Alkoxide Ligands Research Articles

Phosphate diester hydrolysis promoted by new Cu(II) alkoxide complexes

Three Cu(II) complexes of N3 donor pyridyl-amine ligands, bis-(2-pyridin-2-yl-ethyl)-amine (L1), which does not have an alcohol pendant, and 2-[bis-(2-pyridin-2-yl-ethyl)-amino]-ethanol (L2) and 3-[bis-(2-pyridin-2-yl-ethyl)-amino]-propan-1-ol (L3), both containing alcohol pendants, have been synthesized and structurally characterized. The reactivities of CuL1 (1), CuL2 (2), and CuL3 (3) for promoting the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) have been investigated, showing that 3 is much more active (observed second order rate constant k=6.1×10−1M−1s−1 at pH 8.4, 50°C) than 1 (k=9.4×10−3M−1s−1 at pH 8.4, 50°C) or 3 (observed second order rate constant k=5.2×10−3M−1s−1 at pH 8.4, 50°C) at promoting the hydrolysis of BNPP. The much higher reactivity of 3 compared to 2 and 1 may be explained by the higher flexibility of the alkoxide tether in 3, which may allow the nucleophilic alkoxide O-atom in 3 better access to attack a metal-bound BNPP substrate, whereas 2, which has a shorter alkoxide tether, may resort to hydrolyzing the BNPP substrate with a metal-bound hydroxide instead of utilizing its alkoxide moiety because of constraints in the ligand tether. The similar reactivities of 1 and 2 suggest that this is a possible scenario. Another possible explanation for the difference in reactivity between 3 and 2 may be attributed to the alcohol pendant in 3 being better oriented to serve as a proton acceptor from a Cu(II)-bound aqua ligand, which would facilitate its deprotonation and the subsequent nucleophilic attack of the Cu(II)-hydroxide moiety towards the substrate. The reactivities of 2 and 3 towards hydrolyzing BNPP are also compared with that of their Zn(II) analogues, showing that the Cu(II) complexes of the same ligand sets are more hydrolytically active than the corresponding Zn(II) complexes. Single crystal X-ray structures of 1, 2, and 3 are also reported, which show that complexes 2 and 3 form dimers with bridging alkoxide ligands in the solid state, while 1 forms a dimer with bridging chloride ligands in the solid state.

Structural Characterization of Manganese(II) Compounds with Methylalumoxane (MAO) and AlMe2 Units

Manganese complexes with AlMe3 have been synthesized with the aim of investigating Mn(II) active centers in catalytic ethylene polymerization. Reactions of [Mn4(μ-Cl)Cl3(μ-OCH2CH2OCH3)4(HOCH2CH2OCH3)3]2 (1) with an excess of AlMe3 at room temperature in toluene give the heterometallic cage [Mn3Al(μ3-OCH2CH2OCH3)(μ-Cl)3(μ-OCH2CH2OCH3)2(thf)2(Me)2Cl] (2; 30%). A similar reaction carried out under reflux led to the formation of the methylalumoxane species [MnAl3(μ3-O)(μ-OCH2CH2OCH3)3(Me)6] (3; 51%). X-ray diffraction studies have shown complexes 2 and 3 to contain AlMe2+ and [Al3(μ3-O)(Me)6]+ moieties capped by [Mn3(μ3-OCH2CH2OCH3)(μ-Cl)3(μ-OCH2CH2OCH3)2(thf)2Cl]- and [Mn(μ-OCH2CH2OCH3)3]- macrounits, respectively. The tendency of a manganese center to exchange aluminum methyl groups in 2 and 3 is strongly reduced by alkoxide ligands.

Terdentate RuX(CNN)(PP) (X = Cl, H, OR) Complexes: Synthesis, Properties, and Catalytic Activity in Fast Transfer Hydrogenation

