Tartrate Ligand Research Articles

Chiral Self-Assembly of Twisted Prisms, Cuboids, and Polyhedral Capped Cages with Tartrate Ligands.

Homochiral triangular prisms, cuboid cages, and capped polyhedral cages are successfully synthesized via coordination-driven self-assembly. Typical tartrate ligands demonstrated notable torsional flexibility and variable coordination numbers, allowing for diverse coordination patterns, including saturated chelation and terminal mono-coordination with half-sandwich rhodium and iridium fragments. The ligand lengths, molar ratios, and metal vertices are meticulously designed and fine-tuned to yield chiral cages with entirely distinct architectures. Tartrate ligand exhibits abundant hydrogen bonding interactions and chiral induction capabilities, these supramolecular assemblies are characterized by single-crystal X-ray diffraction, nuclear magnetic resonance, and circular dichroism spectroscopy. An efficient method is developed for constructing chiral structurally versatile cage-like entities, facilitating self-assembly in complicated multi-component systems.

Electroless Copper Plating on a Cotton Surface: Effect of Metal Ion Ligand Stability Constant on Reduction Deposition.

Electroless plating facilitates the metallization of nonconductive substrate surfaces, and of note, the precise control of the bath stability constant influences the deposition process of metal particles. In this paper, trisodium citrate, potassium sodium tartrate, nitrogen triacetic acid, thiourea, and ethylenediamine tetraacetic acid disodium were selected as coordination agents, and the effect of the metal ion ligand stability constant on the reduction deposition was studied. Coordination bonds can be established between the Cu2+ and O/N/S particles in the ligand because paired electrons in O/N/S hybrid orbitals tend to occupy empty Cu2+ hybrid orbitals and establish coordination bonds. More importantly, the copper-potassium sodium tartrate ligand exhibits the lowest stability constant and lowest reduction barrier. As an exception, a consecutive Cu-plated coating with an excellent crystallinity property was deposited on the cotton surface when potassium sodium tartrate was used as the coordination agent in the plating solution. The deposition amounts are 55.2% and 74.1% after 1 and 4 h of electroless copper plating, respectively. The surface resistivity of Cu-plated cotton is 0.38 Ω/cm2, and additionally, the surface resistivity ratio before and after 1000 cycles fluctuated between 0.9 and 1.1, indicating that the Cu-plated cotton exhibits outstanding flexibility. In this paper, the deposition rate can be optimized by adjusting the copper particle ligand stability constant in the plating solution, aiming to achieve optimal results.

Structures and Electronic Properties of Titanium Oxo Clusters with Tartrate Ligand: A Density Functional Theory Study

Titanium oxo clusters are considered a model for nanotitanium dioxide and have gradually become a new field of functional materials in solar cells. In this work, the precise structural information of two types of tartrate-based titanium-oxo clusters was investigated by quantum chemical approaches. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations provided the precise structural parameters of Ti4(C4H2O6)(AS)2(OiPr)10 and Ti4(C4H2O6)(ATS)2(OiPr)10. Our work provided reliable results in agreement with the experimental results. In both molecules, the Ti-O bonds were mainly divided into three categories: the bonds in the first category were the coordination bonds between the central skeleton Ti atom and the tartaric acid group, whose bond lengths were 2.000 – 2.080 Å; the bonds in the second category were the coordination bond formed by the Ti atom and the oxygen on the AS ligand sulphonic acid group, whose bond lengths were 2.148 – 2.310 Å; the bonds in the third category were the Ti-O bonds with shorter bond lengths, which were the coordination bond formed by the Ti atom and the oxygen on the isopropylalkoxy group with their bond lengths in the value range from 1.777 Å to 1.795 Å. These kinds of Ti-O bonds also can be found in other Ti4 titanium-oxo clusters. Molecular orbitals showed that electron transfer from the AS/ATS ligand to the {Ti4} central skeleton was induced by sunlight with maximum UV-Vis absorption of 605/629 nm. Our calculation results provide strong guidance for the discovery of new efficient titanium oxo in solar cells.

