Schiff base tantalum(V) complexes

Summary Complexes of pentachlorotantalum with the Schiff bases: bis(vanillin)benzidine, bis(vanillin)-o-dianisidine, bis(acetylacetone)benzidine, bis(p-dimethylaminobenzaldehyde)-o-dianisidine, bis(anisaldehyde)-1, 3-propanediamine and bis(p-dimethylaminobenzaldehyde)-o-phenylenediamine have been prepared and characterized by molar conductance, decomposition temperature, elemental and t.g. analyses and i.r. spectral measurements. The conductances reveal that pentachlorotantalum (1 mole) interacts with all the ligands (1 mole), all five chloride ions thus forming simple adducts. A comparative study of the i.r. spectra of the parent ligands and their complexes allows the coordination sites to be ascertained. The studies show that tantalum(V) chloride prefers to form complexes of high coordination number.     

Benzoyl hydrazone derivatives of niobium(V) and tantalum(V) ethoxides

Summary Niobium(V) and tantalum(V) pentaethoxides react with monofunctional benzoyl hydrazones (BHy) in refluxing benzene to give products of the type, M(OEt)_5–n(BHy)_n (where M=Nb or Ta and n=1, 2 or 3). The complexes have been characterised on the basis of elemental analysis, spectral (i.r. and n.m.r.) and molecular weight data.     

Novel poly(methimazolyl)borate complexes of niobium(V) and tantalum(V)

The reactions of [MCl4(eta-C5H5)](M = Nb, Ta) with Ph3Sn{HB(mt)3} (mt = methimazolyl) provide structurally characterised complexes of the new chlorobis(methimazolyl)borate ligand, [MCl3(eta-C5H5){kappa2-S,S'-HClB(mt)2}].     

Electrochemical behavior of tantalum(V) oxide in chloride-fluoride melts

Electrochemical behavior of KCl-KF-Ta₂O₅ melts is studied by cyclic voltammetry. It is a Ta(V) oxyfluoride (KTaO₂F₂, K₃TaO₂F₄, or K₃TaOF₆), yielded by the Ta₂O₅-KF interaction in the melt, that presumably participates in the electrochemical process, rather than Ta₂O₅ proper. The Ta(V) oxyfluoride complex is reduced to tantalum metal at 973 K via three irreversible diffusion-controlled stages. The product of the electrokinetic transfer coefficient and the number of electrons participating in the third stage is determined. The Ta(V) reduction becomes one-staged at 1073 K. Tantalum-containing ternary systems with molybdenum, cobalt, or silicon are prepared by electrolyzing Ta₂O₅-containing melts.     

Co-ordination complexes of niobium and tantalum. XVIII. Peroxo-EDTA Niobates(V) and Tantalates(V)

Preparation and Properties of peroxoethylenediaminetetraacetato niobates(V) and tantalates(V) are reported. These stable, crystalline complexes can be recrystallized from aqueous and hydrogen peroxide solutions. According to chemical and spectral evidence the complexes correspond to the formula M[M^V (O₂)₃EDTA] nH₂O₂, M^I = K, NH₄; M^V = Nb, Ta; n ≤ 1. Niobium and tantalum compounds are isomorphous.     

Determination of tantalum with hexafluorotantalate(V)-selective coated graphite electrode

A hexafluorotantalate(V)-selective coated-graphite electrode was prepared by coating a graphite rod with brilliant green-hexafluorotantalate(V) extract in 1-chloronaphthalene in a PVC matrix. Potential measurements were made against an HF-resistant plastic sleeve (Ag/AgCl) external reference electrode. The concentrations of sulfuric and hydrofluoric acids, for the optimum response of the electrode to hexafluorotantalate(V), were found to be 1M each in the test solutions. The electrode responded to hexafluorotantalate(V) in the linear range 5.0 × 10^–6-5.0 × 10^–3 M, with a slope of -58 mV per decade and detection limit of 8.0 × 10^–7 M within 5–15s. The relative standard deviation for six determinations of 1.0 × 10^–4 M tantalum(V) was 2%. The life-time of the electrode was 60 days. The effects of forty diverse ions on the electrode response to the hexafluorotantalate(V) were studied and the electrode was found to be highly selective to hexafluorotantalate(V). Niobium, the element that commonly occurs with tantalum ores, showed a very low level of interference. The newly developed coated-graphite electrode has been applied to the determination of tantalum in tantalite-columbite ores and several synthetic matrices by direct, sample addition, standard addition, and Gran's plot potentiometric techniques with reasonable precision (2–4%) and accuracy.     

