Influence of solution pH and reaction atmosphere on the morphology of SrTiO<Subscript>3</Subscript> nanocubes synthesized by thermohydrolysis

The influence of synthetic conditions on the shape and size of SrTiO₃ (STO) nanocubes was studied. These were synthesized in aqueous solution using Sr(OH)₂ as the Sr²⁺ source and titanium(IV) bis(ammonium lactate)dihydroxide (TALH) as the Ti^IV source in the presence of oleic acid and hydrazine. A large excess of OH^? at a pH of the precursor solution higher than 12 is necessary for the formation of STO nanocubes without the need for any calcination. Performing the synthesis in a N₂ atmosphere additionally prevents the formation of SrCO₃ impurity, leading to the creation of uniformly sized STO nanocubes in a reproducible manner. Such size-regulated STO nanocubes are found to align over a large area.     

Complex formation of indium(<Emphasis Type="SmallCaps">iii</Emphasis>) with citric acid in aqueous solution

Complex formation of In³⁺ ion with citric acid in an aqueous solution was studied by pH-metric titration at the molar ratio of the reactants [In³⁺] : [H₄Cit] = 1 : 1, 1 : 2, and 1 : 3 in a range of pH 2—10. The mathematical simulation of equilibria in an indium(iii)—citric acid system was performed using the CPESSP program package. The formation of 1 : 1, 1 : 2, and 1 : 3 indium(iii) citrate complexes with different degrees of nuclearity and protonation was established. The equilibrium constants of formation of the complexes were calculated. The predomination of the polynuclear indium(iii) citrate complexes at the equimolar metal to ligand ratio of was observed in almost the whole pH range studied (3—10).     

Site‐Selective Ligand Exchange on a Heteroleptic TiIV Complex Towards Stepwise Multicomponent Self‐Assembly

A series of heteroleptic [Ti 1 ₂X]^? complexes have been selectively constructed from a mixture of Ti^IV ions, a pyridyl catechol ligand (H₂ 1 ; H₂ 1 =4‐(3‐pyridyl)catechol), and various bidentate ligands (HX) in the presence of a weak base, in addition to a previously reported [Ti 1 ₂(acac)]^? (acac=acetylacetonate) complex. Comparative studies of these Ti^IV complexes revealed that [Ti 1 ₂(trop)]^? (trop=tropolonate) is much more stable than the [Ti 1 ₂(acac)]^? complex, which allows the replacement of acac with trop on the [Ti 1 ₂(acac)]^? complex. This Ti^IV‐centered site‐selective ligand exchange reaction also takes place on a heteronuclear Pd^II? Ti^IV ring complex with the preservation of the Pd^II‐centered coordination structures. Intra‐ and intermolecular linking between two Ti^IV centers with a flexible or a rigid bis‐tropolone bridging ligand provided a tetranuclear and an octanuclear Pd^II? Ti^IV complex, respectively. These higher‐order structures could be efficiently constructed only through a stepwise synthetic route.     

Acetylacetonat-Komplexe des SiIV und TiIV,brenzcatechinat-komplexe des SbIII,SiIV, TiIV und SnIV in methanolischer Lösung

The complex formation of acetylacetone with Si^IV and Ti^IV and of pyrocatechol with Sb^III, Si^IV, Ti^IV and Sn^IV have been investigated by pH-measurements in absolute methanolic solutions containing (CH₃)₄NCl, LiCl of Litiumtosylate (μ = 1; 20,0°).     

Titanium reduction in a soluble ziegler—natta catalyst

The spontaneous reduction of Ti^IV to Ti^III in soluble Ziegler—Natta catalysts of type Cp₂RTiCl·R’AlCl₂ (Cp = h⁵ -cyclopentadienyl, R and R’ = methyl or ethyl) was studied both spectrally and chromatographically. Varied were R, R’, Al/Ti ratio, total concentration, solvent, and added olefin. Kinetic order in [Ti] could be varied from zero to second order by changing solvent. This can be explained by a mechanism in which a Cp₂ RTiCl—R’AlCl₂—olefin complex forms in the rate determining step and ligand R is expelled as half alkane half olefin. The expelled olefin may either polymerize or catalyze reduction by forming the rate-determining complex. Different apparent kinetic orders arise from differences in the olefin competitive reactions. The reaction products appear to form in a rapid bimolecular reaction following the rate-determining step. Evidence is presented that neither free radicals nor Cp₂RR’Ti are reduction intermediates. The intermediate is postulated to be a Ti^IV transient hydride formed by a reverse insertion step.     