Terdentate ruthenium(II) complexes of general formula RuX(CNN)(dppb) (X = chloride, hydride, alkoxide; dppb = Ph2P(CH2)4PPh2), where CNN is a deprotonated 2-aminomethyl-6-arylpyridine ligand, have been prepared. The orthometalated derivative RuCl(b)(dppb) (1) has been obtained by reaction of RuCl2(PPh3)(dppb) with N,N-dimethyl-2-aminomethyl-6-(4-methylphenyl)pyridine (Hb) in 2-propanol and in the presence of triethylamine by elimination of PPh3 and HCl. Similarly, RuCl(a)(dppb) (2) and the chiral analogue RuCl(c)(dppb) (3), containing primary amine ligands, have been isolated starting from 2-aminomethyl-6-(4-methylphenyl)pyridine (Ha) and (R)-2,2-dimethyl-1-(6-phenylpyridin-2-yl)propylamine (Hc), respectively. The synthesis of the functionalized pyridines Ha−Hc is here described, whereas the crystal structure of 3 has been determined through an X-ray diffraction experiment. Treatment of 1−3 with sodium or potassium isopropoxide gives the corresponding hydrides RuH(b)(dppb) (4), RuH(a)(dppb) (5), and RuH(c)(dppb) (6) from the ruthenium isopropoxide complexes, via a β-H elimination process. Studies in solution show that the isopropoxides bearing a NH donor group are in equilibrium with the corresponding hydrides (5 and 6). Reaction of 5 with benzophenone leads to the alkoxide Ru(OCHPh2)(a)(dppb) (7), which has been proven to interact with benzhydrol in C6D6, leading to the adduct 7·(HOCHPh2), the alkoxide ligand, and the alcohol being in rapid exchange. Complexes 2 and 3 display a remarkable high catalytic activity for the transfer hydrogenation of ketones to alcohol in 2-propanol using a very small amount of catalyst. With the chiral complex 3 (0.005 mol %) methyl-aryl ketones can be quickly reduced (TOF ranging from 5.4 × 105 to 1.4 × 106 h-1) with an enatiomeric excess up to 88%.

New insight in the role of modifying ligands in the sol-gel processing of metal alkoxide precursors: A possibility to approach new classes of materials

This paper summarizes recent literature data and presents new experimental data on the mechanisms of chemical modification, hydrolysis and polycondensation of the alkoxides and demonstrates possibilities to approach new classes of materials, exploiting these mechanisms. Low reactivity of silicon alkoxides is improved by either basic catalysis exploiting an SN2 mechanism or acidic catalysis facilitating a proton-assisted SN1 mechanism as well as by modification with chelating ligands. Metal alkoxides are much stronger Lewis bases compared to silicon alkoxides and the acidity of water is strong enough to achieve their rapid hydrolysis via proton-assisted SN1 pathway even in the absence of additional catalysts. Introduction of the modifying chelating ligands is leading generally to increased charge distribution in the precursor molecules. Modifying chelating ligands are also appreciably smaller than the alkoxide ligands they replace. The modification with chelating ligands is thus facilitating the kinetics of hydrolysis and polycondensation. The size and shape of the primary particles formed in sol-gel treatment of metal alkoxides are defined not by kinetic factors in their hydrolysis and polycondensation but by the interactions on the phase boundary, which is in its turn directed by the ligand properties. The products of the fast hydrolysis and condensation sequence consist of micelles templated by self-assembly of ligands (mainly oxo-species). This concept provides explanations for commonly observed material properties and allows for the development of new strategies for the preparation of materials. We discuss the formation of inverted micelles, obtained by the appropriate choice of solvents, which allows for the formation of hollow spheres. The modifying β-diketonate ligands act as the surfactant and form an interface between the hollow sphere and the solvent. Retention of ligands inside the gel particles is possible only if ligands possessing both chelating and bridging properties are applied. Application of such ligands, for example, diethanolamine, permits to prepare new transition metal oxide based microporous membranes.

Combining thermolysis of molecular precursor with sol-gel process to zinc gallate nanoparticles

Zinc gallate nanoparticles were prepared by combining thermolysis of single molecular precursor with sol-gel method in the presence of a capping agent. The size and shape of zinc gallate nanoparticles could be controlled by changing the reaction temperature. Single molecular precursor with alkyl and alkoxide ligand was used for zinc gallate nanoparticles to ensure the correct stoichiometry of the product.

