Electrochemical Studies of Tantalum(V) Chlorides and Bromides in Acetonitrile. Electrochemical Generation of Tantalum(IV) Species

The electrochemical properties of tantalum halides, TaX₅ Et₄NTaCI₆ and Bu₄NTaBr₆ (X = Cl^? and Br^?) were investigated in rigorously dried acetonitrile using cyclic voltammetry and constant potential electrolysis. A special vacuum electrochemical cel1 equipped with a sample loading device and an AI₂O₃ column was used in order to prevent the hydrolysis of tantalum halides with the small amount of water present in “dry” acetonitrile. Two one-electron reduction waves were observed for al1 tantalum(V) species which correspond to consecutive Ta(V) → Ta(IV) and Ta(IV) → Ta(III) reduction. The E_1/2 values for the Ta(V) → Ta(IV) reduction process are criticalIy dependent on the number of CI^? ligands coordinated to the tantalum atom. As the number of Cl^? is increased from 4 to 5 and 6, it becomes more difficult, by 0.4 V, to reduce Ta(V) to Ta(IV). Constant potential electrolysis at –20°C generates Ta(IV) species; thus TaCl₆ ^2-. TaBr₆ ^2-, TaCl₅NCMe^?, TaBr₅NCMe^? and TaCI₄(NCMe)₂, are obtained in acetonitrile solution after electrolysis. It was found that the reduction product of TaCl₅NCMe depends upon the temperature. At a higher temperature (0°C) the initial electron transfer step is fol1owed by a chemical reaction in which some of the product, TaCI₅NCMe^?, reacts with the starting material, TaCI₅NCMe. to produce TaCl₆ ^? and TaCl₄(NCMe)₂. At lower temperatures (–20°C) the rate of the folIowing chemical reaction is much slower and the reduction product is almost exclusively TaCl₅NCMe^?.     

New octahedral Ta(V) hydrazido-substituted compounds for atomic layer deposition: Syntheses,X-ray diffraction structures of TaCl(NMe2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2], and fluxional behavior of the amido and hydrazido ligands in solution

The synthesis and structural characterization of new tantalum(V) compounds containing a single hydrazido(I) ligand are reported. Hydrazinolysis of TaCl(NMe₂)₄ using trimethylsilyl(dimethyl)hydrazine affords the compound TaCl(NMe₂)₃[N(TMS)NMe₂] in essentially quantitative yield. Metathetical replacement of the chloride ligand in TaCl(NMe₂)₃[N(TMS)NMe₂] by LiNMe₂ gives the all-nitrogen coordinated compound Ta(NMe₂)₄[N(TMS)NMe₂]. VT ¹H NMR studies support the existence of low-energy pathways involving rotation about the Ta–N bonds of the ancillary amido and hydrazido ligands in both hydrazido-substituted compounds. X-ray crystallographic analyses confirm the octahedral disposition about the tantalum metal in TaCl(NMe₂)₃[N(TMS)NMe₂] and Ta(NMe₂)₄[N(TMS)NMe₂] and the presence of an η²-hydrazido(I) ligand. Preliminary data using Ta(NMe₂)₄[N(TMS)NMe₂] as an ALD precursor for the preparation of tantalum nitride and tantalum oxide thin films are presented.     

Ta(CNDipp)6: An Isocyanide Analogue of Hexacarbonyltantalum(0)

Hexakis(2,6‐diisopropylphenylisocyanide)tantalum is the first isocyanide analogue of the highly unstable Ta(CO)₆ and represents the only well‐defined zerovalent tantalum complex to be prepared by conventional laboratory methods. Two prior examples of homoleptic Ta⁰ complexes are known, Ta(benzene)₂ and Ta(dmpe)₃, dmpe=1,2‐bis(dimethylphosphano)ethane, but these have only been accessed via ligand co‐condensation with tantalum vapor in a sophisticated metal‐atom reactor. Consistent with its 17‐electron nature, Ta(CNDipp)₆ undergoes facile one‐electron oxidation, reduction, or disproportionation reactions. In this sense, it qualitatively resembles V(CO)₆, the only paramagnetic homoleptic metal carbonyl isolable under ambient conditions.     

