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.     

Internal rotation in peroxynitrous acid (ONOOH)

Using higher levels of wave-function-based electronic structure theory than previously applied, as well as density functional theory (B-LYP and B3-LYP functionals), all theoretical models conclude that three ONOOH conformers are stationary point minima, in disagreement with some of the previous studies that we survey. In order of increasing energy, these are the cis-cis, cis-perp, and trans-perp conformers. Basis sets including diffuse functions seem to be needed to obtain a qualitatively correct representation of the internal rotation potential energy surface at higher levels of theory. Internal rotation about the peroxide bond involving the cis-cis, cis-gauche transition structure (TS), cis-perp, and cis-trans TS conformers is studied in detail. To help ascertain the relative stability of the cis-perp conformer, multireference configuration interaction energy calculations are carried out, and rule of thumb estimates of multireference character in the ground-state wave functions of the ONOOH conformers are considered. CCSD(T)/aug-cc-pVTZ physical properties (geometries, rotational constants, electric dipole moments, harmonic vibrational frequencies, and infrared intensities) are compared with the analogous experimental data wherever possible, and also with density functional theory. Where such experimental data are nonexistent, the CCSD(T) and B3-LYP results are useful representations. For example, the electric dipole moment |mu(e)| of the cis-cis conformer is predicted to be 0.97+/-0.03 D. CCSD(T) energies, extrapolated to the aug-cc-pVNZ limit, are employed in isodesmic reaction schemes to derive zero Kelvin heats of formation and bond dissociation energies of the ONOOH stationary point minima. In agreement with recent gas-phase experiments, the peroxide bond dissociation energies of the cis-cis and trans-perp conformers are calculated as 19.3+/-0.4 and 16.0+/-0.4 kcalmol, respectively. The lowest energy cis-cis conformer is less stable than nitric acid by 28.1+/-0.4 kcalmol at 0 K.     

Use of the titanium(IV) chelate of salicylic acid for a selective absorptiometric determination of titanium

Titanium(IV) forms a 1:1 chelate with salicylic acid in 40% aqueous ethanolic solution. The chelate is extractable into chloroform. A selective extraction method for the estimation of titanium in the presence of a variety of commonly interfering species is described. The addition of pyridine is found to increase the sensitivity and selectivity and makes it unnecessary to use alcohol in the extraction. The extracted species is a pyridine adduct of the titanium-salicylic acid chelate.     

Mechanistic studies on the reaction between cob(II)alamin and peroxynitrite: evidence for a dual role for cob(II)alamin as a scavenger of peroxynitrous acid and nitrogen dioxide

Peroxynitrite/peroxynitrous acid (ONOO(-)/ONOOH; pK(a(ONOOH)) =6.8) is implicated in multiple chronic inflammatory and neurodegenerative diseases. Both mammalian B(12)-dependent enzymes are inactivated under oxidative stress conditions. We report studies on the kinetics of the reaction between peroxynitrite/peroxynitrous acid and a major intracellular vitamin B(12) form, cob(II)alamin (Cbl(II)), using stopped-flow spectroscopy. The pH dependence of the reaction is consistent with peroxynitrous acid reacting directly with Cbl(II) to give cob(III)alamin (Cbl(III)) and (.)NO(2) , followed by a subsequent rapid reaction between (.)NO(2) and a second molecule of Cbl(II) to primarily form nitrocobalamin. In support of this mechanism, a Cbl(II)/ONOO(H) stoichiometry of 2:1 is observed at pH 7.35 and 12.0. The final major Cbl(III) product observed (nitrocobalamin or hydroxycobalamin) depends on the solution pH. Analysis of the reaction products in the presence of tyrosine-a well-established (.)NO(2) scavenger-reveals that Cbl(II) reacts with (.)NO(2) at least an order of magnitude faster than tyrosine itself. Given that protein-bound Cbl is accessible to small molecules, it is likely that enzyme-bound and free intracellular Cbl(II) molecules are rapidly oxidized to inactive Cbl(III) upon exposure to peroxynitrite or (.)NO(2).     