Tetrakisisopropoxy tantalum(V)O,O′-alkylene dithiophosphate complexes

Summary Tetrakisisopropoxytantalum(V) alkylene dithiophosphato complexes, (G=–CMe₂CMe₂–, –CHMeCHMe–, –CH₂CMe₂CH₂– and –CH₂CEt₂CH₂–) have been prepared from equimolar ratios of tantalum(V) isopropoxide and alkylene dithiophosphoric acids in benzene. These moisture-sensitive compounds, which are soluble in common organic solvents and are monomeric, have been characterized by elemental analysis, molecular weight determinations and by their i.r. and n.m.r. spectra. An octahedral geometry is suggested in which the ligand is bidentate.     

Ammonolysis of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives

Ammonolyses of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives [Ti(eta5-C5Me5)X3] (X = NMe2, Me, Cl) have been carried out in solution to give polynuclear nitrido complexes. Reaction of the tris(dimethylamido) derivative [Ti(eta5-C5Me5)(NMe2)3] with excess of ammonia at 80-100 degrees C gives the cubane complex [[Ti(eta5-C5Me5)]4(mu3-N)4] (1). Treatment of the trimethyl derivative [Ti(eta5-C5Me5)Me3] with NH3 at room temperature leads to the trinuclear imido-nitrido complex [[Ti(eta/5-CsMes)(mu-NH)]3(mu3-N)] (2) via the intermediate [[Ti(eta5-C5Me5)Me]2(mu-NH)2] (3). The analogous reaction of [Ti(eta5-C5Me5)Me3] with 2,4,6-trimethylaniline (ArNH2) gives the dinuclear imido complex [[Ti(eta5-C5Me5)Me])2(mu-NAr)2] (4) which reacts with ammonia to afford [[Ti(eta5-C5Me5)(NH2)]2(mu-NAr)2] (5). Complex 2 has been used, by treatments with the tris(dimethylamido) derivatives [Ti(eta5-C5H5-nRn)(NMe2)3], as precursor of the cubane nitrido systems [[Ti4(eta5-C5Me5)3(eta5-C5H5-nRn)](mu3-N)4] [R = Me n = 5 (1), R = H n = 0 (6), R = SiMe3 n = 1 (7), R = Me n = 1 (8)] via dimethylamine elimination. Reaction of [Ti(eta5-C5Me5)Cl3] or [Ti(eta5-C5Me5)(NMe2)Cl2] with excess of ammonia at room temperature gives the dinuclear complex [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) where an intramolecular hydrogen bonding and a nonlineal nitrido ligand bridge the "Ti(eta5-C5Me5)Cl(NH3)" and "Ti(eta5-C5Me5)Cl2" moieties. The molecular structures of [[Ti(eta5-C5Me5)Me]2 (mu-NAr)2] (4) and [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) have been determined by X-ray crystallographic studies. Density functional theory calculations also have been conducted on complex 9 to confirm the existence of an intramolecular N-H...Cl hydrogen bond and to evaluate different aspects of its molecular disposition.     

Addition compounds of monophenoxy-and diphenoxy-niobium(V) and tantalum(V) chlorides with some phosphoryl and amine oxide ligands

Summary Compounds of composition MCl_5–n(OPh)_n · L (M = Nb or Ta; n = 1 or 2 and L = monodentate ligand) have been prepared by reacting phosphoryl and amine oxide ligands with phenoxy-niobium(V) and -tantalum(V) chlorides and characterized by their analytical data, molecular weights, molar conductance and i.r. spectra.     

Ethylene polymerizations with novel tantalum(V) aminopyridinato complex/MAO systems

The use of two kinds of tantalum(V) aminopyridinato complexes, bis(2-benzylaminopyridinato)trichlorotantalum(V) and trichlorobis[2,6-di(phenylamino)pyridinato-N,N′]-tantalum(V), activated by methylaluminoxane was studied in polymerization of ethylene. The activities of these homogeneous catalyst systems are comparable to those of metallocenes. The weight-average molecular weights (M̄_w) of the produced polyethylenes are between 60 000 and 200 000 and M̄_w/M̄_n ≈ 2.     

The structure of (dibenzylideneacetone)(pentamethylcyclopentadienyl)rhodium(I)

The structure of (dibenzylideneacetone)(pentamethylcyclopentadienyl)rhodium(I), Rh(C₅Me₅)(dba), (dba = PhCHCHCOCHCHPh, Ph = C₆H₅, Me = CH₃), has been determined from three-dimensional X-ray data collected by counter methods. The structure has been refined by least-squares techniques to a final R index on F of 0.035 based on 2100 observations above background. The material crystallizes in the monoclinic space group C⁵_2hP2₁/a, with four molecules in a cell of dimensions a=14.348(12), b=14.063(13), c=11.393(10) », β=104.42(3)°. The observed and calculated densities are 1.35(3) and 1.41 g/cm³. The compound is monomeric. The Rh atom is bonded to the C₅Me₅ ring on one side and to the dba molecule through the two olefinic double bonds on the other. The C₅ and Me₅ portions of the C₅Me₅ ring are planar, but not coplanar as the Me group are bent away from the Rh atom by 0.10 » relative to the C₅ plane. The dba molecule is in the s-cis,s-cis conformation with the two double bonds in a plane which is parallel to the planes of the C₅Me₅ ring. The rest of the dba molecule is nonplanar with the CO group pointing away from Rh. The olefinic double bonds have lengthened on coordination to 1.411(9) ». The H atoms on the terminal C atoms of the dba molecule are in very close contact, the refined H?H distance being 1.83(9) ». It is suggested that the strain imposed on the coordinated dba molecule by this interaction might contribute to its ease of dissociation from the complex in solution and hence account for the catalytic properties of the complex.     