Determination of impurities in niobium metal by a radiochemical neutron activation analysis

A radiochemical separation method using an anion exchange resin has been applied to 3N grade Nb for determining nine impurity elements. Five elements (Cr, Fe, Co, Zn and Se) were separated in 2M HF, three elements (Mo, W and Hf) in 32M HF, Nb in 0.5M HF/3M HCl, and Ta in 1M NH₄F/4M NH₄CCl. The contents of the elements were calculated by a single comparator method using two monitors of Au and Co. The main impurity was revealed to be Ta with a content of over 160 ppm.     

Effects of nitrogen composition on the resistivity of reactively sputtered TaN thin films

Nanocrystalline tantalum nitride (TaN) thin films have been deposited by reactive direct current magnetron sputtering technique on Si/SiO₂ (100) substrate with nitrogen flow rate ranging from 0, 3, 5, 7, 9 to 11 standard cubic centimeter per minute (sccm). Structural properties, surface morphology, chemical composition and and resistivity of the TaN films were investigated by X‐ray diffraction (XRD), field emission scanning electron microscopy, X‐ray photoemission spectroscopy (XPS) and four‐point probe measurements, respectively. In the XRD spectra, a classical formation sequence of tantalum nitride phases in the order of Ta‐Ta₂N‐TaN‐Ta₄N₅ and decreasing amount of metallic Ta were observed with increasing nitrogen flow. The electrical resistivity of the TaN film was found to increase with increasing N/Ta ratio as a result of the increased electron scattering from interstitial N atoms. In the XPS analysis, two groups of Ta4f doublets relating to different TaN phases were observed in the core level spectra of TaN films. No strong coupling was observed between the Ta4f doublets and the Ta4p and the N1s groups. The appropriate nitrogen flow was believed to be helpful in the bonding and formation of stoichiometric TaN. Copyright © 2014 John Wiley & Sons, Ltd.     

Catalytic diversity of diene complexes of niobium and tantalum on polymerizations of ethylene,norbornene, and methyl methacrylate

Half‐metallocene diene complexes of niobium and tantalum catalyzed three‐types of polymerization: (1) the living polymerization of ethylene by niobium and tantalum complexes, MCl₂(η⁴‐1,3‐diene)(η⁵‐C₅R₅) ( 1‐4 ; M = Nb, Ta; R = H, Me) combined with an excess of methylaluminoxane; (2) the stereoselective ring opening metathesis polymerization of norbornene by bis(benzyl) tantalum complexes, Ta(CH₂Ph)₂(η⁴‐1,3‐butadiene)(η⁵‐C₅R₅) ( 11 : R = Me; 12 : R = H) and Ta(CH₂Ph)₂(η⁴‐o‐xylylene)(η⁵‐C₅Me₅) ( 16 ); and (3) the polymerization of methyl methacrylate by butadiene‐diazabutadiene complexes of tantalum, Ta(η²‐RN=CHCH=NR)(η⁴‐1,3‐butadiene)(η⁵‐C₅Me₅) ( 25 : R = p‐methoxyphenyl; 26 : R = cyclohexyl) in the presence of an aluminum compound ( 24 ) as an activator of the monomer.     

Formation of PPy/Ta2O5/Ta structure by electropolymerization

Electropolymerization of pyrrole on tantalum (Ta) electrodes was carried out in buffer solutions (0.04 M phosphoric acid, 0.04 M acetic acid, 0.04 M boric acid and 0.2 M sodium hydroxide) containing 0.1 M sodium ptoluenesulfonate (TsONa) under galvanostatic conditions and it was found that a polypyrrole (PPy) and a tantalum oxide (Ta₂O₅) layer are formed on a Ta electrode by an electrochemical oxidation process. The conditions of this simultaneous formation were studied in respect to current density (i_d), pyrrole concentration ([Py]), pH and the amount of electricity. Under certain conditions ([Py] = 0.25 M, pH = 1.8, i_d = 10–20 mA cm^?2, the amount of electricity = 1 C), 6–8 μm thick PPy films were efficiently formed on homogeneous 30–50 nm thick Ta₂O₅ layers. The PPy film showed a high electrical conductivity (110 s cm^?1), adhered well and covered the Ta₂O₅ layer. The resulting PPy/Ta₂O₅/Ta system is therefore proved to have excellent properties as a capacitor.     