Mixed ligand titanium(IV) complexes. Chlorobis(dithiocarbamato)bis(methylsalicylato)titanium(IV) andbis(dithiocarbamato)bis(methylsalicylato)titanium(IV)

Summary Dichlorobis(methylsalicylato)titanium(IV) reacts with potassium or amine salts of dialkyl or diaryl dithiocarbamates in 11 and 12 molar ratios in anhydrous benzene (room temperature) or in boiling CH₂Cl₂ to yield mixed ligand complexes: (AcOC₆H₄O)₂ Ti(S₂CNR₂)Cl (1) and (AcOC₆H₄O)₂ Ti(S₂CNR₂)₂ (2), R=Et, n-Pr, n-Bu, cyclo-C₄H₈ and cyclo-C₅H₁₀. These compounds are moisture sensitive and highly soluble in polar solvents. Molecular weight measurement in conjunction with i.r.,¹H and¹³C n.m.r. spectral studies suggest coordination number 7 and 8 around titanium(IV) in (1) and (2) respectively.     

Coprecipitation of titanium(IV) and Iron(III) complex salts from peroxide-fluoride solutions

Because the salts (NH₄)₃TiO₂F₅ and (NH₄)₃FeF₆ are isostructural, precipitation of the titanium peroxy salt from the (NH₄)₂TiF₆-(NH₄)₃FeF₆-NH₄F-H₂O₂ system is accompanied by crystal-chemical substitution of ions. As a result of partial peroxolysis, the iron salt coprecipitates with ammonium peroxypentafluorotitanate. If the degree of precipitation is higher than 20%, TiO₂ samples of high whiteness can be prepared from the hydrolysis product isolated from such solutions, after annealing at 850°C.     

Schiff base derivatives of titanium(IV) and zirconium(IV)

The reactions of a few bifunctional and tridentate Schiff bases with titanium-(IV) and zirconium(IV) isopropoxides in equimolar and bimolar ratios are described. The resulting compounds have been obtained in almost quantitative yields and are of the general formulae M(SB)_x(OPrⁱ)_4?2X (where M = Ti or Zr; SB- = anion of the Schiff base SBH₂ and x = 1 or 2). Their molecular weights have been determined ebullioscopically and IR spectra recorded.     

A self-assembled complex with a titanium(IV) catecholate core as a potential bimodal contrast agent

A ditopic chelating ligand (H(6)4) that bears catechol and diethylenetriamine-N,N,N',N',N'-pentaacetate (DTPA) has been designed and shown to specifically bind lanthanide(III) ions at the DTPA core ([Ln(H(2)4)(H(2)O)](-)) and further self-assemble with titanium(IV), thereby giving rise to the formation of a supramolecular metallostar complex with a lanthanide(III)-to-titanium(IV) ratio of 3:1, [(Ln4)(3)Ti(H(2)O)(3)](5-) (Ln=La, Eu, Gd). The efficacy of the metallostar complex as a potential bimodal optical/magnetic resonance imaging (MRI) agent has been evaluated. Nuclear magnetic relaxation dispersion (NMRD) measurements for the [(Gd4)(3)Ti(H(2)O)(3)](5-) complex have demonstrated an enhanced r(1) relaxivity that corresponds to 36.9 s(-1) mM(-1) per metallostar molecule at 20 MHz and 310 K, which is a result of a decreased tumbling rate. The ability of the complex to bind to human serum albumin (HSA) was also examined by relaxometric measurements. In addition, upon UV irradiation the [(Gd4)(3)Ti(H(2)O)(3)](5-) complex exhibits broad-band green emission in the range 400-750 nm with a maximum at 490 nm. Taking into account the high relaxivity and luminescence properties, the [(Gd4)(3)Ti(H(2)O)(3)](5-) complex is a good lead compound for the development of efficient bimodal contrast agents.     