Structure, optical transition analysis and magnetic study of lanthanide based metal-organic framework: holmium bi-tartrate trihydrate

This study presents the hydrothermal synthesis of holmium bi-tartrate trihydrate single crystals. The crystal structure and morphology were determined using single crystal x-ray analysis and field emission scanning electron microscopy, respectively. Fourier transform infrared spectroscopy was employed to identify the ligand phase. The thermal stability was assessed through differential scanning calorimetry. The effect of ligand on the incorporated paramagnetic center is discerned by reckoning the magnetic moment of the same. Judd-Ofelt theory was utilized to analyze optical transitions and calculate excited state properties. The complex exhibited violet and red emissions, attributed to ligand-based and metal-based luminescence , respectively. The absence of antenna effect was due to a significant energy gap between the resonance energy state, 5 S 2 of the Ho(III) ion and the triplet state of the coordinated tartrate ligand. The synthesized MOF's prolonged radiative lifetime holds promise for laser technologies and environmental monitoring applications.

(R,R)-Tartrate as a polytopic ligand for UO22+: Mono- and diperiodic coordination polymers including di- and tetranuclear subunits

(R,R)-Tartaric acid (H4tart) has been reacted with uranyl nitrate hexahydrate under solvo-hydrothermal conditions, in the presence of either [Ni(R,S-Me6cyclam)]2+ (R,S-Me6cyclam = 7(R),14(S)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), quinH+ (quinuclidinium), or [Ni(bipy)3]2+ (bipy = 2,2ʹ-bipyridine), to give three complexes characterized by their crystal structure. [H2NMe2]2[(UO2)2(Htart)2(HCOO)2Ni(R,S-Me6cyclam)]⋅2H2O (1) crystallizes as a monoperiodic coordination polymer in which dimeric [UO2(Htart)]22– subunits are linked by nickel(II) cations. The structure of the dimers matches that expected from solution experiments but not previously found in the solid state. Both [Hquin][(UO2)2(tart)(CH3COO)] (2) and [Ni(bipy)3][(UO2)2(tart)(CH3COO)]2 (3) crystallize as diperiodic networks based on tetranuclear [(UO2)2(tart)(CH3COO)]22– subunits which contain a central U2O2 ring involving μ2-bridging alkoxide groups. Further chelation of the two lateral uranyl cations by the carboxylate groups of adjacent subunits yields a sql-type network. Due to the different bulk of the counterions, the layers are quasi-planar in 2 and corrugated in 3. Comparison of these polynuclear subunits with those formed by the related ligands citrate, R,S-malate and R-citramalate shows that while the dimeric form is common to all, the tetranuclear subunit is specific to the tartrate ligand.

Anion Effect and Ligand Preference in the Precipitation of Ni(II) Complex from Methanolic Solution: Case of Tartrate vs Pyrazine

This research aims to incorporate pyrazine in the synthesis of Ni(II)-tartrate-pyrazine metal organic frameworks or Ni(II)-T-P MOF as candidate of porous material. Synthesis of the targeted MOF was conducted at room temperature in a methanolic solution by mixing Ni(II) solution, L-tartaric acid (T), and pyrazine (P) solutions sequentially in various molar ratios (Ni(II):T:P = 1:1:0; 1:0:2; 1:2:2; and 1:2:4) using two different Ni(II) salts (chloride and nitrate). Solid products were characterized by infrared spectroscopy, qualitative anion identification test, melting point test, and scanning electron microscopy. The result shows that type of nickel salt affects the precipitation of Ni(II) complex, in which pale blue solids were precipitated out only from the chloride reactions (1:0:2, 1:2:2, and 1:2:4). IR and SEM analyses from the 1:2:2 and 1:2:4 reactions show identical result as also shown by the 1:0:2 reaction, whereas qualitative anion identification test result suggests that the chloride is uncoordinated to the Ni(II). The targeted Ni(II)-T-P is unsuccessfully obtained, instead the product is proposed to be [Ni(pyrazine)x(H2O)(6-x)]Cl2. although the tartaric acid was doubled and firstly reacted with the Ni(II), the pyrazine still has higher preference to coordinate to the Ni(II) center than the tartrate ligand.

