Reductions of oxo species by aquatitanium(II) as catalyzed by titanium(IV)

Titanium(II) solutions, prepared by dissolving titanium wire in triflic acid + HF, contain equimolar quantities of Ti(IV). These preparations react readily with oxidizing species such as VO(2)(+), substituted benzoquinones, and the biguanide complex of Mn(IV). Reactions with excess Ti(II) yield Ti(III). Reductions of 1,4-benzoquinones and the Mn(IV) complex are catalyzed by added Ti(IV) but stoichiometry is unaffected. Rate laws for the Ti(IV)-catalyzed reactions are consistent with formation of a Ti(II)-Ti(IV) complex (K(formation) 4 x 10(2) M(-1)), which, in some cases, also partakes in contributing deprotonated and halogen-catalyzed paths.     

Reductions by titanium(II) as catalyzed by titanium(IV)

The cobalt(III) complexes, [(NH3)5CoBr]2+ and [(NH3)5CoI]2+ are reduced by Ti(II) solutions containing Ti(IV), generating nearly linear (zero-order) profiles that become curved only during the last few percent of reaction. Other Co(III)-Ti(II) systems exhibit the usual exponential traces with rates proportional to [Co(III)]. Observed kinetics of the biphasic catalyzed Ti(II)-Co(III)Br and Ti(II)-Co(III)I reactions support the reaction sequence: [Ti(II)(H20)n]2+ + [Ti(IV)F5]- (k1)<==>(k -1) [Ti(II)(H2O)(n-1)]2+ + [(H2O)Ti(IV)F5]-, [Ti(II)(H2O)(n-1)]2+ + Co(III) (k2)--> Ti(III) + Co(II) with rates determined mainly by the slow Ti(IV)-Ti(II) ligand exchange (k1 = 9 x 10(-3) M(-1) s(-1) at 22 degrees C). Computer simulations of the catalyzed Ti(II)-Co(III) reaction in perchlorate-triflate media yield relative rates for reduction by the proposed active [Ti(II)(H2O)(n-1)]2+ intermediate; k(Br)/k(I) = 8.     

Reactions of molybdenum(VI) with metal ion reductants

The reactions of aqueous H2MoO4 at low pH with titanium(II), titanium(III), europium(II), vanadium(II), and germanium(II), as monitored at 430 nm, give biphasic profiles featuring a sharp rise in absorbance followed by a marked decrease (Fig. 1). The final product is the dimeric Mo(V) cation, [Mo2O4]2+, and the strongly absorbing intermediate is taken as a monomeric Mo(V) species. The molar absorbances of the transients from different reductants are not the same, nor are the rate laws governing the fadings. None of the decay curves exhibits evidence of a second order dependence on the transient. The kinetic behaviors of these systems are consistent with the intervention of successor complexes of the type [Chemical structure: see text], (formed by inner sphere reductions of Mo(VI)), which decompose, via first-order processes, to a monomeric Mo(V) species. The latter then experiences rapid dimerization, which is kinetically silent. The possibility that Ge(II) bypasses the unstable tripositive state by reducing Mo(VI) to Mo(IV) (which then undergoes rapid Mo(VI)-Mo(IV) comproportionation) is considered.     

Reductions by aquatitanium(II)

Solutions of titanium(II), prepared by dissolving titanium wire in mixtures of hydrofluoric and triflic acids, reduce quinones, nitrosodisulfonate anion, and complexes of cobalt(III). When the oxidant is taken in excess, these reactions yield Ti(IV), whereas with excess reductant, the principal product is Ti(III). These reactions are compared with those by Ti(III). Despite differences in rate laws, it is clear that rate ratios for the two reductants (kTiII/kTiIII) fall well below 10(4), the minimum selectivity corresponding to estimated differences in formal potentials, and in some instances, Ti(II), the stronger reductant, reacts more slowly. For both Ti(III) and Ti(II), reductions within the series [Co(NH3)5X]2+(where X=F, Cl, Br, and I), the fluoro complex reacts much more rapidly than its congeners, and the bromo and iodo complexes are slowest, an order similar to that for Eu2+ reductions, but opposite to that for Cr(II) and Cu(I). The [Co(NH3)5Br]2+ reaction with excess Ti(II) proceeds at rates very nearly independent of [oxidant] during the first 80-90% reaction, implying that initiation occurs via unimolecular conversion of Ti(II) to an activated cationic reducing species, in the same manner as the earlier described reduction of I3- by Ge(II) in aqueous HCl.     