Kinetics and mechanism of sulphurous acid oxidation by hexachloroiridate(IV) in hydrochloric acid

Summary The reaction between sulphurous acid and hexachloroiridate(IV) appears to take place through the formation of an intermediate complex followed by decomposition to give oxidation products. The rate is retarded as the hydrogen ion concentration increases. Thermodynamic parameters associated with the equilibrium step as well as with the slowest step have been calculated. The probable mechanism of the reaction is discussed.     

Spectrophotometric determination of titanium(IV) as a mixed thiocyanate—monooctyl-α-anilinobenzylphosphonate complex

Extraction of titanium(IV) thiocyanate complexes by monooctyl-α-anilinobenzylphosphonate (MOABP) dissolved in chloroform has been investigated as a function of hydrochloric and sulfuric acid concentration. Chloroform solutions follow Beer's law and are stable for at least 24 h. A sensitive and reproducible spectrophotometric determination of titanium is possible. Two complexes were identified; the first formed at acid concentrations less than 1 M, has the ratio Ti:SCN:MOABP 1:1:2 and the second, formed at higher acidity, has the ratio Ti:SCN:MOABP 1:2:2. The method is based on the extraction of titanium thiocyanate with MOABP from either 0.5 M H2SO4, and measurement of the absorbance at 336 nm (? = 11 000 l mol^-1 cm^-1 ), or from 6.5 M HCl, and measurement of the absorbance at 420 nm (? = 22 000 l mol^-1 cm^-1). The polymerization of thiocyanate has been studied; isoperthiocyanic acid has been identified as the polymerization product. Although both procedures give reproducible results, extraction from 6.5 M HCl is more sensitive, fewer elements interfere, and the precipitation of isoperthiocyanic acid is avoided.     

Kinetics and mechanism of benzene oxidation by peroxymonosulfate catalyzed with a binuclear manganese(IV) complex in the presence of oxalic acid

It is established that Oxone (peroxymonosulfate, 2KHSO₅ · KHSO₄ · K₂SO₄) oxidizes benzene to p-quinone very efficiently and selectively in a homogeneous solution in aqueous acetonitrile in the presence of a catalyst, i.e., dimeric manganese(IV) complex [LMn(O)₃MnL](PF₆)₂ where L is 1,4,7-trimethyl-1,4,7-triazacyclononane, and a cocatalyst, i.e., oxalic acid. The dependences of the maximum rate of quinone accumulation on the initial concentrations of reagents are studied. It is proposed that benzene is oxidized by the manganyl particle containing the Mn(V)=O fragment that forms upon the reaction of the reduced form of the starting dimeric manganese complex with Oxone.     

Kinetics of water oxidation with cerium(IV) compounds catalyzed by a tetranuclear ruthenium complex

The kinetics and mechanism of water oxidation with cerium(IV) compounds catalyzed by a tetranuclear ruthenium complex containing two polyoxotungstate ligands are reported. Four water molecules are oxidized via an eight-electron process to form two oxygen molecules.     

Kinetics of oxidation of hydrazine by a dioxo-bridged dimanganese(III,IV) complex ion

In aqueous acidic media containing an excess of Hbipy⁺–bipy buffer in the pH 3.5–4.5 range, the complex ion [(bipy)₂Mn^III(-O)₂Mn^IV(bipy)₂]³⁺ (1) coexists in rapid equilibrium with its diaqua derivative [Mn^III,IV ₂ (-O)₂(bipy)₃(H₂O)₂]³⁺ (1a) (bipy = 2,2-bipyridine). An excess of N₂H₅ ⁺ quantitatively reduces the mixture to Mn^II, itself being oxidised to N₂. The first order rate constant, k _o decreases with increasing C _bipy (C _bipy = [Hbipy⁺] + [bipy]) but increases with increasing [N₂H₅ ⁺] and [H⁺]. The observed kinetic dependence can be explained in terms of a reaction between (1a) and N₂H₅ ⁺. Replacement of solvent H₂O with D₂O decreases k _o substantially and the effect suggests simultaneous transfer of an electron and a proton in the rate-determining step. The relevance of this observation to the delayed oxidation of H₂O in the hydrazine-treated photosystem II is discussed.