Quaternary Germanides RE3TRh4Ge4 (RE = Ce,Pr, Nd; T = Nb,Ta) – A New Coloring Variant of the Aristotype AlB2

The quaternary germanides RE₃TRh₄Ge₄ (RE = Ce, Pr, Nd; T = Nb, Ta) were synthesized from the elements by arc‐melting and subsequent annealing in a muffle furnace. The structure of Ce₃TaRh₄Ge₄ was refined from single‐crystal X‐ray diffractometer data: new type, Pbam, a = 719.9(2), b = 1495.0(3), c = 431.61(8), wR₂ = 0.0678, 1004 F² values, and 40 variables. Isotypy of the remaining phases was evident from X‐ray powder patterns. Ce₃TaRh₄Ge₄ is a new superstructure variant of the aristotype AlB₂ with an ordering of cerium and tantalum on the aluminum site, whereas the honey‐comb network is built up by a 1:1 ordering of rhodium and germanium. This crystal‐chemical relationship is discussed based on a group‐subgroup scheme. The distinctly different size of tantalum and cerium leads to a pronounced puckering of the [Rh₄Ge₄] network, which shows the shortest interatomic distances (253–271 pm Rh–Ge) within the Ce₃TaRh₄Ge₄ structure. Another remarkable structural feature concerns the tantalum coordination with six shorter Ta–Rh bonds (265–266 pm) and six longer Ta–Ge bonds (294–295 pm). The [Rh₄Ge₄] network fully separates the tantalum and cerium atoms (Ce–Ce > 387 pm, Ta–Ta > 431 pm, and Ce–Ta > 359 pm). The electronic density of states DOS from DFT calculations show metallic behavior with large contributions of localized Ce 4f as well as itinerant ones from all constituents at the Fermi level but no significant magnetic polarization on Ce could be identified. The bonding characteristics described based on overlap populations illustrate further the crystal chemistry observations of the different coordination of Ce1 and Ce2 in Ce₃TaRh₄Ge₄. The Rh–Ge interactions within the network are highlighted as dominant. The bonding magnitudes follow the interatomic distances and identify differences of Ta bonding vs. Ce1/Ce2 bonding with the Rh and Ge substructures.     

Na0.74Ta3O6, ein niedervalentes Oxotantalat mit einer Ta–Ta‐Mehrfachbindung

Na_0.74Ta₃O₆, a Low‐Valent Oxotantalate with Multiple Ta–Ta Bonds The title compound was prepared in a sealed tantalum tube through the reaction of Ta₂O₅, tantalum and Na₂CO₃ in a NaCl flux at 1570 K within 5 d. The crystal structure of Na_0.74Ta₃O₆ (a = 713.5(1), b = 1027.4(2), c = 639.9(1) pm, Immm, Z = 4) was determined by single crystal X‐ray means. The structure is isomorphous with NaNb₃O₅F [1]. The characteristic structural units are triply bonded Ta1₂ dumb‐bells with eight square‐prismatically co‐ordinated O ligands. Four Ta2, each octahedrally surrounded by O atoms, are side‐on bonded weakly to the binuclear Ta₂O₈ complex, thus forming a Ta₆ propellane‐like cluster. The lattice parameters of three additional M_xTa₃O₆ phases, M = Li, Mn, and Yb, are reported.     