Aqueous spectroscopy and redox properties of carboxylate-bound titanium

The aqueous chemistry of Ti(III) and Ti(IV) in two different chemical environments is investigated given its relevance to environmental, materials, and biological chemistry. Complexes of titanium with the carboxylate ligands citrate and oxalate, found ubiquitously in Nature, were synthesized. The redox properties were studied by using cyclic voltammetry. All the titanium citrate redox couples are quasi-reversible. Electrospray mass spectrometry of the Ti(III) citrate solution shows the presence of a 1:2 Ti/cit complex in solution, in contrast to the predominant 1:3 Ti/cit complex with Ti(IV). The change in the coordination of the ligand to the metal on reduction may explain the quasi-reversible behavior of the electrochemistry. The redox potentials for Ti(IV) citrate in water vary with pH. At pH 7, the approximate E(1/2) is less than -800 mV. This stated change in redox properties is considered in light of the previously reported Ti(IV) citrate solution speciation. Analogous speciation behavior is suggested from the EPR spectroscopy of Ti(III) citrate aqueous solutions. The g tensors are deduced for several pH-dependent species from the simulated data. The X-ray crystal structure of a Ti(III)(2) oxalate dimer Ti(2)(mu-C(2)O(4))(C(2)O(4))(2)(H(2)O)(6).2H(2)O (3), which crystallizes from water below pH 2, is reported. Complex 3 crystallizes in a monoclinic P2(1)/c space group with a = 9.5088(19) Angstroms, b = 6.2382(12) Angstroms, c = 13.494(3) Angstroms, V = 797.8(3) Angstroms(3), and Z = 2. The infrared spectroscopy, EPR spectroscopy, and cyclic voltammetry on complex 3 are reported. The cyclic voltammetry shows an irreversible redox couple approximately -196 mV which likely corresponds to the Ti(IV)(2)/Ti(III)Ti(IV) couple. The EPR spectroscopy on solid complex 3 shows a typical S = 1 triplet-state spectrum. The solid follows non-Curie behavior, and the antiferromagnetic coupling between the two metal centers is determined to be -37.2 cm(-1). However, in solution the complex follows Curie behavior and supports a Ti(III)Ti(IV) oxidation state for the dimer.     

Studies on the liquid-liquid extraction of iron(III) and titanium(IV) with 3-phenyl-4-benzoyl-5-isoxazolone

The extraction behaviour of iron(III) and titanium(IV) from acidic chloride solutions has been investigated using 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) in xylene as an extractant. The results demonstrate that these metal ions are extracted into xylene as Fe(PBI)(3) and TiO(PBI)(2). The equilibrium constants of the extracted complexes have been deduced by non-linear regression analysis by taking into account complexation of metal ion with inorganic ligands in the aqueous phase and all plausible complexes extracted into the organic phase. IR and proton NMR ((1)H NMR) spectra were used to further clarify the nature of complexes extracted into organic phase. The effect of the nature of the diluent on the extraction of iron(III) and titanium(IV) has been studied and correlated with dielectric constants. The extraction behaviour of titanium(IV) has also been compared with that of other metal ions, viz. magnesium(II), vanadium(V), chromium(VI), iron(III), manganese(II), zinc(II) and zirconium(IV), which are associated with the titanium in waste chloride liquors of the titanium-mineral-processing industry.     