A New Structural Motif for Biological Iron: Iron K-Edge XAS Reveals a [Fe4-μ-(OR)5(OR)9-10] Cluster in the Ascidian Perophora annectens

The Phlebobranch ascidian Perophora annectens surprisingly exhibited a biological Fe/V ratio of approximately 15:1 on multichannel X-ray fluorescence analysis of two independent collections of organisms. Iron K-edge X-ray absorption spectroscopy (XAS) indicated a single form of iron. The XAS K-edge of the first collection of blood cells was shifted approximately +1 eV relative to that of the second, indicating redox activity with average iron oxidation states of 2.67+ and 2.60+. The first-derivative iron XAS K-edge features at 7120.5, 7124, and 7128 eV resembled the XAS of magnetite but not of ferritin or of dissolved Fe(II) or Fe(III). Pseudo-Voigt fits to blood-cell iron K-edge XAS spectra yielded 12.4 integrated units of preedge intensity, indicating a noncentrosymmetric environment. The non-phase-corrected extended X-ray absorption fine structure (EXAFS) Fourier transform spectrum showed a first-shell O/N peak at 1.55 angstroms and an intense Fe-Fe feature at 2.65 angstroms. Fits to the EXAFS required a split first shell with two O at 1.93 angstroms and three O at 2.07 angstroms, consistent with terminal and bridging alkoxide ligands, respectively. More distant shells included three C at 2.87 angstroms, two Fe at 3.08 angstroms, three O at 3.29 angstroms, and one Fe at 3.8 angstroms. Structural models consistent with these findings include a [Fe4(OR)13](2-/3-) broken-edged Fe4O5 cuboid or a [Fe4(OR)14](3-/4-) "Jacob's ladder" with three edge-fused Fe2(OR)2 rhombs. Either of these models represents an entirely new structural motif for biological iron. Vanadium domination of blood-cell metals cannot be a defining trait of Phlebobranch tunicates so long as P. annectens is included among them.

Synthesis and characterization of a large heterometallic alkoxide cation: Self-assembly and rational route to [formula omitted

Ionic or ionogenic alkoxides are attractive potential catalysts for polymerization reactions. Direct interaction between CoCl 2 and [Ta(OMe) 5] produced, by self-assembly, a small amount of ionic aggregate clathrate with acetonitrile, [ Co 2 Ta 2 ( OMe ) 12 ( MeCN ) 4 ] 2 + [ CoCl 3 ( MeCN ) ] 2 - · 2 MeCN ( 1 ) . The structure of 1 contains a planar M 4( μ 3-OR) 2(μ-OR) 4-core that can be considered as a fragment of hexagonal packing of metal and ligand atoms completed by 6 (3 + 3) terminal alkoxide ligands at the tantalum and 4 (2 + 2) terminal acetonitrile molecules at the cobalt atoms that are bearing the (+1) positive charge each. A rational synthesis of this compound was achieved by reaction between [NaTa(OMe) 6] with four equivalents of CoCl 2 in toluene/acetonitrile to give a new crystallographic form [Co 2Ta 2(OMe) 12(MeCN) 4] [CoCl 3(MeCN)] 2 ( 2), not containing interstitial solvent molecules.

Sequential Reaction Intermediates in Aliphatic C−H Bond Functionalization Initiated by a Bis(μ-oxo)dinickel(III) Complex

The reaction of [Ni2(OH)2(Me2-tpa)2]2+ (1) (Me2-tpa = bis(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) with H2O2 causes oxidation of a methylene group on the Me2-tpa ligand to give an N-dealkylated ligand and oxidation of a methyl group to afford a ligand-based carboxylate and an alkoxide as the final oxidation products. A series of sequential reaction intermediates produced in the oxidation pathways, a bis(mu-oxo)dinickel(III) ([Ni2(O)2(Me2-tpa)2]2+ (2)), a bis(mu-superoxo)dinickel(II) ([Ni2(O2)2(Me2-tpa)2]2+ (3)), a (mu-hydroxo)(mu-alkylperoxo)dinickel(II) ([Ni2(OH)(Me2-tpa)(Me-tpa-CH2OO)]2+ (4)), and a bis(mu-alkylperoxo)dinickel(II) ([Ni2(Me-tpa-CH2OO)2]2+ (5)), was isolated and characterized by various physicochemical measurements including X-ray crystallography, and their oxidation pathways were investigated. Reaction of 1 with H2O2 in methanol at -40 degrees C generates 2, which is extremely reactive with H2O2, producing 3. Complex 2 was isolated only from disproportionation of the superoxo ligands in 3 in the absence of H2O2 at -40 degrees C. Thermal decomposition of 2 under N2 generated an N-dealkylated ligand Me-dpa ((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) and a ligand-coupling dimer (Me-tpa-CH2)2. The formation of (Me-tpa-CH2)2 suggests that a ligand-based radical Me-tpa-CH2* is generated as a reaction intermediate, probably produced by H-atom abstraction by the oxo group. An isotope-labeling experiment revealed that intramolecular coupling occurs for the formation of the coupling dimer. The results indicate that the rebound of oxygen to Me-tpa-CH2* is slower than that observed for various high-valence bis(mu-oxo)dimetal complexes. In contrast, the decomposition of 2 and 3 in the presence of O2 gave carboxylate and alkoxide ligands, respectively (Me-tpa-COO- and Me-tpa-CH2O-), instead of (Me-tpa-CH2)2, indicating that the reaction of Me-tpa-CH2* with O2 is faster than the coupling of Me-tpa-CH2* to generate ligand-based peroxyl radical Me-tpa-CH2OO*. Although there is a possibility that the Me-tpa-CH2OO* species could undergo various reactions, one of the possible reactive intermediates, 4, was isolated from the decomposition of 3 under O2 at -20 degrees C. The alkylperoxo ligands in 4 and 5 can be converted to a ligand-based aldehyde by either homolysis or heterolysis of the O-O bond, and disproportionation of the aldehyde gives a carboxylate and an alkoxide via the Cannizzaro reaction.

