Nb Complexes Research Articles

Methane Activation by Laser-Ablated V, Nb, and Ta Atoms: Formation of CH3−MH, CH2MH2, CH⋮MH3-, and (CH3)2MH2

Methane activation by group 5 transition-metal atoms in excess argon and the matrix infrared spectra of reaction products have been investigated. Vanadium forms only the monohydrido methyl complex (CH3-VH) in reaction with CH4 and upon irradiation. On the other hand, the heavier metals form methyl hydride and methylidene dihydride complexes (CH3-MH and CH2=MH2) along with the methylidyne trihydride anion complexes (CHMH3-). The neutral products, particularly the methylidene complex, increase markedly on irradiation whereas the anionic product depletes upon UV irradiation or addition of a trace of CCl4 or CBr4 to trap electrons. Other absorptions that emerge on irradiation and annealing increase markedly at higher precursor concentration and are attributed to a higher-order product ((CH3)2MH2)). Spectroscopic evidence suggests that the agostic Nb and Ta methylidene dihydride complexes have two identical metal-hydrogen bonds.

Novel water-soluble peroxo complexes of Nb(V) and Ta(V)

Novel water-soluble peroxo complexes of Nb(V) and Ta(V)

Physical metallurgy of high strength low alloy strip steel production using compact strip processing

The physical metallurgy of high strength low alloy (HSLA) steel production by compact strip processing or direct slab casting is reviewed. Specimens were extracted from 50 mm thickness cast Nb + Ti(+ V)HSLA slabs and quenched to room temperature, before and after the slab was heat treated in a tunnel furnace at 1150°C. These specimens were analysed to track changes in composition and microstructure during processing, in particular the formation of complex (Nb,Ti)(C,N) precipitates during solidification and subsequent heat treatment. It is shown that partial dissolution of the precipitates takes place during thermal equilibration, increasing the solute content of niobium and titanium before hot rolling. Tailoring of the industrial thermomechanical processing route, to improve the Charpy toughness and hardness of thin slab cast niobium HSLA linepipe and other grades, is also described.

Solid-State93Nb and13C NMR Investigations of Half-Sandwich Niobium(I) and Niobium(V) Cyclopentadienyl Complexes

Solid-state 93Nb and 13C NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)4 and CpNb(V)Cl4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from 93Nb MAS and static NMR spectra for seven different complexes. It is demonstrated that 93Nb NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)4 complexes. There are dramatic differences in the 93Nb quadrupolar coupling constants (C(Q)) between the Nb(I) and Nb(V) complexes, with C(Q) between 1.0 and 12.0 MHz for Cp'Nb(CO)4 and C(Q) = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line 93Nb NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state 93Nb and 13C NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)Cl4.THF. 13C MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond 93Nb, 13C J-coupling, 1J(93Nb,13C). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.

Magnetism and electronic structure of triplet binuclear niobium complexes in inorganic glasses, organic ligand environment, and polymers

We investigated paramagnetic properties of binuclear niobium complexes Nb–O–Nb with two nonequivalent Nb4+ ions in lithium–niobium phosphate glasses (LNPG), in the environment of catechol/ortho-quinone ligands and in polyethylene. Experimental electron paramagnetic resonance spectrum analysis revealed nonequivalent distribution of the charge and electron spin density between two Nb atoms. Mechanochemical interaction of LNPG with an organic donor–acceptor mixture catechol/ortho-quinone followed by organic solvent extraction leads to the formation of a new binuclear complex with catechol/ortho-quinone ligands. This complex can be further incorporated into polyethylene matrix to form the complex with properties close to the complex in LNPG.

Agostic versus Hypervalent Si‐H Interactions in Half‐Sandwich Complexes of Nb and Ta

AbstractFor Abstract see ChemInform Abstract in Full Text.