Structural studies of mono(pentamethylcyclopentadienyl)lanthanide complexes

The mono(pentamethylcyclopentadienyl) lanthanide complexes [(C₅Me₅)Yb(μ-I)(μ-η ⁵?: η ⁵-C₅Me₅)Yb(C₅Me₅)]_n (1), {[(C₅Me₅)Sm]₃(μ-Cl)₄(μ ₃-Cl)(μ ₃-OH)(THF)}₂ (2), {[(C₅Me₅)Sm]₂ (μ-OH)(μ-Cl)₄(μ ₃-Cl)Mg(THF)₂}₂ (3), [(C₅Me₅)₂Sm](μ-Cl)₆(μ ₃-Cl)₂(μ ₄-Cl)[(C₅Me₅)Sm]₄ (4), {[(C₅Me₅)Nd]₃(μ ₃-Cl)₄(μ ₄-Cl)₂(μ ₃-O₂CPh)₂K₂(η ⁶-C₇H₈)}₂ (5), [(C₅Me₅)Nd(C₈H₈)]₂(μ-dioxane) (6), [(C₅Me₅)Yb(MeO^tBu)]₂(μ-η ⁸?:?η ⁸-C₈H₈) (7), [(C₅Me₅)Dy(μ-I)₂]₃ (8), and [(C₅Me₅) Tm(MeCN)₆]I₂ (9), have been identified by X-ray crystallography. 1 is unusual in that it has a μ-η ⁵?:?η ⁵-C₅Me₅ ring that generates a local bent metallocene environment around ytterbium. Complexes 2–5 demonstrate the versatility of bridging chlorides in generating a variety of structures for mono(pentamethylcyclopentadienyl) lanthanide halides. Complex 6 shows how dioxane can generate a crystallographically-analyzable complex by bridging two mixed-ligand metallocene units that do not readily crystallize with THF. The structure of 7 shows how methyl tert-butyl ether (MTBE) ligates a lanthanide. Complex 8 is a trimeric cyclopentadienyl lanthanide halide unusual in that it has six bridging halides that roughly define a trigonal prism. Complex 9 constitutes an organometallic example of a lanthanide in which acetonitrile completely displaces iodide counterions.     

Synthetic and structural studies of monocyclopentadienyl cyclometalated aryl tantalum(V) compounds

Cyclometalated aryl tetra- or trichlorido cyclopentadienyl tantalum complexes [TaXCl(3){C(6)H(4)(2-CH(2)NMe(2))-κ(2)C,N}] (X = Cl 1, η(5)-C(5)H(5)2, η(5)-C(5)H(4)(SiMe(3)) 3, η(5)-C(5)Me(5)4) containing a five-membered TaC(3)N chelate ring were synthesized by reaction of the TaXCl(4) (X = Cl, η(5)-C(5)H(5), η(5)-C(5)H(4)(SiMe(3)), η(5)-C(5)Me(5)) with the appropriate lithium aryl reagent [Li{C(6)H(4)(2-CH(2)NMe(2))}]. The reported complexes were studied by IR and NMR spectroscopy and the X-ray molecular structures of compounds 2, 3 and 4 were determined by diffraction methods. These compounds were theoretically analyzed by the DFT method and their structures were rationalized. The preferential coordination of the 2-{(dimethylamino)methyl}phenyl ligand was justified by an analysis of the molecular orbitals of the Ta(η(5)-C(5)H(5))Cl(3) and C(6)H(4)(2-CH(2)NMe(2)) fragments. In addition, the exchange pathways that account for the NMR equivalency of the Me(2)N- methyl groups and -CH(2)- hydrogen atoms of the coordinated C(6)H(4)(2-CH(2)NMe(2))-κ(2)C,N ligand were theoretically studied.     

Sulfinylcalix[4]arene-impregnated amberlite XAD-7 resin for the separation of niobium(V) from tantalum(V)

Amberlite XAD-7 resin was impregnated with p-tert-butylsulfinylcalix[4]arene. Niobium(V) was collected on the impregnated resin in yields of more than 90% around pH 5.4, whereas tantalum(V) was negligibly collected. The collected niobium(V) was desorbed with 9 M sulfuric acid nearly quantitatively, hence the separation of niobium(V) from tantalum(V) was successfully achieved.