Phosphate diester hydrolysis promoted by Zinc(II) complexes with alkoxide pendants

Two new Zn(II) complexes of tripodal pyridyl-amine ligands with alcohol pendants, 2-[bis-(2-pyridin-2-yl-ethyl)-amino]-ethanol (L 1) and 3-[bis-(2-pyridin-2-yl-ethyl)-amino]-propan-1-ol (L 2), have been synthesized and structurally characterized. The reactivities of ZnL 1 ( 1) and ZnL 2 ( 2) for promoting the hydrolysis of bis( p-nitrophenyl)phosphate (BNPP) have been investigated, and both complexes are highly active in catalyzing the hydrolysis of BNPP, with 2 ( k = 1.8 × 10 −1 M −1 s −1) being about two orders of magnitude more active than 1 ( k = 2.7 × 10 −3 M −1 s −1) in promoting the hydrolysis of BNPP. The differences in reactivity can be attributed to the disparate abilities of the alkoxide nucleophiles in the pendant groups in ligands L 1 and L 2 to attack the substrate phosphate diester BNPP due to differences in their tether lengths. X-ray crystal structures of 1 and 2 are also reported, which show that the complexes form dimers with bridging alkoxide ligands in the solid state.

MOCVD and ALD of High‐k Dielectric Oxides Using Alkoxide Precursors

AbstractA number of high‐permittivity (κ) dielectric oxides are currently being investigated as alternatives to SiO2 as the dielectric insulating layer in sub‐0.1 μm CMOS technology. Metal‐organic (MO)CVD and atomic layer deposition (ALD) are promising techniques for the deposition of these high‐κ dielectric oxides. In this paper it is shown how the use of “designed” metal alkoxide precursors, containing bidentate donor‐functionalized alkoxide ligands, in MOCVD leads to a marked improvement in the physical properties of the MOCVD precursors and process parameters. However, the use of such ligands is not as beneficial in the ALD process, highlighting the very different requirements of MOCVD and ALD precursors.

Electronic ménages a trois: A molecular orbital perspective of protonated ferryl intermediates and synthetic models

Presented here is a molecular orbital perspective of various S = 1 iron(IV)-hydroxo compound II intermediates as well as of synthetic heme and nonheme analogues. A key conceptual issue concerns how the iron(IV) center in these species coexists with highly reducing alkoxide, thiolate, phenolate, and hydroperoxide ligands. We suggest that a clue to this conundrum involves a three-way splitting of the spin density among the iron and two π-basic ligands, which effectively delocalizes the high positive charge away from the iron.