Zero-Field Splittings and Redox Potentials in an Isostructural Series of Dinuclear FeIITiIV,FeIIITiIV, and MnIITiIV Complexes with a Face-Sharing Bridging Motive

The ligand H₆ioan has been used to synthesize the three dinuclear complexes [(ioan)Mn^IITi^IV], [(ioan)Fe^IITi^IV], and [(ioan)Fe^IIITi^IV]⁺. The face-sharing bridging mode of the three phenolates provides short M-Ti^IV distances of ≈3.0 Å. Mössbauer spectra of [(ioan)Fe^IIITi^IV]⁺ show a magnetically split six-line spectrum at 3 K in zero magnetic field demonstrating a slow magnetic relaxation. Magnetic measurements provide a zero-field splitting of |D|=5 cm⁻¹ in [(ioan)Fe^IITi^IV]. EPR spectroscopy demonstrates sizable zero-field splittings of the S=5/2 spin systems of [(ioan)Mn^IITi^IV] (D=0.246 cm⁻¹) and [(ioan)Fe^IIITi^IV]⁺ (D<−1 cm⁻¹) that can be related to enforced covalency of the M-O^ph bonds. [(ioan)Fe^IIITi^IV]⁺ exhibits a reversible reduction at −0.26 V vs. Fc⁺/Fc demonstrating the facile accessibility of Fe^III and Fe^II. In contrast to an irreversible oxidation in [(ioan)Ni^IITi^IV] at 0.78 V vs. Fc⁺/Fc, the reversible oxidation at 0.25 V vs. Fc⁺/Fc in [(ioan)Mn^IITi^IV] indicates even the access of Mn^III. These results indicate that pentanuclear complexes [(ioan)FeM¹M²M¹Fe(ioan)]ⁿ⁺ are meaningful targets to access electron delocalization in mixed-valence systems over five ions due to the facile accessibility of both Fe^II and Fe^III in the terminal positions. This study provides important local spin-Hamiltonian and Mössbauer parameters that will be essential for the understanding of the potentially complicated electronic structure in the anticipated pentanuclear complexes.     

Synthesis and spectral properties of titanium(iv) polynuclear hydroxo complexes stabilized in solution by the [PW11O39]7− heteropolyanion

Unsaturated heteropolyanions (HPA) [PW₁₁O₃₉]⁷⁻ stabilize Ti^IV hydroxo complexes in aqueous solutions (Ti: PW₁₁ [PW₁₁O₃₉]⁷⁻⪯12, pH 1–3). Spectral studies (optical,¹⁷O and³¹P NMR, and IR spectra) and studies by the differential dissolution method demonstrated that Ti^IV hydroxo complexes are stabilized through interactions of polynuclear Ti^IV hydroxo cations with heteropolyanions [PW₁₁TiO₄₀ ⁵⁻ formed. Depending on the reaction conditions, hydroxo cations Ti _n−1O _x H _y ^m+ either add to oxygen atoms of the W−O−Ti bridges of the heteropolyanions to form the complex [PW₁₁TiO₄₀·Ti _n−1O _x H _y ] ^k− (at [HPA]=0.01 mol L⁻¹) or interact with Ti^IV of the heteropolyanions through the terminal o atom to give the polynuclear complexes [PW₁₁O₃₉Ti−O−Ti _n−1O _x H _y ]^q− (at [HPA]=0.2 mol L⁻¹). When the complexes of the first type were treated with H₂O₂, Ti^IV ions added peroxo groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 914–920, May, 1997.     

Mechanistic Insight into Peroxo‐Shunt Formation of Biomimetic Models for Compound II,Their Reactivity toward Organic Substrates,and the Influence of N‐Methylimidazole Axial Ligation