Crystallization of Mn(II) and Cd(II) Complexes in A Water-Methanol System: Tartrate vs Nicotinamide Ligand Selectivity

Ligand selectivity of tartrate vs nicotinamide in a water-methanol system has been observed in the crystallization of Mn(II) and Cd(II) complexes. These complexes were crystallized at room temperature by a layered solution technique using a water-methanol mixture solvent in a M(II):tartrate:nicotinamide (M = Mn, Cd) molar ratio of 1:1:2. Complexes of M(II)-nicotinamide and M(II)-tartrate were also prepared for data comparison. Analysis of the crystals by infrared spectroscopy, powder-X-ray diffraction and qualitative anion test showed that in a presence of both tartrate and nicotinamide, the Mn(II) forms neutral Mn(II)-tartrate hydrate complex, whereas the Cd(II) forms ionic Cd(II)-nicotinamide chloride complex. In the case of Mn(II) complex, tartrate tend to coordinate as ligand than the nicotinamide, although molar ratio of nicotinamide was doubled than that of tartrate ligand. In contrast, the neutral nicotinamide ligand is more predominant to coordinate in the Cd(II) complex than the anionic tartrate. The tartrate-nicotinamide ligand selectivity in the crystallization of Mn(II) and Cd(II) complexes is likely due to the use of tartrate salt as precursor and the choice of solvent mixture. In addition, powder-XRD analysis confirms that there was no indication of M(II)-tartrate and M(II)-nicotinamide that co-crystallized together at the same time by both metal ions.

Spectroscopic properties of lanthanide based metal-organic framework [Nd(C4H5O6)(C4H4O6)][3H2O]: Theoretical and experimental approaches

The spectroscopic properties of a lanthanide based MOF, [Nd(C4H5O6)(C4H4O6)][3H2O] have been discussed in detail. Semiempirical quantum methods are used to calculate the singlet and triplet excited states, and Uv and IR spectra of the organic part of the MOF. The experimental absorption and luminescent spectra of the MOF are also reported. The experimental absorption spectrum shows the characteristic transition of Nd(III) ion as well as the transitions of the ligands. The experimental line strengths of all the observed transition are calculated. The standard Judd-Ofelt theory is used to calculate intensity parameters, transition probabilities, branching ratios, and radiative lifetimes for [Nd(C4H5O6)(C4H4O6)][3H2O] MOF. The JO parameters are used to calculate various excited state properties. Upon excitation by ultraviolet and visible photons, the complex emits in blue and green regions respectively. The blue emission is ligand (tartrate) based assigned to S1→ S0 transition whereas green emission is metal based assigned to 4G9/2 + 4G7/2 → 4I11/2 transitions. No antenna effect is observed in NTT-MOF. It is because of the large gap between the triplet energy level of the tartrate ligand and the resonance energy level 4F3/2 of the Nd(III) ion.

Microwave synthesis of surface functionalized ErFeO3 nanoparticles for photoluminescence and excellent photocatalytic activity

Herein, we report the origin of inherent multicolor photoluminescence in functionalized and surface modified ErFeO3 nanoparticles. Moreover, we have found unprecedented photocatalytic property of functionalized ErFeO3 nanoparticles in the degradation of a model water-contaminant. Along with theoretical confirmation and careful investigation through UV–visible absorption and fluorescence study says that the ligand-to-metal charge-transfer from tartrate ligand to the lowest unoccupied energy levels of Fe3+ or Er3+ metal ions in the NPs and f-f transitions (Er3+) play the key role in the emergence of multiple photoluminescence from the ligand functionalized ErFeO3 NPs.