Re‐evaluation of the X‐ray Crystal Structure of [Ta4F20] and the Synthesis and Characterization of a Series of Mixed‐Metal Pentafluorides of Niobium and Tantalum

The direct fluorination of intimately mixed niobium and tantalum powders gives a range of mixed‐metal pentafluorides [Nb_xTa_4‐xF₂₀] (x = 1 2 , 2 3 , 3 4 ) as discreet species isostructural with tantalum pentafluoride (x = 0 1 ). The crystal structures of 1–4 are indistinguishable by X‐ray crystallography. Complex 1 crystallizes in the monoclinic space group C2/m with a = 9.5462(13), b = 14.3578(19), c = 5.0174(7) Å, β = 97.086(2)°, Z = 2. The geometry about the tantalum atom is distorted octahedral with 2 short and 2 slightly longer Ta‐F_terminal, and 2 Ta‐F_bridging distances. The angles at the bridging fluorine atoms are 172.9(5)°.     

Unusual reactivity of tantalum pentakis(dimethylpyrazolate) with CS2: scission of the C=S bond and formation of dmpz3CS– ligand

The reaction beetwen tantalum pentakis(3,5-dimethyl-pyrazolate) and CS₂ afforded novel complex [Ta(=S)(dmpz)₂{(dmpz)₃CS}]. In this reaction the molecule of CS₂ undergoes scission with the migration of one sulfide to the tantalum atom while three pyrazolate residues migrate to carbon with the formation of unusual (dmpz)₃CS^– ligand. The structure of the product was established by X-ray diffraction.     

Asymmetric hydroamination of aminoallenes catalyzed by titanium and tantalum complexes of chiral sulfonamide alcohol ligands

A series of bidentate sulfonamide alcohol ligands with varying steric and electronic properties was synthesized. The titanium and tantalum complexes of these ligands, prepared in situ from either Ti(NMe₂)₄ or Ta(NMe₂)₅, were used as catalysts for the asymmetric hydroamination of 6-methyl-hepta-4,5-dienylamine, giving exclusively the α-vinylpyrrolidine product. The titanium derived catalysts gave products with low stereoselectivity, up to 11%ee favoring (+)-2-(2-methyl-propenyl)-pyrrolidine, while the tantalum derived catalysts gave products with higher and opposite stereoselectivity, up to 34%ee favoring (−)-2-(2-methyl-propenyl)-pyrrolidine.     

Induction and Acceleration of Bonelike Apatite Formation on Tantalum Oxide Gel in Simulated Body Fluid

Untreated tantalum metal forms bonelike apatite layer on its surface in a simulated body fluid (SBF) after a long period. The apatite formation on the tantalum metal is significantly accelerated, when the metal was previously subjected to NaOH and heat treatments to form an amorphous sodium tantalate on its surface. The fast formation of the apatite on the NaOH- and heat-treated tantalum metal was explained as follows. The sodium tantalate on the surface of the metal releases the Na⁺ ion via exchange with H₃O⁺ ion in SBF to form a lot of Ta-OH groups on its surface. Thus formed Ta-OH groups induce the apatite nucleation and the released Na⁺ ion accelerates the apatite nucleation by increasing ionic activity product of the apatite in SBF due to increase in OH^– ion concentration. In the present study, in order to confirm this explanation, apatite formations on sodium tantalate gels with different Na/Ta atomic ratios, which were prepared by a sol-gel method were investigated. It was found that even Na₂O-free tantalum oxide gel forms the apatite on its surface in SBF. This proves that the Ta-OH groups abundant on the gel can induce the apatite nucleation. The apatite-forming ability of the gels increased with increasing Na/Ta atomic ratios of the gels. The sodium-containing tantalum oxide gels released the Na⁺ ion, the amount of which increased with increasing Na/Ta atomic ratios of the gels. The released Na⁺ ion gave an increase in pH of SBF. These results prove that the apatite nucleation induced by the Ta-OH groups is accelerated with the released Na⁺ ion by increasing ionic activity product of the apatite in SBF.     

