Zirconium Complexes Research Articles

Allyl Complexes of Tungsten from the Rearrangement of Transient Cyclopropyl Precursors

This article explores the contrasting reactivity of cyclopropyl complexes of early transition metals. [Cp*W(NO)(CH2R)(c‐C3H5)] (R = SiMe3, Ph, t‐Bu) generated in THF solution from [Cp*W(NO)(CH2R)Cl] and [Mg(c‐C3H5)2(dioxane)x] readily rearrange to η3‐allyl derivatives [Cp*W(NO)(CH2R)(η3‐C3H5)] by an intramolecular ring opening reaction. Both direct and catalyzed pathways are revealed by kinetic studies. Computational modeling indicates the ring opening reaction is preferred on thermodynamic grounds for tungsten whereas kinetic products arising from β‐H abstraction reactions of cyclopropane are observed for related zirconium and niobium complexes reported previously. The energetic accessibility and the nature of the LUMO in the tungsten complexes promote distal CC bond cleavage in the cyclopropyl ring.

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene-π Interactions: Ligand Binding and Ethylene Polymerization.

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene−π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands.

Titanium, zirconium and hafnium complexes bearing amino-benzotriazole phenolate ligands as efficient catalysts for ring-opening polymerization of lactides

Group 4 catalysts incorporated in 2-(2H-benzotriazol-2-yl)-4-methyl-6-(morpholinomethyl)phenolate ligand (C1MOBTP) were synthesized and structurally characterized. The reaction of Ti(OiPr)4 or M(OiPr)4(iPrOH) (M = Zr, Hf) with C1MOBTP-H (2.0 M equiv.) in toluene produced the hexa-coordinated bis-adduct metal complex [(C1MOBTP)2M(OiPr)2] (M = Ti for 1, Zr for 2 and Hf for 3). Experimental results indicate that hafnium complex 3 displayed the higher catalytic activity than those of di-C1MOBTP-featuring complexes 1 and 2 for the controlled L-LA polymerization. Hafnium alkoxide 3 catalyzes the ring-opening polymerization effectively not only in a “controlled” fashion but also an “immortal” manner, giving the corresponding polymers with the expected molecular weights and narrow molecular weight distributions.

Directing-Group-Free C7-Alkylations of N-Alkylindoles by Zirconium Complexes

Key words zirconium catalysis - regioselectivity - alkylation - alkylindoles

Construction of SiO2@UiO-66 core–shell microarchitectures through covalent linkage as flame retardant and smoke suppressant for epoxy resins

The SiO2@metal organic framework (Universitetet i Oslo-66, UiO-66) core–shell microspheres were constructed through covalent linkage between amine groups in UiO-66-NH2 and epoxy groups on the surface of silica. The morphology and size of the SiO2@UiO-66 core–shell microspheres could be simply controlled by tuning the ratio between UiO-66-NH2 and epoxy terminated silica (E-SiO2). As observed by TEM, the SiO2@UiO-66 hybrids showed better dispersion state within epoxy matrix compared to either E-SiO2 or UiO-66-NH2. The incorporation of SiO2@UiO-66 hybrids slightly promoted the thermal degradation of the resultant epoxy composites but improved residual yield. The dynamic mechanical analysis results indicated that the SiO2@UiO-66 hybrids slightly increased the glass transition temperature and the modulus. The SiO2@UiO-66 hybrids exhibited higher efficiency in reducing the heat release rate and the smoke production rate compared to either E-SiO2 or UiO-66-NH2. The influence of the component ratio in SiO2@UiO-66 on flame retardancy of the epoxy composites was also studied by cone calorimeter. Specifically, the SiO2@UiO-66 hybrid with medium ratio (SiO2@UiO-66-2) exhibited maximum reduction in peak heat release rate (−31%), total heat release (−23%) and total smoke production (−16%). The char residues were investigated by the Fourier transform infrared spectra, scanning electron microscopy and X-ray photoelectron spectroscopy, which demonstrated that the enhanced flame retardancy of EP/SiO2@UiO-66-2 was attributable to the continual morphology and high thermal resistance originated from the presence of the silicon and zirconium complex. These favorable characteristics including high flame retardant efficiency and good smoke suppression make SiO2@UiO-66 hybrids promising for flame retardant polymers application.

New insights into the predicament of DFT assisted optimized energy, stability and distortions of optimized topologies of some novel complexes of Zirconium (IV) and enhancement of antimicrobial potential

A comprehensive density functional theory (DFT/B3LYP method) investigation was done to study the generation, optimized topologies, bonding, reactivity, stability, distortions and enhancement of antimicrobial potential of a number of organic–inorganic hybrid complexes of zirconium (IV). Zirconium (IV) complexes having the general formulae ZrL(Cl)2 and Zr(L)2 were prepared by the reaction of zirconium tetrachloride with dipotassium salts of sterically constrained oximes of heterocyclic β‐diketones in different stoichiometric ratios in refluxing anhydrous THF. Plausible topologies of these complexes were suggested with the aid of physico‐chemical and spectral studies. The optimized topologies of the complexs of the types ZrL(Cl)2 and Zr(L)2 were given by comprehensive DFT investigation. The molecular orbitals, molecular electrostatic potential, chemical reactivity, energy optimization, stability and distortions of these newly generated complexes were studied by DFT. These complexes were screened against bacterial isolate E. Coli and fungal isolates viz. A. Niger and T. Reesei. The chemical reactivity of these complexes is corroborated by small energy gap. Few principal quantum chemical descriptors were correlated with the enhancement of antimicrobial potential of these sterically constrained complexes of zirconium (IV). The presence of ‐CH2CH3, ‐C6H5 and –C6H4Cl (p) groups in complexes of the types ZrL(Cl)2 and Zr(L)2 provides an opportunity to examine the effect on optimized energy, stability and distortions of optimized topologies.

