic techniques. The prepared Au@SiO₂ nanoparticles were further functionalized with amino, carboxylate, alkyl groups to facilitate the rapid translation of the nanoparticles to a wide range of end applications. The functional groups were identified by XPS, and zeta potential measurements.     

Barium coordination polymers based on fluorinated and fluorine-free benzene-dicarboxylates: Mechanochemical synthesis and spectroscopic characterization

A series of new Ba-based coordination polymers (CPs) were mechanochemically synthesized by milling Ba-hydroxide samples with perfluorinated and fluorine-free benzene-dicarboxylic acids, including tetrafluoroisophthalic acid (H₂mBDC-F₄), tetrafluorophthalic acid (H₂oBDC-F₄), isophthalic acid (H₂mBDC) and phthalic acid (H₂oBDC). The new fluorinated CPs: [Ba(mBDC-F₄)·0.5H₂O] (1) and [Ba(oBDC-F₄)·1.5H₂O] (2) are compared to their nonfluorinated counterparts: [Ba(mBDC)·2.5H₂O] (3), and [Ba(oBDC)·1H₂O] (4). These materials are thoroughly characterized using powder X-ray diffraction. The products obtained by milling are all hydrated but vary in their water contents. Compositions and local structures are investigated by elemental analysis, thermal analysis, MAS NMR and attenuated total reflection-infrared spectroscopy. These materials exhibit high thermal stabilities but small surface areas that remain unchanged even after thermal treatments.     

Electronic structure of clusters modeling silica

Self-consistent-field-Xα-scattered wave calculations on clusters Si₂O₇^6? and H₆Si₂O₇ modeling silica have been performed. Incorporation of Si 3d orbitals produces significant changes in the overall valence structure. In addition to σ Si — O bonds, there exists a bonding π character due to the participation of O 2p and Si 3d. Hydrogen terminators do not seem to correct edge effects for these π states.     

Synthesis, structural and spectroscopic characterization, catalytic properties, and thermal transformations of new cyclic di- and trisiloxanediolato tantalum complexes

The reaction between Ta(OEt)5 and 1,1,3,3-tetramethyl-1,3-disiloxanediol, (HOSiMe2OSiMe2OH), leads to new siloxy complexes in which the dimeric nature of Ta(OEt)5 is maintained with both bridging ethoxide and disiloxanediolato bridges. With equal amounts of the reagents, two terminal OEt groups are replaced to form [Ta(OEt)2]2(mu-OEt)2(mu-OSiMe2OSiMe2O)2, 1, whereas with an excess of diol, the remaining terminal OEt groups are also replaced but with a trisiloxanediolato unit to form [Ta(OSiMe2OSiMe2OSiMe2O)]2(mu-OEt)2(mu-OSiMe2OSiMe2O)2, 2. Complexes 1 and 2 catalyze the transformation of HOSiMe2OSiMe2OH to polysiloxanes. Thermal treatment of 1 results in the formation of a 1:2 mixture of Ta2O5/SiO2; no new phases are observed. The molecular structures of 1 and 2 are confirmed by X-ray crystallography.     

Shape-persistent octanuclear zinc salen clusters: synthesis, characterization, and catalysis

We describe a selective and template-controlled synthesis of a series of Zn(8) metal complexes based on a bis-nucleating salen ligand scaffold. Our results, a combination of X-ray analysis and solution studies, show that discrete, shape-persistent metal clusters can be prepared in high yield. Their activity in organic carbonate catalysis is a function of the metal-connecting fragment present in the exterior of the cluster complex. The high stability of the clusters has been confirmed by (1)H, (13)C (DEPTQ) and DOSY NMR, gel permeation chromatography, high-performance liquid chromatography, and mass spectrometry.