Spektrochemische Analyse der Mischpentoxide Nb2O5 + Ta2O5

Zusammenfassung Zur spektrochemischen Analyse der Mischpentoxide von Niob und Tantal wurde ein Verfahren ausgearbeitet, das die Anregung im Gleichstrom-Kohlebogen nach Addink u. Mitarb. benutzt. Für die Niobbestimmung werden die bekannten K-Werte verwendet, während die K-Werte für Ta 2681,87, Ta 2684,28 sowie einige schwächere Nb- und Ta-Linien erst gefunden werden mußten. Die Reproduzierbarkeit des Verfahrens beträgt 10–11%; es erlaubt eine bequeme und rasche Analyse, besonders in Verbindung mit einer chemischen Isolierung von (Nb, Ta)₂O₅.

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Summary A spectrochemical method utilizing the Addink's Constant Temperature D.C. Arc Method has been worked out for the determination of Nb₂O₅ and Ta₂O₅ in combined pentoxides. For the determination of niobium the known K-values are employed, while the K-values for Ta 2681.87, Ta 2684.28 and for some weaker niobium and tantalum lines had to be found. The reproducibility being 10–11%, the method has proved to be convenient for a rapid and simple niobium and tantalum analysis, especially in combination with a chemical isolation of (Nb,Ta)₂O₅.

     

Molekül‐ und Kristallstruktur von Bis[chloro(μ‐phenylimido)(η5‐pentamethylcyclopentadienyl)tantal(IV)](Ta–Ta), [{TaCl(μ‐NPh)Cp*}2]

Molecular and Crystal Structure of Bis[chloro(μ‐phenylimido)(η⁵‐pentamethylcyclopentadienyl)tantalum(IV)](Ta–Ta), [{TaCl(μ‐NPh)Cp*}₂] Despite the steric hindrance of the central atom in [TaCl₂(NPh)Cp*] (Ph = C₆H₅, Cp* = η⁵‐C₅(CH₃)₅), caused by the Cp* ligand, the imido‐ligand takes a change in bond structure when this educt is reduced to the binuclear complex [{TaCl(μ‐NPh)Cp*}₂] in which tantalum is stabilized in the unusual oxidation state +4.     

Structural and thermal investigation of Ta–25 mass% Cu alloy prepared by mechanosynthesis route

A mixture of Ta and 25 mass% Cu elemental powders was subjected to mechanical alloying in a high-energy ball mill up to 60 h. The results are composite particles formed by nanocrystalline Cu and amorphous Ta phases. Thermal stability of amorphous was investigated by DSC. The XRD, FTIR and EDX analyses of Ta–25 mass% Cu powder milled for 60 h performed after DSC at 800 and 900 °C have revealed large amounts of Ta nitride and Ta oxides even though the milling process was done in Ar atmosphere. This is due to high reactivity of Ta fine particles with oxygen and nitrogen from air. During manipulations of the powder (taking samples from vials and its investigation), the adsorption phenomena on its surface occur, and both surface-adsorbed N₂ and O₂ are processed with powder and embedded in it. While heating of Ta–25% Cu milled powder in DSC, nitrogen and oxygen diffusion into tantalum is activated, and Ta₂N and TaO₂/Ta₂O₅ compound forms.

     

Comparative study of bimetal alkoxo complexes of rhenium,niobium, and tantalum by single-crystal x-ray diffraction and IR spectroscopy

Bimetal alkoxo complexes of rhenium, niobium, and tantalum with the general formula M₄O₂(OR)₁₄(ReO₄)₂, where M = Nb or Ta and R = Me or Et, were prepared in ～80% yield by reacting niobium or tantalum alkoxide M(OR)₅ (R = Me or Et) with rhenium heptoxide Re₂O₇ in toluene. Comparative analysis of the molecular structures of the complexes was carried out by means of single-crystal X-ray diffraction and IR spectroscopy. The effect of the organic ligand and crystallization temperature on the geometry of the perrhenate group was studied. The solubility of the aforementioned alkoxo complexes in organic solvents increases with increasing hydrocarbon chain length of the ligand.     