Exploration of a potential difluoromethyl-nucleoside substrate with the fluorinase enzyme

The investigation of a difluoromethyl-bearing nucleoside with the fluorinase enzyme is described. 5′,5′-Difluoro-5′-deoxyadenosine 7 (F2DA) was synthesised from adenosine, and found to bind to the fluorinase enzyme by isothermal titration calorimetry with similar affinity compared to 5′-fluoro-5′-deoxyadenosine 2 (FDA), the natural product of the enzymatic reaction. F2DA7 was found, however, not to undergo the enzyme catalysed reaction with l-selenomethionine, unlike FDA 2, which undergoes reaction with l-selenomethionine to generate Se-adenosylselenomethionine. A co-crystal structure of the fluorinase and F2DA7 and tartrate was solved to 1.8Å, and revealed that the difluoromethyl group bridges interactions known to be essential for activation of the single fluorine in FDA 2. An unusual hydrogen bonding interaction between the hydrogen of the difluoromethyl group and one of the hydroxyl oxygens of the tartrate ligand was also observed. The bridging interactions, coupled with the inherently stronger C–F bond in the difluoromethyl group, offers an explanation for why no reaction is observed.

Ultrafast dynamics at the zinc phthalocyanine/zinc oxide nanohybrid interface for efficient solar light harvesting in the near red region

Phthalocyanine-based light harvesting nanomaterials are attractive due to their low cost, eco-friendly properties and sensitivity in the red region of the solar spectrum. However, for any practical application, phthalocyanines need to be chemically modified for anchoring groups with wide-band semiconducting nanomaterials. In this paper, zinc phthalocyanine (ZnPc) was functionalized with two carboxyl groups containing a biologically important ligand, tartrate, using a facile wet chemistry route and duly sensitized zinc oxide (ZnO) to form nanohybrids for application in photocatalytic devices. The nanohybrids have been characterized using a high-resolution transmission/scanning electron microscope (HRTEM, FEG-SEM), X-ray diffraction (XRD), steady-state infrared/UV–vis absorption and emission spectroscopy for their structural details and optical properties, whereas the ultrafast dynamical events, which are key for understanding the photocatalytic activities, were monitored using picosecond resolution fluorescence techniques. More specifically, vibrational spectroscopy (FTIR) revealed the covalent connection of ZnPc with the host ZnO nanoparticles via the tartrate ligand. The efficiency of the material for photocatalysis under red light irradiation was found to be significantly enhanced compared to bare ZnO. A mechanistic pathway for the formation of photo-induced reactive oxygen species (ROS) in an aqueous medium for the photocatalytic efficacy was investigated. To make a prototype for a potential application in a flow device for water decontamination, we have sensitized ZnO nanorods (ZnO NR) with tartrate-functionalized ZnPc. The molecular proximity of ZnPc to the ZnO surface has been confirmed by picosecond resolution Förster Resonance Energy Transfer (FRET) from the intrinsic emission of surface defects of ZnO NR to the attached ZnPc. The excited-state electron transfer dynamics, as revealed by a picosecond resolution fluorescence study (TCSPC), is in good agreement with the enhanced charge separation at the interface of the nanohybrid. The enhanced photoresponse, wavelength-dependent photocurrent of the sensitized nanorods and photodegradation of a model water pollutant in a prototype device format confirmed the potential use of the nanohybrids in water purification.