High‐valent iron‐oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe^III(OH) porphyrin ([meso‐tetrakis(2,4,6‐trimethyl‐3‐sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H₂O₂ with (TMPS)Fe^III(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe^IV?O(OH) point to the overall associative nature of the process. A pH‐dependence study on the formation of (TMPS)Fe^IV?O(OH) revealed a very high reactivity of OOH^? toward (TMPS)Fe^III(OH) in comparison to H₂O₂. The influence of N‐methylimidazole (N‐MeIm) ligation on both the formation of iron(IV)‐oxo species and their oxidising properties in the reactions with 4‐methoxybenzyl alcohol or 4‐methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe^III(H₂O)(N‐MeIm) is highly reactive toward H₂O₂ to form the iron(IV)‐oxo species, (TMPS)Fe^IV?O(N‐MeIm). The latter species can also be formed in the reaction of (TMPS)Fe^III(N‐MeIm)₂ with H₂O₂ or in the direct reaction of (TMPS)Fe^IV?O(OH) with N‐MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe^IV?O(OH) and (TMPS)Fe^IV?O(N‐MeIm) do not display a pronounced effect of the N‐MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH^? substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH^? or N‐MeIm) in the trans position to the oxo group in the iron(IV)‐oxo species does not significantly affect the activation barriers calculated for C?H dehydrogenation of the selected organic substrates.     

A computational study and fragment analysis of the back-bonding in Titanocenyl Complexes containing a five-member L,L′-cyclic ligand, L,L′ = O,O′; S,S′ or Se,Se′

A DFT computational study is performed on different Cp₂Ti^IV(L,L′-BID) complexes with L,L′-BID = dioxolene, dithiolene or diselenolene. A fragment analysis of the titanocene-ligand bonding in the DFT optimized geometries showed that out of plane folding for maximum Ti ← L π donation increases Cp₂Ti^IV(O,O′-BID) (35°) < Cp₂Ti^IV(S,S′-BID) (43–49°) < Cp₂Ti^IV(Se,Se′-BID) (48–53°).     

Equilibres d'ionisation et photoréduction du vanadium(V) dans l'acide sulfurique concentré

By spectrophotometry of dilute solutions of V₂O₅ in concentrated sulfuric acid V^V? H₂SO₄ complexes are shown to convert slowly to V^IV monomers. Spectral, kinetic and equilibrium studies on these solutions suggest that this conversion involves a photoreduction of V^V to V^IV dimeric species.     

Methoxokomplexe drei-, vier- und fünfwertiger Zentralatome

The complex formation with CH₃O^? of As^III, Sb^III, Ge^IV, Nb^IV, Se^IV, Te^IV, Ti^IV, Sn^IV and Mo^V has been investigated in absolute methanolic solutions containing (CH₃)₄NCl, LiCl, or Lithiumtosylate (μ = 1; 20.0°) by means of pH-titrations. The relations between the stoichiometry of the reactions and the shape of the buffer regions, as well as the concentration-dependance of these buffer regions are discussed.     

Photoexcitation of octacyano complexes of molybdenum(IV) and tungsten(IV) with ethylene diamine. Kinetics and reaction mechanism

Summary The kinetics of thermal reactions of photochemically generated aquaheptacyano Mo^IV and W^IV complexes and their protonated and deprotonated forms have been studied spectrophotometrically in buffer solutions at pH 5.0–10.0, and ionic strength, 8×10^–2 M at 25°C. A reaction scheme for the photochemical and thermal reactions of the Mo^IV and W^IV octacyano complexes with ethylene diamine is proposed. Rate constants and quantum yields for these systems are maximal at pH 8.0. At pH>8.0, the reverse reaction, generating octacyano complexes from heptacyano species, is faster; at low pH the ligand is protonated and is less reactive. Quantum yields are higher for Mo than for W owing to the shorter life time of excited state species. This is because physical deactivation is expected to be more rapid in the heavier element due to enhanced spin-orbit coupling. Furthermore Mo-induced splitting is larger in [W(CN)₈]^4– as compared to [Mo(CN)₈]^4– which results in greater bond strength for tungsten.     

DOTP versus DOTA as Ligands for Lanthanide Cations: Novel Structurally Characterized CeIV and CeIII Cyclen-Based Complexes and Clusters in Aqueous Solutions