Extraction and separation of bismuth(III) from steel and bismuth-base alloys

Summary Separation of bismuth(III) from iron(III), molybdenum(VI), vanadium(V), chromium(VI), titanium(IV), antimony(III), lead(II), beryllium(II), uranium(VI), hafnium(IV), indium(III) and zirconium (IV) is achieved by solvent extraction with high molecular weight amines from sodium succinate solution adjusted to suitable pH. Bismuth(III) is stripped from the organic phase and determined spectrophotometrically. The method is shown to be applicable to bismuth alloys.

---

Extraktion und Trennung von Wismut(III) aus Stahl und wismuthältigen Legierungen

Zusammenfassung Wismut(III) läßt sich von Fe(III), Mo(VI), V(V), Cr(VI), Ti(IV), Sb(III), Pb(II), Be(II), U(VI), Hf(IV), In(III) und Zr(IV) durch Extraktion mit hochmolekularen Aminen aus Natriumsuccinat bei geeignetem pH trennen. Bi(III) wird dann von der organischen Phase getrennt und spektralphotometrisch bestimmt. Das Verfahren eignet sich für Wismutlegierungen.

     

Untersuchungen zur anwendung ternärer komplexe in der photometrie—I Die bestimmung des Titan(IV) mit Tiron in gegenwart von äthylendiamintetraessigsäure

Two model systems and three analytical procedures based on them have been investigated analytically and characterized statistically with the aim of evaluating the application of ternary complexes in photometry. From measurements on the systems Ti(IV), Tiron (TiR^8?₃, procedure I), Ti(IV), Tiron, EDTA (TiR^8?₃, procedure II), Ti(IV), Tiron, EDTA [TiO(HY)R^5?, procedure III], the molar absorptivities, standard deviations, coefficients of variation, calibration data, limits of detection and determination have been calculated, and the possible interferences of 45 ions have been examined. Procedure III is shown to be the least sensitive of the three, but to offer a higher selectivity towards titanium in the presence of Cr(III), Cu(II), Fe(III), Mn(II), Mo(VI), Ni(II), U(VI) and W(VI). The reasons for this are discussed, and some suggestions are offered concerning the intended application of ternary complexes.     

Syntheses and characterization of titanium(IV) and titanium(III) complexes with (2-dimethylphosphino)ethane-1-thiolate and (3-dimethylphosphino)propane-1-thiolate as ligands

Reactions of Cp(2)TiCl(2) (Cp = eta(5)-cyclopentadienide) with 2 or 1 equiv of hybrid P-S ligands (L), (CH(3))(2)P(CH(2))(n)()S(-) (n = 2, dmpet; n = 3, dmppt), produced new dicyclopentadienyltitanium(IV) complexes with L, Cp(2)Ti(L-kappaS)(2) (1, L = dmpet; 2, L = dmppt) and [Cp(2)Ti(L-kappa(2)S,P)]BPh(4) (3, L = dmpet; 4, L = dmppt). The Ti(III) complexes, Cp(2)Ti(L-kappa(2)S,P) (5, L = dmpet; 6, L = dmppt), were prepared by the reaction of Cp(2)Ti(eta(3)-C(3)H(5)) with 1 equiv of L. The structures of complexes 1-6 were confirmed by X-ray diffraction analyses. It was found that complexes 3 and 5 were isostructural around Ti(IV) and Ti(III) centers: the Ti(IV)-S bond length in 3 (2.3498(9) A) is shorter by 0.14 A than Ti(III)-S in 5 (2.4877(7) A), while Ti(IV)-P (2.534(1) A) was merely 0.05 A shorter than Ti(III)-P (2.5844(7) A). The redox potential between 3 and 5 in acetonitrile was -1.14 V vs the ferricinium/ferrocene couple. A heterobimetallic complex that has the frame of complex 1, [Cp(2)Ti(dmpet)(2)Cu]PF(6) (7), was also isolated and structurally characterized: the Ti-Cu distance (2.95(1) A) was shorter than that in [Cp(2)Ti(SC(2)H(4)PPh(2))(2)Cu]BF(4), previously reported by White and Stephan. Structural characterization was also carried out for CpTi(dmpet-kappaS)(2)(dmpet-kappa(2)S,P) (8) and CpTiCl(2)(dmppt-kappa(2)S,P) (9), which were obtained by the reactions of Cp(or Cp)TiCl(3) (Cp = eta(5)-C(5)Me(5)(-)) with n equiv (n = 1-3) of L. The mutual site-exchange reaction between phosphorus atoms on a coordinated dmpet in the kappa(2)S,P mode and on two other coordinated dmpet's in the kappaS mode within complex 8 was analyzed by the variable-temperature (31)P[(1)H] dynamic NMR method. The kinetic parameters for this process, k(ex)(298) = 1.9 x 10(5) s(-)(1), DeltaH = 48 kJ mol(-)(1), and DeltaS = 17 J mol(-)(1) K(-)(1), as well as the rather long Ti(IV)-P distance (2.652(1) A), indicate the fluxional nature of the coordination geometry in complex 8.     