Tantalum isocyanide complexes: TaI(CNDipp)6 (Dipp is 2,6‐diisopropylphenyl) and ionic [Ta(CNDipp)7][Ta(CNDipp)6], a formal disproportionation product of the 17‐electron Ta0 metalloradical Ta(CNDipp)6

Treatment of tetraethylammonium hexacarbonyltantalate, [Et₄N][Ta(CO)₆], with 1.1 equivalents of molecular iodine (I₂) in tetrahydrofuran (THF) at 200 K, followed by the addition of 6.0 equivalents of 2,6‐diisopropylphenyl isocyanide (CNDipp) and slow warming to 293 K over a 24 h period gave the tantalum(I) iodide derivative hexakis(2,6‐diisopropylphenyl isocyanide‐κC)iodidotantalum(I), [TaI(C₁₃H₁₇N)₆] or TaI(CNDipp)₆, 1 . Recrystallization of this substance from pentane provided deep‐red nearly black parallelepipeds of the product, which was characterized by single‐crystal X‐ray diffraction. Addition of 1 in THF at 200 K to a suspension of an excess (5.8 equivalents) of caesium graphite (CsC₈), followed by warming, filtration, and solvent removal, afforded a dark‐green oily solid of unknown composition, from which several red–brown rhombohedral plates of the ditantalum salt heptakis(2,6‐diisopropylphenyl isocyanide‐κC)tantalum hexakis(2,6‐diisopropylphenyl isocyanide‐κC)tantalate, [Ta(C₁₃H₁₇N)₇][Ta(C₁₃H₁₇N)₆] or [Ta(CNDipp)₇][Ta(CNDipp)₆], 2 , were harvested. Salt 2 is a unique substance, as it is the only known example of a salt containing a homoleptic cation, [ML_x]⁺, and a homoleptic anion, [ML_y]^?, with the same transition metal and π‐acceptor ligand L. In solution, 2 undergoes full comproportionation to afford the recently reported 17‐electron paramagnetic zerovalent tantalum complex Ta(CNDipp)₆, the only known isolable TaL₆ complex of Ta⁰.     

Synthesis and properties of dicyclopentadienyltantalum hydride olefin compounds

Reactions of Cp₂TaCl₂ with RMgCl (R = n-C₃H₇, i-C₃H₇, n-C₄H₉, s-C₄H₉, n-C₅H₁₁ and C₅H₉) give tantalum hydride π-olefin complexes Cp₂Ta(H)L (L = C₃H₆, C₄H₈, C₅H₁₀ and C₅H₈). Two isomers of Cp₂Ta(H)C₃H₆ were obtained. The complexes are useful starting materials for the synthesis of other tantalum hydride species, e.g. Cp₂Ta(H)PEt₃ and Cp₂TaH₃.     

An AES study of the room-temperature oxidation of TaSix after bombardment with Ar+ ions of different energies

Tantalum silicide films of ∼200 nm thick and composition TaSi₂ were obtained by co-sputtering in a Varian 3120 S-gun magnetron system. The films were then introduced in an AES spectrometer and bombarded with Ar⁺ ions of different energies in order to obtain surfaces of different compositions as a consequence of preferential sputtering effects and their dependence on the energy of the primary ions. Lowering the energy of the Ar⁺ ions resulted in surfaces very rich in tantalum. The interactions of these surfaces with oxygen at low pressures (10⁻⁸−10⁻⁵ Torr) and at room temperature then have been studied comparatively by Auger electron spectroscopy. Reference experiments with pure Si and Ta allowed the comparison with those of the different silicide surfaces. It is found that the oxygen uptake depends on the Ta content so that the richer in Ta the surface is, the higher the O₂ incorporation. Furthermore, the uptake rate at the different TaSi_x surfaces resembles better the measured rate for pure Ta than that observed for pure Si. It has been observed also that the oxidation of Si is enhanced over that of pure silicon in all the surfaces studied here. Besides, the enhancement depends on the tantalum content.