The coordination and redox chemistry of aqueous Ce^IV/III macrocyclic compounds were studied by using the ligands DOTA and DOTP (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid), respectively). The hydrolysis tendency of the tetravalent cation in the presence of DOTA is shown to result in the formation of a highly ordered, fluorite-like [Ce^IV₆(O)₄(OH)₄(H₂O)₈(DOTAH)₄] oxo-hydroxo structure both in solution and in the solid state. The lifetime of the analogous species formed in the presence of DOTP was found to be much shorter. Spectroscopic measurements of the latter suggest its similarity to the former. Its gradual decomposition in solution leads to the accumulation of the in-cage complexes [Ce^IVDOTP] and [Ce^IIIDOTP(H₂O)], which were crystallographically characterized in this study. The redox energetics and spectroscopic characteristics for the transition between these two in-cage complexes in aqueous solutions were studied as well. Together with the crystallographic structures of the above-mentioned species, the in-cage [Ce^IVDOTA(H₂O)] complex structure is presented herein for the first time. An elaborative analysis of the X-ray crystallographic structural data obtained for the in-cage complexes studied herein and similar structures published previously suggests that hard-bonding cyclen-derived ligands are, counter-intuitively, better suited for encapsulating, and perhaps kinetically stabilize softer cations than harder ones with DOTP, marked as a possible adequate chelator for the study of the aqueous properties of Ln^II and Ac^III cations.     

Structural Studies of Bis(cyclopentadienyl)molybdenum‐Amino Acid Complexes

The crystal structures of molybdocene‐amino acid compounds of the type [Cp₂Mo^IV(κN, κO‐AA)]⁺Cl^—·xH₂O with AA = D ‐phenylalaninato (x = 1.5), DL ‐leucinato (x = 2) and DL ‐valinato (x = 1) have been determined (Cp = η⁵‐C₅H₅). The compounds feature an almost planar, five‐membered chelate ring of the aminocarboxylate moiety (deprotonated amino acid) with the molybdenum atom. In the phenylalaninato complex π‐stacking between the phenyl rings is found. The complexes were proven kinetically stable at pH < 1 for at least 24 h.     

Kinetics and mechanism of oxidation of 2‐mercaptosuccinic acid by bis(μ‐oxo)‐ manganese(III,IV)‐cyclam complex in aqueous medium: Influence of externally added copper(II)

Kinetic studies on the oxidation of 2‐mercaptosuccinic acid by dinuclear [Mn₂^III/IV(μ‐O)₂(cyclam)₂](ClO₄)₃] ( 1 ) (abbreviated as Mn^III–Mn^IV) (cyclam = 1,4,8,11‐tetraaza‐cyclotetradecane) have been carried out in aqueous medium in the pH range of 4.0–6.0, in the presence of acetate buffer at 30°C by UV–vis spectrophotometry. In the pH region, two species of complex 1 (Mn^III–Mn^IV and Mn^III–Mn^IVH, the later being μ‐O protonated form) were found to be kinetically significant. The first‐order dependence of the rate of the reactions on [Thiol] both in presence and absence of externally added copper(II) ions, first‐order dependence on [Cu²⁺] and a decrease of rate of the reactions with increase in pH have been rationalized by suitable sequence of reactions. Protonation of μ‐O bridge of 1 is evidenced by the perchloric acid catalyzed decomposition of 1 to mononuclear Mn(III) and Mn(IV) complex observed by UV–vis and EPR spectroscopy. The kinetic features have been rationalized considering Cu(RSH) as the reactive intermediate. EPR spectroscopy lends support for this. The formation of a hydrogen bonded outer‐sphere adduct between the reductant and the complex in the lower pH range prior to electron transfer reactions is most likely to occur. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 170–177 2004     

Determination of particle size in aqueous suspensions of hydrogels of iron(III), indium(III), aluminum, chromium(III), titanium(IV), and zirconium(IV) oxohydroxides

Dispersion of hydrogels of Fe^III, In^III, Al^III, Cr^III, Ti^IV, and Zr^IV oxohydroxides in an aqueous medium was studied by sedimentation analysis. The hydrogel dispersion depends on the metal nature, pH of precipitation, and suspension concentration. The systems are predominantly polydisperse, and the gel particle size ranges from 2 to 140 μm. The data obtained suggest that the gel particles are formed by three-dimensional networks consisting of polymeric chains metal-oxygen and contain cavities filled with water. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1083–1088, May, 2005.     

Photolysis and Thermolysis of Platinum(IV) 2,2′‐Bipyridine Complexes Lead to Identical Platinum(II)–DNA Adducts

Two Pt^IV and two Pt^II complexes containing a 2,2′‐bipyridine ligand were treated with a short DNA oligonucleotide under light irradiation at 37 °C or in the dark at 37 and 50 °C. Photolysis and thermolysis of the Pt^IV complexes led to spontaneous reduction of the Pt^IV to the corresponding Pt^II complexes and to binding of Pt^II 2,2′‐bipyridine complexes to N7 of guanine. When the reduction product was [Pt(bpy)Cl₂], formation of bis‐oligonucleotide adducts was observed, whereas [Pt(bpy)(MeNH₂)Cl]⁺ gave monoadducts, with chloride ligands substituted in both cases. Neither in the dark nor under light irradiation was the reductive elimination process of these Pt^IV complexes accompanied by oxidative DNA damage. This work raises the question of the stability of photoactivatable Pt^IV complexes toward moderate heating conditions.     