Surface Modification of <inline-formula> <tex-math notation="TeX">\({\boldsymbol {\alpha }}-\) </tex-math></inline-formula>Fe<sub>2</sub>O<sub>3</sub> Nanoparticles to Develop as Intrinsic Photoluminescent Probe and Unprecedented Photocatalyst

Herein, we demonstrate the emergence of intrinsic multicolor fluorescence in α-Fe 2 O 3 nanoparticles (NPs) from blue, cyan, to green, upon functionalization and further surface modification with a small organic ligand, Na-tartrate. Furthermore, we have found an excellent photocatalytic property of the functionalized α-Fe 2 O 3 NPs in the decoloration of a model water contaminant. Detailed characterization of α-Fe 2 O 3 NPs before and after functionalization were carried out by X-ray diffraction and transmission electron microscopy. Proper investigation through UV-visible absorption and photoluminescence study along with theoretical support from literature reveals that ligand-to-metal charge transfer from tartrate ligand to lowest unoccupied energy level of Fe 3+ of the NPs and d-d transitions centered over Fe 3+ ions in the NPs play the key role in the generation of multiple fluorescence from the ligand functionalized α-Fe 2 O 3 NPs. Vibrating sample magnetometry measurements exhibit magnetic behavior of Fe 2 O 3 NPs changed considerably after surface modification. We believe that the developed ferromagnetic, multicolor fluorescent α-Fe 2 O 3 NPs would pioneer new opportunities toward diverse applications.

Facile functionalization of Fe2O3 nanoparticles to induce inherent photoluminescence and excellent photocatalytic activity

Herein, we report the emergence of intrinsic multicolor photoluminescence in Fe2O3 nanoparticles (NPs) ranging from blue, cyan, to green, upon facile functionalization and further surface modification with a small organic ligand, Na-tartrate. Moreover, we have found unprecedented photocatalytic property of the functionalized Fe2O3 NPs in the degradation of a model water-contaminant. Meticulous investigation through UV-visible absorption and fluorescence study along with theoretical support from literature unfolds that ligand-to-metal charge-transfer transition from the tartrate ligand to the lowest unoccupied energy level of Fe3+ of the NPs and d−d transitions centered over Fe3+ ions in the NPs play the key role in the emergence of multiple photoluminescence from the ligand functionalized Fe2O3 NPs. Moreover, vibrating sample magnetometry measurements demonstrate that the surface modification changes the magnetic behaviour of Fe2O3 NPs upon functionalization. We believe that the great potential of our versatile, ferromagnetic, multicolor photoluminescent Fe2O3 NPs would stimulate the development of numerous opportunities toward their biological and technological applications.

Surface Modification of MnFe2O4 Nanoparticles to Impart Intrinsic Multiple Fluorescence and Novel Photocatalytic Properties

The MnFe2O4 nanoparticle has been among the most frequently chosen systems due to its diverse applications in the fields ranging from medical diagnostics to magnetic hyperthermia and site-specific drug delivery. Although numerous efforts have been directed in the synthesis of monodisperse MnFe2O4 nanocrystals, unfortunately, however, studies regarding the tuning of surface property of the synthesized nanocrystals through functionalization are sparse in the existing literature. Herein, we demonstrate the emergence of intrinsic multicolor fluorescence in MnFe2O4 nanoparticles from blue, cyan, and green to red, upon functionalization and further surface modification with a small organic ligand, Na-tartrate. Moreover, we have found an unprecedented photocatalytic property of the functionalized MnFe2O4 nanoparticles in the degradation of a model water contaminant. Detailed characterization through XRD, TEM, and FTIR confirms the very small size and functionalization of MnFe2O4 nanoparticles with a biocompatible ligand. Proper investigation through UV-visible absorption, steady-state and time-resolved photoluminescence study reveals that ligand-to-metal charge-transfer transition from the tartrate ligand to the lowest unoccupied energy level of Mn(2+/3+)or Fe(3+) of the NPs and Jahn-Teller distorted d-d transitions centered over Mn(3+) ions in the NPs play the key role behind the generation of multiple fluorescence from the ligand-functionalized MnFe2O4 nanoparticles. VSM measurements indicates that the superparamagnetic nature of MnFe2O4 nanoparticles remains unchanged even after surface modification. We believe that the developed superparamagnetic, multicolor fluorescent MnFe2O4 nanopaticles would open up new opportunities as well as enhance their beneficial activities toward diverse applications.