Anodic stripping voltammetric determination of titanium(IV) using a carbon paste electrode modified with cetyltrimethylammonium bromide

A method is described for the voltammetric determination of titanium(IV) using a carbon paste electrode modified in situ with cetyltrimethylammonium bromide. The cationic micellar surfactant adsorbs onto the electrode particularly at negative potentials, simultaneously preconcentrating titanium(IV) as the oxalate complex with reduction to titanium(III). Anodic stripping voltammetry exploiting reoxidation can be used for the determination of trace levels of titanium(IV). Linearity between current and concentration exists between 5 and 160 mug l(-1) Ti(IV) (preconcentration time 2 min). The limit of detection (calculated as 3sigma) is 0.1 mug l(-1), with a preconcentration time of 10 min.     

Selective extraction and spectrophotometric determination of titanium(IV) with 2-thenoyltrifluoroacetone

A new rapid method is suggested for simultaneous extraction and spectrophotometric determination of titanium with 2-thenoyltrifluoroacetone (TTA). The greenish-yellow chelate of titanium(IV)-TTA can be extracted with isoamvl alcohol-benzene and measured at 430 mm. The extraction can be carried out in presence of Al, Cr(III), Th, Zr, Mo, W, borate, phosphate, citrate, tartrate and EDTA.     

Titanium(II) and titanium(III) tetrahydroborates. Crystal structures of [Li(Et2O)2][Ti2(BH4)5(PMe2Ph)4], Ti(BH4)3(PMe2Ph)2, and Ti(BH4)3(PEt3)2

The molecular structures of the titanium(III) borohydride complexes Ti(BH4)3(PEt3)2 and Ti(BH4)3(PMe2Ph)2 have been determined. If the BH4 groups are considered to occupy one coordination site, both complexes adopt distorted trigonal bipyramidal structures with the phosphines in the axial sites; the P-Ti-P angles deviate significantly from linearity and are near 156 degrees. In both compounds, two of the three BH4 groups are bidentate and one is tridentate. The deduced structures differ from the one previously described for the PMe3 analogue Ti(BH4)3(PMe3)2, in which two of the tetrahydroborate groups were thought to be bound to the metal in an unusual "side-on" (eta(2)-B,H) fashion. Because the PMe3, PEt3, and PMe2Ph complexes have nearly identical IR spectra, they most likely have similar structures. The current evidence strongly suggests that the earlier crystal structure of Ti(BH4)3(PMe3)2 was incorrectly interpreted and that these complexes all adopt structures in which two of the BH4 groups are bidentate and one is tridentate. The synthesis of the titanium(III) complex Ti(BH4)3(PMe2Ph)2 affords small amounts of a second product: the titanium(II) complex [Li(Et2O)2][Ti2(BH4)5(PMe2Ph)4]. The [Ti2(BH4)5(PMe2Ph)4]- anion consists of two Ti(eta(2)-BH4)2(PMe2Ph)2 centers linked by a bridging eta(2),eta(2)-BH4 group that forms a Ti...(mu-B)...Ti angle of 169.9(3) degrees. Unlike the distorted trigonal bipyramidal geometries seen for the titanium(III) complexes, the metal centers in this titanium(II) species each adopt nearly ideal tbp geometries with P-Ti-P angles of 172-176 degrees. All three BH4 groups around each Ti atom are bidentate. One of the BH4 groups on each Ti center bridges between Ti and an ether-coordinated Li cation, again in an eta(2),eta(2) fashion. The relationships between the electronic structures and the molecular structures of all these titanium complexes are briefly discussed.     