Titanium‐mediated living radical styrene polymerizations. V. Cp2TiCl‐catalyzed initiation by epoxide radical ring opening: Effect of solvents and additives

AbstractThe effects of solvents, additives, ligands, and solvent in situ drying agents as well as catalyst and initiator concentrations have been investigated in the Cp2TiCl‐catalyzed radical polymerization of styrene initiated by epoxide radical ring opening. On the basis of the solubilization of Cp2Ti(III)Cl and the polydispersity of the resulting polymer, the solvents rank as follows: dioxane ≥ tetrahydrofuran > diethylene glycol dimethyl ether > methoxybenzene > diphenyl ether ≥ bulk > toluene ≫ pyridine > dimethylformamide > 1‐methyl‐2‐pyrrolidinone > dimethylacetamide > ethylene carbonate, acetonitrile, and trioxane. Alkoxide additives such as aluminum triisopropoxide and titanium(IV) isopropoxide are involved in alkoxide ligand exchange with the epoxide‐derived titanium alkoxide and lead to broad molecular weight distributions, whereas similarly to strongly coordinating solvents, ligands such as bipyridyl block the titanium active site and prevent the polymerization. By contrast, softer ligands such as triphenylphosphine improve the polymerization in less polar solvents such as toluene. Although mixed hydrides such as lithium tri‐tert‐butoxyaluminum hydride, sodium borohydride, and lithium aluminum hydride react with bis(cyclopentadienyl)titanium dichloride to form mixed titanium hydride species ineffective in polymerization control, simple hydrides such as lithium hydride, sodium hydride, and especially calcium hydride are particularly effective as in situ trace water scavengers in this polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2015–2026, 2006

Fiber-optic infrared reflectance spectroelectrochemical studies of osmium and ruthenium nitrosyl porphyrins containing alkoxide and thiolate ligands

We have examined the redox behavior of the osmium and ruthenium compounds (OEP)M(NO)(OEt) and (OEP)M(NO)(SEt) (OEP = octaethylporphyrinato dianion; M = Os, Ru) by cyclic voltammetry and infrared spectroelectrochemistry. The compound (OEP)Os(NO)(OEt) undergoes a single reversible oxidation process in dichloromethane. In contrast, the thiolate compound (OEP)Os(NO)(SEt) undergoes a net irreversible oxidation resulting in formal loss of the SEt ligand. Extended Hückel calculations on crystal structures of these two compounds provide insight into the nature of their HOMOs. In the case of the alkoxide compound, the HOMO is largely metal centered, with 70% of the charge located in the metal's orbital and approximately 25% on the porphyrin ring. However, the HOMO of the thiolate compound consists of a pi bonding interaction between the metal dxz orbital and the px orbital on the sulfur, and a pi antibonding interaction between the metal d orbital and a pi* orbital on NO. The redox behavior of the Ru analogues have been determined, and are compared with those of the Os compounds.

Group 4 Metal Complexes of Nitrogen-Bridged Dialkoxide Ligands: Synthesis, Structure, and Polymerization Activity Studies

Neutral soluble titanium(IV) and zirconium(IV) complexes of new amino-dialkoxide ligands {OCR2CH2N(CH2Ph)CH2CR2O}2- ({ONOR}2-) have been prepared. Alcohol and alkane elimination reactions from {ONOR}H2 (R = Me, 1; p-tol, 2) and salt metathesis routes from Li2{ONOMe} in situ-generated afford {ONOMe}Ti(OiPr)2 (3), {ONOR}2M (R = Me, M = Ti, 5; Zr, 6; R = p-tol, M = Zr, 7), {ONOMe}Zr(CH2Ph)2 (8), {ONOMe}ZrCl2(THF)n (n = 0, 9; n = 1, 10), {ONOMe}Zr(NMe2)2 (11), and {ONOR}TiCl2 (R = Me, 13; R = p-tol, 14) in good yields. X-ray crystallographic studies showed that 3, 8, 13, and 14 adopt in the solid state mononuclear structures, with coordination of the nitrogen atom to the metal center. Dinuclear structures with bridging alkoxide and amido ligands were observed for 6 and a mixed amido-{ONOMe}Zr complex (12) isolated in the preparation of 11. NMR data are consistent with the existence of a single monomeric species in toluene solution for all complexes, except 11 and dichlorozirconium complexes 9 and 10, for which mixtures of monomeric and possibly aggregated species are observed in toluene and THF. The dynamic behavior of monomeric species 3, 8, 13, and 14 in toluene was investigated by variable-temperature NMR spectroscopy. The activation parameters determined by line-shape analysis, in particular for 8 (ΔH⧧ = 20.0 ± 1 kcal·mol-1; ΔS⧧ = 13.1 ± 2 cal·mol-1·K-1) and 13 (ΔH⧧ = 17.4 ± 1 kcal·mol-1; ΔS⧧ = 11.4 ± 2 cal·mol-1·K-1), suggest a process involving dissociation of nitrogen, inversion of configuration at nitrogen, and amine re-coordination. The bulkiness of the R substituents affects the fluxional behavior of {ONOR}TiCl2 (R = Me, 13; R = p-tol, 14). The THF-free dichlorozirconium complex 9 in combination with MAO generates highly active but very unstable ethylene polymerization catalyst systems.