Emergence of Multicolor Photoluminescence in La0.67Sr0.33MnO3 Nanoparticles

Herein, we report the emergence of multicolor photoluminescence in a mixed-valence manganite nanoparticle La0.67Sr0.33MnO3 (LSMO NP) achieved through electronic structural modification of the nanoparticles upon functionalization with a biocompatible organic ligand, sodium tartrate. From UV–vis absorption, X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence study, and Raman spectroscopic measurements, it is revealed that ligand-to-metal charge transfer transitions from highest occupied molecular orbital (HOMO, centered in tartrate ligand) to lowest unoccupied molecular orbital (LUMO, centered in Mn3+/4+ of the NPs), and d–d transitions involving Jahn–Teller sensitive Mn3+ ions in the NPs plays the central role behind the origin of multiple photoluminescence from the ligand functionalized LSMO NPs.

Synthesis and crystal structure of three new cadmium tartrates with open frameworks

Four metal–organic coordination polymers: {[Cd3(C4H4O6)3(H2O)]·H2O}nI, {[Cd3(C4H3O6)2(H2O)2]·5.5H2O}nII, {[Cd2(C4H4O6)2(H2O)]·3H2O}nIII, and [Cd(C4H4O6)]nIV were obtained under hydrothermal conditions and characterized by IR spectroscopy and single crystal X-ray diffraction. The structural analysis reveals that compounds I–III exhibit new 3D open frameworks filled with water molecules, which according to thermogravimetric analysis evolve at temperatures significantly lower than onset temperatures for decomposition. Compound I exhibits an 8-c net; uninodal net (eci net) with Schlafli symbol {36·412·510}, where Cd atoms are connected by μ4,κ6-mode and μ4,κ5-mode tartrate ligands. Compound II is a rare example of a tartrate trianion complex formed by μ5,κ6-mode ligands creating a (6,3)-connected net with Schlafli symbol (4·62)2(42·610·83). Compound III exhibits a 3,3,4,4-connected, 4-nodal net with Schlafli symbol {4·82}{4·84·10}, built up by dimers of two octahedral Cd atoms linked by μ4,κ6-mode and μ3,κ5-mode tartrate ligands. Compound IV was previously reported although being obtained under different synthetic conditions and can be described as a 5,5,11-connected, 3-nodal net with the Schlafli symbol of {32·46·52}2{34·414·516·618·72·8}{34·43·52·6}. None of the compounds reported here are topologically related, evidencing the versatility of the tartrate ligand for the framework formation of coordination polymers.

Hydrothermal synthesis and structural characterization of a new 3D chiral coordination polymer [Cd2(C4H4O6)2] n

A new chiral coordination polymer [Cd2(C4H4O6)2]n (I) has been synthesized and characterized by elemental analysis, IR, and X-ray single-crystal diffraction. The X-ray diffraction analysis reveals that I crystallizes in the orthorhombic system, space group P212121. The adjacent Cd(1) and Cd(2) centers are linked by one tartrate ligand through tridentate coordination to form a dimer. The dimer is further connected to the other dimer via tartrate ligands to construct an infinite three-dimensional (3D) coordination polymer. The unit cell parameters for I: a = 7.4984(17), b = 7.9106(18), c = 19.560(4) A, V = 1160.2(5) A3, Z = 4.