Complexes of gallium(III), antimony(III), titanium(IV), and cobalt(II) with acenaphthenequinonimine

Reactions of (2,6-diisopropylphenylimino)acenaphthenone (dpp-mian) with gallium(III), antimony(III), titanium(IV), and cobalt(II) chlorides in toluene lead to the formation of compounds of the formulas [(dpp-mian)₂GaCl₂][GaCl₄], (dpp-mian)SbCl₃, (dpp-mian)TiCl₄, and [(dpp-mian)CoCl₂]₂[CoCl₂(EtOH)₄], respectively. The complexes were characterized by IR and NMR spectra, their structure was established by X-ray crystallography.     

Hierarchical assembly of helicate-type dinuclear titanium(IV) complexes

The ligands 4-7-H(2) were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li(4)[(4-7)(6)Ti(2)] and Li(6)[(4/5a)(6)Ga(2)]. The X-ray crystal structures of Li(4)[(4)(6)Ti(2)], Li(4)[(5b)(6)Ti(2)], and Li(4)[(7a)(6)Ti(2)] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.     

Asymmetric alkylation of aldehydes catalyzed by novel dinuclear bis-BINOLate titanium(IV) complexes

Bis-BINOLs 1a,b in which two BINOL units are tethered by o- and m-phenylenebis(ethynyl) groups form stable dinuclear bis-BINOLate titanium(IV) complexes 2a,b by treatment with titanium tetraisopropoxide. In the presence of excess titanium tetraisopropoxide, 2a and 2b (2–20 mol %) catalyze diethylzinc addition to aromatic and aliphatic aldehydes in an efficient manner to give the ethylation products with high enantioselectivities. While more than 1 equiv of titanium tetraisopropoxide (with respect to a substrate aldehyde) is generally employed for obtaining high turnover frequency and selectivity in reactions catalyzed by a parent (BINOLate)Ti(OⁱPr)₂, the amount can be reduced as low as 0.2 equiv in the reactions catalyzed by 2a,b.     

Reactivity of boranes with a titanium(IV) amine tris(phenolate) alkoxide complex; formation of a Ti(IV) tetrahydroborate complex, a Ti(III) dimer and a Ti(IV) hydroxide Lewis acid adduct

Treatment of the titanium(IV) alkoxide complex [Ti(Oi Pr)(OC6Me2H(2)CH2)3N] (2) with BH3.THF, as part of a study into the utility and reactivity of (2) in the metal mediated borane reduction of acetophenone, results in alkoxide-hydride exchange and formation of the structurally characterised titanium(iv) tetrahydroborate complex [Ti{BH4}(OC6Me2H2CH2)3N] (3). Complex (3) readily undergoes reduction to form the isolable titanium(III) species [Ti(OC6Me2H2CH2)3N]2 (4). Reaction of (2) with B(C6F5)3 results in formation of the Lewis acid adduct [Ti(OC6Me2H2CH2)3N][HO.B(C6F5)3] (5). In comparison, treatment of the less sterically encumbered alkoxide Ti(Oi Pr)4 with B(C6F5)3 results in alkoxide-aryl exchange and formation of the organometallic titanium complex [Ti(Oi Pr)3(C6F5)]2 (6). The molecular structures of 3, 4, 5 and 6 have been determined by X-ray diffraction.     

Extractive photometric determination of vanadium(V) as thiocyanato and azido mixed-ligand complexes with N-hydroxy-N-p-chlorophenyl-N'-(2-methyl-4-chlorophenyl) benzamidine hydrochloride

Ternary systems involving thiocyanate or azide (X) and N-hydroxy-N-p-chlorophenyl-N'-(2-methyl-4-chlorophenyl)benzamidine hydrochloride (HOAm) are used for the extraction-photometric determination of vanadium(V) as a VOX(2)(OAm)HOAm complex. A strong synergistic effect is observed. Mn(II), Cr(III), Ti(IV), Zr(IV), Mo(VI) and W(VI) do not interfere. The method has been applied to standard steel samples.     