Synthesis of cyclopentadienyl gadolinium and samarium alkoxide complexes

The mixed ligand complexes [{Gd(OCMe 2- i-Pr) 2Cp} 2], [{Sm(OCMe 2- i-Pr) 2Cp} 2], and [Sm 2(OCMe 2- i-Pr) 3Cp 3] were synthesized by reacting [LnCp 3] with HOCMe 2- i-Pr. X-ray crystallographic studies show that the three complexes each have two bridging alkoxide ligands linking two metal atoms. The coordination geometries at the metals are distorted tetrahedral.

Chelation Effect in Polymerization of Cyclic Esters by Metal Alkoxides: Structure Characterization of the Intermediate Formed by Primary Insertion of Lactide into the Al−OR Bond of an Organometallic Initiator

Well-defined dialkylaluminum alkoxides derived from 2-methoxyethanol [R2Al(μ-OCH2CH2OMe)]2 [where R = Me (1a) and tBu (1b)] and rac-ethyl lactate [Me2Al(μ-OCH(Me)CO2Et)]2 (2) were used as model complexes mimicking intermediate species in the initiation and propagation steps in the ring-opening polymerization of cyclic esters with metal alkoxides. Particularly, the effect of Lewis base termini of both the supporting ligand and the first polymer chain repeating unit on the aluminum alkoxides activity in the polymerization of ε-caprolactone (ε-CL) and rac-lactide (LA) were explored. It revealed that the initiation process is affected by the extent of chelation to a similar degree for ε-CL and LA as the character of the donor-functionalized alkoxide ligand determines the access of an incoming cyclic ester molecule to the effective coordinate site. On the other hand, a significant chelation effect on the propagation step was observed only in the polymerization of LA. The difference in the reactivity is rationalized on the basis of the first structurally authenticated intermediate [Me2Al(μ-OCH(Me)C(O))2O(CH2)2OMe]2 (1-LA) resulting from primary insertion of a lactide molecule into the aluminum−alkoxide bonds of the [Me2Al(μ-OCH2CH2OMe)]2 initiator. The direct relationship between the extent of intramolecular coordination and initiation and propagation steps of polymerization is also presented.

Syntheses and X-ray structures of cerium amide complexes

Reaction of CeCl 3 with an excess of LiTMP (TMP-H = 2,2,6,6-tetramethylpiperidine) gave [Ce(μ-OCH CH 2)(TMP) 2] 2. The alkoxide ligand originated from THF, which was used as the solvent in the reaction. CeCl 3 reacted with 3 equiv. of LiTMP in THF to produce, after crystallization, a 3:2 crystalline mixture of Ce(TMP) 3 and [Ce(μ-OCH CH 2)(TMP) 2] 2. Ce(TMP) 3 could not be isolated cleanly or separated from the mixture except by handpicking the crystals under a microscope. The homoleptic amide complex Ce[N- t-Bu(SiMe 3)] 3 was obtained by reacting CeCl 3(THF) 2 with 3 equivalents of LiN- t-Bu(SiMe 3). In the solid state, [Ce(μ-OCH CH 2)(TMP) 2] 2 is a dimer with a Ce(μ-OR) 2Ce four-member ring, while Ce(TMP) 3 and Ce[N- t-Bu(SiMe 3)] 3 are trigonal planar monomers. There is structural evidence to suggest the presence of a Ce–vinyl group interaction in [Ce(μ-OCH CH 2)(TMP) 2] 2.