Composition and structure of complex Nb - Cr carbide coatings on carbon steels

Coatings of chromium and niobium carbides deposited on steels 20, 45, 60, U8, and U10 have been studied. The structure, chemical and phase compositions of diffusion coatings were determined by x-ray layer-by-layer analysis and electron-probe microanalysis for simultaneous saturation with niobium and chromium. The coatings were found to have a layered structure with layers of markedly different compositions. The lattice constants of the phases were determined over various depth of the coatings. The microhardness of the layers ranged from 16.5 to 21.0 GPa. The cavitation-corrosion resistance of complex niobium-chromium coatings was higher than that of single-phase niobium carbide coatings.

A versatile entry into aqueous niobium chemistry: isolation and structure of the intermediate Nb 4(μ 2-O) 2(μ 2-OC 2H 5) 4Cl 4(OC 2H 5) 4(HOC 2H 5) 4

The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. From the zinc-reduced solution, the major intermediate, Nb 4(μ 2-O) 2(μ 2-OC 2H 5) 4Cl 4(OC 2H 5) 4(HOC 2H 5) 4, was isolated and the crystal structure determined by X-ray crystallography. The complex crystallizes in the orthorhombic space group Pccn, with Z=4, a=21.0105(9), b=11.0387(5), c=19.1389(8), V=4438.9(3) Å 3, M r=1090.19, R 1=0.0327 and wR 2=0.0876. The structure revealed a centrosymmetric tetrameric Nb(IV) complex, consisting of a pair of edge-sharing bi-octahedral Nb 2(μ 2-OC 2H 5) 4Cl 2(OC 2H 5) 2(HOC 2H 5) 2 units that are joined by two axial oxo ligands. The Nb–Nb distance of 2.7458(3) Å is consistent with a single metal–metal bond.

A versatile entry into aqueous niobium chemistry: isolation and structure of the intermediate Nb4($mu;2-O)2($mu;2-OC2H5)4Cl4(OC2H5)4(HOC2H5)4*1

The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. The major intermediate Nb 4 (μ 2 -O) 2 (μ 2 -OC 2 H 5 ) 4 Cl 4 (OC 2 H 5 ) 4 (HOC 2 H 5 ) 4 was isolated and the crystal structure reported. The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. From the zinc-reduced solution, the major intermediate, Nb 4 (μ 2 -O) 2 (μ 2 -OC 2 H 5 ) 4 Cl 4 (OC 2 H 5 ) 4 (HOC 2 H 5 ) 4 , was isolated and the crystal structure determined by X-ray crystallography. The complex crystallizes in the orthorhombic space group Pccn , with Z =4, a =21.0105(9), b =11.0387(5), c =19.1389(8), V =4438.9(3) Å 3 , M r =1090.19, R 1 =0.0327 and wR 2 =0.0876. The structure revealed a centrosymmetric tetrameric Nb(IV) complex, consisting of a pair of edge-sharing bi-octahedral Nb 2 (μ 2 -OC 2 H 5 ) 4 Cl 2 (OC 2 H 5 ) 2 (HOC 2 H 5 ) 2 units that are joined by two axial oxo ligands. The Nb–Nb distance of 2.7458(3) Å is consistent with a single metal–metal bond.

Separation and simultaneous determination of niobium and tantalum in steel by reversed-phase high-performance liquid chromatography using 2-(2-pyridylazo)-5-diethylamino phenol as a pre-column derivatizing reagent

A method for the simultaneous determination of Nb and Ta in steel and alloy by reversed-phase high-performance liquid chromatography (RP-HPLC) was proposed. 2-(5-Bromo-2-pyridylazo)-5-diethylamino phenol (5-Br-PADAP) was used as a pre-column chelating agent to form a ternary complex with Nb(V) and Ta(V) and tartaric acid. The ternary complexes of Nb(V) and Ta(V) were eluted within 8min on a C18 column with a mobile phase of methanol–water (55:45, v/v) containing 10mmoll−1 acetate buffer (pH3.5) and determined with spectrophotometric detection at 598nm. The detection limits for Nb(V) and Ta(V) were 0.60 and 0.72μgl−1, respectively, when the ratio of signal-to-noise is 3. The proposed method was used to analyze Nb and Ta in cast iron, alloy and stainless steel.