Photochemical properties of peroxo complexes of titanium,intermediate products in the photocatalytic liquid-phase oxidation of alcohols

A study has been made of the photochemical reactions of titanium(IV) peroxo complexes formed in the reaction of titanium tetrachloride with hydrogen peroxide in alcoholic solutions and which are characterized by intense charge transfer bands at 360–425 nm. Irradiation of the solutions with light of = 254 nm leads to the decomposition of the titanium(IV) peroxo complexes, the formation of titanium(III) compounds, as well as the oxidation of the alcohol to aldehyde. In the irradiated frozen solutions associated complexes of titanium(III), organic free radicals formed from the alcohol molecule, as well as peroxy radicals have been identified by EPR. Irradiation with light corresponding to the longwave band of the peroxo complexes leads to their decomposition but titanium(III) compounds and alcohol oxidation products are not formed in this case. In irradiated frozen solutions the formation of paramagnetic titanium(IV) complexes containing the fragment Ti...O ₂ has been established, as well as other paramagnetic particles identified tentatively as coordinated Ti...O ₂ ^· radicals or radical pairs. It is shown that the decomposition of hydrogen peroxide in the presence of titanium(IV) compounds is photocatalytic.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 67–74, January–February, 1988.     

Preparation and crystal structures of a series of titanium(III) 9-BBN hydroborate complexes containing Ti...H agostic interactions

9-BBN hydroborate complexes Ti{(mu-H)2BC8H14}3(THF)2 (1), Ti{(mu-H)2BC8H14}3(OEt2) (2), and [K(OEt2)4]-[Ti{(mu-H)2BC8H14}4] (4) were formed from the reaction of TiCl4 with K[H2BC8H14] in diethyl ether or THF. Ti{(mu-H)2BC8H14}3(PhNH2) (3) was isolated from the reaction of 2 with aniline in diethyl ether. In the formation of these complexes, Ti(IV) is reduced to Ti(III). The coordinated diethyl ether in 2 can be displaced by the stronger bases THF and aniline, to form 1 and 3, respectively. All of the compounds were characterized by single-crystal X-ray diffraction analysis. In complex 1, which contains two coordinated THF ligands, the titanium possesses a 17 electron configuration and there is no evidence for agostic interaction. Complexes 2 and 3 contain only one coordinated ether or aniline ligand, and the titanium possesses a 15 electron configuration. In these compounds, a C-H hydrogen on an alpha carbon on the BC8H14 unit of a 9-BBN hydroborate ligand forms an agostic interaction with the titanium. Criteria for assessing the existence of agostic interactions are discussed. As the potassium salt, the anion of complex 4 is more stable than the complexes 1-3. Organometallic anions of the type [ML4]- for titanium(III) are rare.     

Photolysis of titanium (III) complexes in alcoholic matrices at 77‡k

Using the electron spectroscopy and EPR methods, it was found that during UV irradiation of frozen alcoholic solutions of distorted octahedral complexes [Ti(ROH)₄Cl₂]⁺ and [Ti(ROH)₆]³⁺, distorted tetrahedral hydroxide complexes of titanium (III) are obtained. The mechanism of the process, including successive redox reactions of the titanium compounds has been discussed. As the result of a phototransfer of an electron from the central atom orbitals to the vacant orbitals of the ligands (alcohol molecules) a titanium (IV) compound is obtained containing an anion radical of the alcohol as the ligand. The compound is unstable and undergoes ultraspheric transformations. Rupture of the C-O bond occurs in the anionradical of the alcohol; ethyl radicals and hydroxide complexes of titanium (IV) are thus formed, which by the action of light quanta transform into hydroxide complexes of titanium (III).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 480–484, July–August, 1985.