Origin of 1,4-Regiochemistry in the Dicouplings of Ketones with 6,6-Dimethylcyclohexadienyl Complexes of Titanium and Zirconium: A Mechanism Arising from Five-Electron Donation by Alkoxide Ligands

In an earlier study of the coupling reactions of Ti(C5H5)(6,6-dmch)(PMe3) (dmch = dimethylcyclohexadienyl) with ketones, the initially formed metal complex could not be isolated, but after hydrolysis an unprecedented 1,4-dicoupling product was isolated. To gain further insight into the origin of this regiochemistry, the reaction of a zirconium analogue, Zr(C5H5)(6,6-dmch)(PMe3)2, with acetone was investigated. A solid dimeric product could be isolated and structurally characterized, which revealed that both acetones had coupled, with the unusual 1,4-regiochemistry. This then necessitated the proposal of a new mechanism for these reactions, in which an alkoxide ligand, generated from a coupling of the initially incorporated ketone, serves as a formal five-electron donor, thereby altering the mode of coordination of the diene fragment. This proposal has been supported by the characterization of a mono(benzophenone) coupling product of Ti(C5H5)(6,6-dmch)(PMe3). This coupling product possessed both a large Ti...

Effect of aluminum derivatives in the retarded styrene anionic polymerization

This paper reports on the influence of various aluminum derivatives, i.e. trialkyl aluminum, diisobutylaluminum hydride and diethylaluminum alkoxides, on the anionic polymerization of styrene in hydrocarbons at high temperature. The importance of the structure of the aluminum additives and of the ratio [Al]/[Li] on both the formation of ‘ate’ complexes, the retardation effect and the reactivity profile of the polymerization are investigated by UV–visible spectroscopy and polymerization studies. The presence of bulky alkyl groups and/or alkoxide ligands onto aluminum allows the retarded polymerization to proceed for [Al]/[Li] ratios higher than one. Some of these systems offers a broad polymerization window in which the rate of polymerization is strongly reduced and remains almost constant over a large range of [Al]/[Li] ratios. While for trialkylaluminum and diethylaluminum alkoxide the number of PS chains formed can be related directly to the initial amount of lithium initiator, for diisobutyl aluminum hydride and diisobutylpolystyryl aluminum, the hydride and polystyryl groups act as second chain carrier, thus yielding the formation of up to two polymer chains per ‘ate’ complex.

Catechol derivatives of Group 4 and 5 compounds

The structural modifications of Group 4 and 5 metal alkoxides using the bidentate catechol (C 6H 3(OH) 2-1,3 = cat-H 2) have been investigated. From the reaction of [Ti(μ-ONep)(ONep) 3] 2 (ONep = OCH 2CMe 3), [Zr(μ-OPr i )(ONep) 3 · HONep] 2(OPr i = OCHMe 2), Hf(OEt) 4 (OEt = OCH 2Me) with 2 equivalents, and M(OEt) 5 (M = Nb or Ta) with 3 equiv. of cat-H 2 in pyridine (py), the complete removal of alkoxide ligands and isolation of the first neutral catechol derivatives of the Group 4 and 5 species were observed. The corresponding catechol derivatives were structurally characterized as: Ti 3(μ 3-O)(μ-cat) 3(cat)(cat-H) 2 ( 1), Zr 4(μ 4-O)(μ-cat) 6(μ-cat-H) 2(py) 4 ( 2), Hf 4(μ 4-O)(μ-cat) 6(μ-cat-H) 2(py) 4 ( 3), M(cat) 2(cat-H)(py) (M = Nb ( 4); Ta ( 5)). Compound 1 is trinuclear with each octahedrally bound metal center having three chelating and three chelating bridging cat ligands, along with a μ 3-O ligand, to form a corner-missing cube arrangement. Based on charge balance, two of the cat ligands must retain a hydrogen atom (cat-H). The congeners 2 and 3 adopt similar structures wherein each of the metal centers is eight coordinated, adopting a dodecahedral-like geometry. The metals are joined by a μ 4-O ligand and each cation uses two chelating cat ligands to bridge to two other metal centers as well as coordinating a single py ligand. Again, based on charge balance, two of the ligands must be cat-H ligands. Compounds 1– 3 are insoluble at room temperature in their parent solvent. Compounds 4 and 5 are mononuclear wherein each metal center formally adopts a seven coordination capped-trigonal prism involving three chelating catechol ligands, with one being cat-H, along with a coordinated pyridine ligand. Multinuclear NMR data indicate that the structures of 4 and 5 are retained in solution. TGA/DTA data indicated that compound 1 is a reasonable precursor for nanoparticles. 1 was used to generate nanoparticles of TiO 2 from benzylalcohol.