Activation and cleavage of dinitrogen by three-coordinate metal complexes involving Mo(III) and Nb(II/III).

Density functional calculations have been employed to rationalize why the heteronuclear N(2)-bridged Mo(III)Nb(III) dimer, [Ar((t)Bu)N](3)Mo(mu-N(2))Nb[N((i)Pr)Ar](3)(Ar = 3,5-C(6)H(3)Me(2)), does not undergo cleavage of the dinitrogen bridge in contrast to the analogous Mo(III)Mo(III) complex which, although having a less activated N-N bond, undergoes spontaneous dinitrogen cleavage at room temperature. The calculations reveal that although the overall reaction is exothermic for both systems, the actual cleavage step is endothermic by 144 kJ mol(-1) for the Mo(III)Nb(III) complex whereas the Mo(III)Mo(III) system is exothermic by 94 kJ mol(-1). The reluctance of the Mo(III)Nb(III) system to undergo N(2) cleavage is attributed to its d(3)d(2) metal configuration which is one electron short of the d(3)d(3) configuration necessary to reductively cleave the dinitrogen bridge. This is confirmed by additional calculations on the related d(3)d(3) Mo(III)Nb(II) and Nb(II)Nb(II) systems for which the cleavage step is calculated to be substantially exothermic, accounting for why in the presence of the reductant KC(8), the [Ar((t)Bu)N](3)Mo(mu-N(2))Nb[N((i)Pr)Ar](3) complex was observed to undergo spontaneous cleavage of the dinitrogen bridge. On the basis of these results, it can be concluded that the level of activation of the N-N bond does not necessarily correlate with the ease of cleavage of the dinitrogen bridge.

Synthesis and structure of cyclic hexanuclear oxo-alkoxo–carboxylatoniobium(IV) complexes

The synthesis and crystal structure of novel oxo-alkoxo–carboxylato Nb(IV) complexes, Nb 6(μ 2-O) 3(μ 2-O 2CCX 3) 6(μ 2-OC 2H 5) 6(OC 2H 5) 6 (X=Cl, F) are described. The single-crystal X-ray structure determination carried out, revealed a closed ring of six Nb(IV) ions linked alternately by two carboxylato groups and an oxo group, and two ethoxo groups. The hexanuclear molecule can essentially be described as a trimer of Nb 2(μ 2-OC 2H 5) 2(OC 2H 5) 2, each linked by an μ 2-oxo group and two μ 2-carboxylato groups. The ethoxo-bridged Nb–Nb distances are 2.735(5) Å, which is consistent with a single metal–metal bond, while the oxo- and carboxylato-bridged non-bonding Nb–Nb distances are 3.635(2) Å.

Enantioselective synthesis of organic carbonates promoted by Nb(IV) and Nb(V) catalysts

In this paper, we describe the use of Nb(IV) and Nb(V) catalysts in the carboxylation of either pure enantiomers of chiral epoxides or racemic mixtures. The carboxylation of pure enantiomers with Nb(V)-oxide is very selective (100%), proceeds with a good TON and occurs with total retention of the configuration. Optically active Nb(IV) complexes have been synthesised and used in the conversion of a racemic mixture of epoxides. An enantiomeric excess (ee, up to 22%) has been observed, that varies with the catalyst and the reaction conditions. The enantiomeric excess depends on the liability of the chiral ligands on Nb(IV).