Modification of chiral dimethyl tartrate through transesterification: Immobilization on POSS and enantioselectivity reversal in sharpless asymmetric epoxidation

Modification of dimethyl tartrate has been investigated through transesterification with aminoalcohols to provide reactive functionalities for the covalent bonding of chiral tartrate to polyhedral oligomeric silsesquioxanes. The transesterification of dimethyl tartrate has been widely studied using different catalytic systems and reaction conditions. Through the proper selection of both the catalytic system and the reaction conditions, it is possible to achieve monosubstituted or bis-substituted tartrate derivatives as sole products. All the intermediate chiral tartrate-derived ligands were successfully used in the homogeneous enantioselective epoxidation of allylic alcohols providing moderate enantiomeric excess over the products. Attached amine groups have been used to support the modified tartrate ligands on to a haloaryl-functionalized silsesquioxane moiety. This final chiral tartrate ligand displays reverse enantioselectivity in the asymmetric epoxidation of allylic alcohols with regard to the starting dimethyl tartrate ligand, both molecules having the same chiral sign. However, the POSS-containing ligand can be easily recovered in almost quantitative yield and reused in asymmetric epoxidation reactions. In addition, recovered silsesquioxane-pendant ligand, though displaying decreasing catalytic activity in recycling epoxidation tests, showed very stable enantioselective behavior.

Mechanism of the Asymmetric Sulfoxidation in the Esomeprazole Process: Effects of the Imidazole Backbone for the Enantioselection

Abstractmagnified imageThe asymmetric sulfoxidation reaction of imidazole‐based prochiral sulfides was studied to explore the mechanistic details of the highly efficient esomeprazole process, which is one of the few industrial scale catalytic asymmetric procedures. The synthetic studies revealed that the smallest subunit governing the selectivity in the esomeprazole process is an imidazole ring. Thus, by using the esomeprazole procedure methyl imidazole sulfide could be oxidized as efficiently as its several functionalized derivatives, including pyrmetazol. However, alkylation of the imidazole nitrogen led to a major drop of the enantioselectivity. Our atmospheric pressure chemical ionization‐mass spectrometry (APCI/MS) studies indicate that addition of small amounts of water to the reaction mixture facilitates the formation of mononuclear titanium species, which are the active catalytic intermediates of the selective oxidation reaction. One of the most important features of the esomeprazole procedure is that amine additives increase the enantioselectivity of the oxidation process. The NMR studies of the presumed reaction intermediates show that under catalytic conditions the amines are able to coordinate to titanium and dissociate the coordinated imidazole substrate. The density functional theory (DFT) modelling studies provided new insights in the mechanism of the asymmetric induction. It was found that the oxidation requires a lower activation energy if the imidazole sulfide precursor does not coordinate to titanium. Two possible reaction paths were explored for this out of sphere oxidation mechanism. The most important interaction governing the enantioselection is hydrogen bonding between the NH of the imidazole ring and the chiral tartrate ligand on titanium. Furthermore, the oxidation reaction imposes an important structural constraint to the TS structure involving a linear arrangement of the peroxide oxygens and the sulfur atom. This constraint and the N coordination of imidazole leads to a very strained structure for the inner sphere mechanism of the oxidation, which leads to a much higher activation barrier than the corresponding out of sphere process, and therefore it is unlikely.

Reaction of uranyl nitrate with carboxylic diacids under hydrothermal conditions. Crystal structure of complexes with l(+)-tartaric and oxalic acids

l(+)-tartaric acid reacts with uranyl nitrate in the presence of KOH, under mild hydrothermal conditions, to give the complex [UO 2(C 4H 4O 6)(H 2O)] ( 1), the first uranyl tartrate to be crystallographically characterized. Each tartrate ligand bridges three uranyl ions, one of them in chelating fashion through proximal carboxylate and hydroxyl groups. The resulting assemblage is two-dimensional, with the uranyl pentagonal bipyramidal coordination polyhedra separated from one another. Prolonged heating of an uranyl tartrate solution resulted in oxidative cleavage of the acid and formation of the oxalate complex [(UO 2) 2(C 2O 4) 2(OH)Na(H 2O) 2] ( 2). The bis-bidentate oxalate and bridging hydroxide groups ensure the formation of sheets with corner-sharing uranyl pentagonal bipyramidal coordination polyhedra, in which six-membered metallacycles encompass the sodium ions. These sheets are assembled into a three-dimensional framework through further oxo-bonding of the sodium ions.