Syntheses and supramolecular structures of two 5‐nitrosalicylate titanocene complexes

Two 4‐coordinated titanocene complexes, [(η⁵‐C₅H₅)₂Ti(O,O′)(5‐NO₂‐OCC₆H₃)] (I) and [(η⁵‐C₅H₅)₂Ti(2‐OH‐5‐NO₂‐O₂CC₆H₃)₂] (II), have been synthesized by reaction of Cp₂TiCl₂ and 5‐nitrosalicylic acid in aqueous media. Single‐crystal X‐ray analyses of I and II display the mononuclear forms of Ti^IV, and geometries at titanium atoms are distorted tetrahedrons, while the coordination environment at Ti^IV in complex I is different from that in complex II. Crystallographic characterization revealed that each of the complexes exhibits a three‐dimensional framework constructed through weak interactions, which are H‐bonding, π–π stacking and C–H·π interactions, but they differ greatly when forming the three‐dimensional network structure in both complexes. The results show that the dramatic change of conditions has great effect on the molecular structure of 5‐nitrosalicylate titanocene, thereby significantly influencing the weak interactions and the specific framework structure. Copyright © 2005 John Wiley & Sons, Ltd.     

Calculation of chemical equilibria in CaCl<Subscript>2</Subscript>-heparin-sodium adenosinetriphosphate and MgCl<Subscript>2</Subscript>-heparin-sodium adenosinetriphosphate systems in physiological salines

Chemical equilibria in aqueous solutions of disodium adenosinetriphosphate (Na₂H₂ATP), high-molecular-weight heparin (H₄Hep) and Na₂H₂ATP and in MCl₂-H₄Hep-Na₂H₂ATP-H₂O-NaCl (M = Ca²⁺ or Mg²⁺) solutions in the 0.15 M NaCl background were studied using computer simulation and pH titration at 2.3 ≤ pH ≤ 10.5. Formation constants were determined for two protonated ATP species, three protonated complex species of heparin with ATP of equimolar stoichiometry, and mixed-ligand calcium and magnesium complexes with heparin and adenosinetriphosphate. The formation constants for mixed-ligand calcium(II) complexes with heparin are more than two times those of homoligand calcium complexes with heparin. As a result, the Ca²⁺ ion concentration at 6.8 ≤ pH ≤ 7.4 (the pH range of blood plasma stability) decreases from 40 to 100 wt % depending on the ratio of the initial concentrations of CaCl₂, H₄Hep, and Na₂H₂ATP.     

Mechanistic disparity in the electron transfer reactions of thiosulfate with di‐μ‐oxo‐bis(1,4,8,11‐tetraazacyclotetradecane)‐dimanganese(III,IV) and di‐μ‐oxo‐bis(1,4,7,10‐tetraazacyclododecane)‐dimanganese(III,IV) complexes

A comparative kinetic study of the reactions of two mixed valence manganese(III,IV) complexes of macrocyclic ligands, [L¹Mn^IV(O)₂Mn^IIIL¹], 1 (L¹ = 1,4,8,11‐tetraazacyclotetradecane) and [L²Mn^IV(O)₂Mn^IIIL²], 2 (L² = 1,4,7,10‐tetraazacyclododecane) with thiosulfate has been carried out by spectrophotometry in aqueous buffer at 30°C. Reaction between complex 1 and thiosulfate follows a first‐order rate saturation kinetics. The pH dependency and kinetic evidences suggest the participation of two complex species of Mn^III(μ‐O)₂Mn^IV under the experimental conditions. Detailed kinetic study shows that reduction of 2 proceeds through an autocatalytic path where the intermediate (Mn^III)₂ species has been assumed to catalyze the reaction. The difference in the reaction mechanisms is ascribed to the difference in stability of the intermediate complex species, the evidence for which comes from the electrochemical behavior of the complexes and time dependent EPR spectroscopic measurements during the reduction of 2 . © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 119–128, 2004