The Niobaziridine−Hydride Functional Group: Synthesis and Divergent Reactivity

A multistep synthetic strategy enables the isolation of the niobaziridine-hydride complex Nb(H)(eta2-tBu(H)C=NAr)(N[Np]Ar)2 (1, Np = neopentyl, Ar = 3,5-C6H3Me2), which functions as a reactive synthon for its tautomer, the three-coordinate, trisamide species Nb(N[Np]Ar)3 (2). Treatment of 1 with various small molecules has demonstrated its capacity to effect two-electron reduction chemistry. Most noteworthy is the reaction between 1 and elemental phosphorus (P4), providing in high yield the bridging diphosphide complex (mu2:eta2,eta2-P2)[Nb(N[Np]Ar)3]2. However, unsaturated organic functionality including nitriles and aldehydes can insert into the Nb-H bond of 1, leaving the niobaziridine ring intact, thus demonstrating that dual pathways of reactivity are available to the niobaziridine-hydride functional group.

Spectroscopic and Structural Characterizations of Novel Water‐Soluble Peroxo[polyaminocarboxylato bis(N‐oxido)]niobate(V) Complexes

AbstractNew water‐soluble peroxo complexes of niobium(V) with polyaminocarboxylic acids (PAC) have been prepared and characterized by IR and NMR (1H, 13C, 15N) spectroscopy, as well as by thermal analysis. The synthesis in presence of excess H2O2 leads to the oxidation of the nitrogen atoms of the PAC ligand into N‐oxide groups. The compounds obtained correspond to the general formula (AI)3[Nb(O2)2(LO2)]·xH2O·yH2O2 [AI = NH4+ or CN3H6+ (gu) and L = edta, pdta] in which H4LO2 refers to the bis(N‐oxide) derivative of the PAC ligand. The crystal structures of the guanidinium derivatives of the (edta)‐ and (pdta)Nb complexes have been determined, both showing an eight‐coordinate Nb atom with two bidentate peroxo ligands and a quadridentate PAC bis(N‐oxido) ligand, resulting in a distorted dodecahedral geometry. The structure of the guanidinium derivative of the edta bis(N‐oxido) ligand is also described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Water-soluble niobium peroxo complexes as precursors for the preparation of Nb-based oxide catalysts

In the frame of research aimed at developing new synthetic procedures of multimetallic Nb-based catalysts, peroxo complexes of niobium(V) of general formula A I 3[Nb(O 2) 4] and A I 3[Nb(O 2) x (H y L)]· nH 2O (A I: NH 4 +, CN 3H 6 + (gu); L: oxalate, tartrate, citrate) have been prepared and characterized on the basis of elemental and thermal analysis, FTIR and 13 C -NMR spectra. The crystal structure of (gu) 3[Nb(O 2) 4] and (gu) 3[Nb(O 2) 2(C 2O 4) 2]·2H 2O have been determined. The application of the obtained Nb complexes as precursors for the preparation of silica-supported Nb–Mo–O catalysts has been demonstrated. Combining Nb peroxo-carboxylato compounds with analogous Mo(VI) compounds in a silica-impregnation method carried out in aqueous medium leads to the formation of the supported Nb 2Mo 3O 14 phase.

A Study on the Development of Chemical Vapor Deposition Precursors. 4. Syntheses and Characterization of New N-Alkoxo-β-ketoiminate Complexes of Niobium and Tantalum

Partial replacement of ethoxide ligands with N-alkoxo-β-ketoiminates in the tantalum or niobium ethoxides resulted in M(N-alkoxo-β--ketoiminate)(OEt) 3 (M = Ta and Nb) complexes. Most of these complexes are liquid and showed enhanced thermal and chemical stability, especially toward moisture. These complexes have octahedral geometry with a meridional N-alkoxo-β-ketoiminate ligand and showed fluxional behaviors. All of these precursors, especially the ones with methyl groups on both the N-alkoxo and β-ketoiminate backbones, demonstrate considerably higher deposition rates than M 2 (OEt) 10 (M = Ta and Nb), one of the most popular precursors for Ta 2 O 5 and Nb 2 O 5 films. Thermogravimetric analysis experiments showed that successive decomposition of ethoxide ligands is followed by decomposition of N-alkoxo-β-ketiminate in the pyrolysis process. Introduction of a methyl group onto the N-hydroxyethyl backbone enhanced the thermal stability at lower temperatures and the pyrolysis rate at higher temperatures. X-ray diffraction patterns indicate that the deposited films are not crystallized until the substrate temperature goes over 650 °C. Well-crystallized longish grains were formed in the Ta 2 O 5 film deposited at 700 °C, but it was found from the depth profile spectra by Auger electron spectroscopy (AES) that the nitrogen and carbon impurities are 2.3% and 4.2%, respectively, for the Ta 2 O 5 film deposited at 550 °C. Heat treatment at 700 °C in oxygen removed the nitrogen impurity in this Ta 2 O 5 completely, and the residual carbon was proved to be negligible from the AES and X-ray photoelectron spectroscopy results.

Changes of OH − bands induced by laser irradiation in LiNbO 3:Fe single crystals

By FTIR spectroscopy in undoped and Fe-doped LiNbO 3 single crystals of same origin and different histories it was found that laser irradiations in Fe-doped crystals cause the appearance of an OH − band at ∼3507 cm −1. This band is assumed to be caused by the formation of Fe 2+(Li)–OH −–Fe 3+(Nb) complexes via proton migration. This process precedes the microscopic structural laser-damages studied earlier.

1,4-Diaza-1,3-diene niobium chlorides: syntheses and X-ray crystal structures of ( t-Bu-DAD)NbCl 3(THF), ( t-Bu-DAD) 2NbCl, and [( t-Bu-DAD)NbCl 4](H 3N t-Bu)

The new 1,4-diaza-1,3-diene niobium complexes ( t-Bu-DAD)NbCl 3(THF) ( 2) and ( t-Bu-DAD) 2NbCl ( 3) [ t-Bu-DAD( t-Bu)NCHCHN( t-Bu)] have been prepared in 75–82% isolated yields by replacing of two or four chlorine atoms of NbCl 5 by the sterically demanding t-Bu-DAD ligand. In addition, we also describe the ionic Nb(V) complex [( t-Bu-DAD)NbCl 4](H 3N t-Bu) ( 6) which was formed as a byproduct during the preparation of 2. The new compounds have been characterized in solution by NMR measurements as well as by X-ray analyses in solid state. The structural parameters within the t-Bu-DAD ligand of 6 are in agreement with a chelating enediamido dianion. On the other hand the NC and CC bond distances of the t-Bu-DAD ligand of 2 are rather comparable to those found in complexes having a radical–anionic t-Bu-DAD ligand. The higher steric crowding at the niobium center caused by two 1,4-diaza-1,3-diene ligands in 3 leads to a significant asymmetric distortion of the 1,3-diaza-2-niobacyclopent-4-ene rings. However, none of the 1,4-diaza-1,3-diene ligands of 2, 3 or 6 is coordinated in the sterically less demanding η2-C,N bonding mode as found in the binuclear complex ( t-Bu-DAD) 5Nb 2 ( 4).

Determination of trace amounts of niobium in steel by reversed-phase high-performance liquid chromatography with methyl isobutyl ketone extraction and pre-column derivatization

A method for the determination of trace amounts of niobium in steel samples by reversed-phase high-performance liquid chromatography (RP-HPLC) was developed. The iron matrix was removed from sample solution by using methyl isobutyl ketone as an extraction agent. 2-(5-Bromo-2-pyridylazo)-5-diethylamino phenol (5-Br-PADAP) was used as a pre-column chelating agent to form ternary complex with Nb(V) and tartaric acid. The Nb(V) complex was eluted within 3min with a mobile phase of methanol–water (56:44 (v/v)) containing 10mmol/l acetate buffer (pH 3.5) on a C18 column and determined with spectrophotometric detection at 600nm. The detection limit of this method for Nb in iron or/and steel sample was 0.015μg/g. The proposed method was applied to the analysis of trace amounts of niobium in cast iron.