A New Class of Pyridine-Amide Containing Ti and Zr Based Catalysts for Olefin Polymerization: Influence of Ligand Substituents

Two tridendate pyridine-amide ligands in di-deprotonated form have been used to synthesize new titanium and zirconium complexes. The molecular structure of pentafluoro containing Titanium and Zirconium complexes were determined by DFT calculations. These new metal complexes have been further evaluated for ethylene polymerization activity in the presence of co-catalyst methylaluminoxane (MAO) under atmospheric pressure, producing polyethylenes with moderate molecular weights and unimodal molecular weight distribution. Remarkably, zirconium complexes show higher activity than titanium complexes under the same polymerization condition. The catalytic activity of all the complexes were found to be temperature dependent and reach their highest efficiency at 70 °C. The influence of the nature of the ligands and reaction parameters upon the catalytic activities has been studied. Additionally, the pentafluoro containing Zirconium and Titanium complexes were further evaluated for the copolymerization of ethylene and propylene and exhibited good catalytic activity. A new class of titanium and zirconium based metal complexes were synthesised from pyridine-amide containing ligands. The synthesised complexes have been evaluated for ethylene polymerization in the presence of co-catalyst methylaluminoxane (MAO) under atmospheric pressure. Interestingly, zirconium complexes show high catalytic activities than titanium complexes under the same polymerization condition.

Carbazolyl-Substituted [OSSO]-Type Zirconium(IV) Complex as a Precatalyst for the Oligomerization and Polymerization of α-Olefins

The dibenzyl zirconium(IV) complex (4) incorporating with a carbazolyl(Cbz)-substituted [OSSO]-type bis(phenolate) ligand was synthesized. Upon activation with dried modified methylaluminoxane (dMMAO), precatalyst 4 at relatively low catalyst loadings was found to promote the 1,2-regioselective oligomerization of 1-hexene to produce the corresponding vinylidene-ended oligomers with moderate turnover frequencies (TOFs) up to 2080 h−1. The 13C NMR analysis of the resulting oligomers revealed the formation of dimer-enriched oligo(1-hexene)s in 39–62% distributions. The precatalyst 4 with dried methylaluminoxane (dMAO) also exhibited good performance in the polymerization of styrene yielding isotactic polystyrenes ([mm] > 99%) with quite large molecular weights (Mw < 508,100 g mol−1) and relatively high catalytic activity (up to 2810 g mmol(4)−1 h−1).

Tales of the Unexpected: The Case of Zirconium(IV) Complexes with Desferrioxamine.

The Zr4+ complexes with desferrioxamine (H3DFO) and its derivatives are the only 89Zr-based imaging agents for proton emission tomography (PET) that have been used so far in clinical trials. Nevertheless, a complete speciation of the Zr4+/H3DFO system in solution has never been performed and the stability constants of the relevant complexes are still unknown. Here we report, for the first time, the speciation of this system in water, performed by potentiometric titrations, and the determination of the stability constants of all complexes formed in the pH range 2.5–11.5. Surprisingly, although desferrioxamine gives rise to very stable 1:1 complexes with Zr4+ (logK = 36.14 for Zr4+ + DFO3− = [ZrDFO]+), 2:2 and 2:3 ones are also formed in solution. Depending on the conditions, these binuclear complexes can be main species in solution. These results were corroborated by small-angle X-ray scattering (SAXS) and MALDI mass spectrometry analyses of complex solutions. Information on complex structures was obtained by means of density functional theory (DFT) calculations.

Use of Chelate Complexes of Lead(II) in Low-Temperature Technologies of Ceramic Piezoelectric Materials

Technology is suggested for fabrication of ceramic piezoelectric materials from the lead-containing ferroelectric phase. The technology includes a low-temperature synthesis of ultradispersed lead titanate and zirconate powders and also of phases of solid solutions on their basis and the low-temperature procedure for sintering of ceramic fabricated from ultradispersed powders of ferroelectric phases. As precursors used in the suggested low-temperature synthesis serve lead glycerate and tartrate. It was found that, when interacting (at temperatures of 240–330°C) with hydroxo-peroxo-aqua complexes of titanium and(or) zirconium(IV) in the course of 30–40 min, the precursors form ultradispersed powders of these phases, which are single-phase and contain particles with average size of less than 100 nm. It was shown that using the suggested technology makes it possible not only to reduce the energy expenditure for separate procedures for synthesis of ultradispersed ferroelectric phases and for synthesis of piezoelectric materials, but also to substantially diminish the discharge of lead(II) compounds into the atmosphere of industrial premises. It was also shown that the technology enables fabrication of piezoelectric materials with prescribed values of electrical parameters.

Directing-Group-Free C7-Alkylations of N-Alkylindoles Mediated by Cationic Zirconium Complexes: Role of Brønsted Acid for Catalytic Manifold.

Highly position selective alkylations of N-alkylindoles at C7-positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7-positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold.

Electronic influence of ligand substituents in the ring-opening polymerization of l-Lactide promoted by OSSO-type zirconium complexes

The zirconium complexes (OSSOCl)ZrOtBu2 (1), (OSSOBr)ZrOtBu2 (2) and (OSSOtBu)ZrOtBu2 (3) bearing OSSO-type ligands (OSSOCl-H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dichlorophenol); OSSOBr-H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dibromophenol); OSSOtBu-H = 1,4-dithiabutanedyl-2,2′-bis(4,6-di-tert-butyl-phenol)) were prepared and characterized. NMR spectroscopy revealed a rigid octahedral coordination geometry with a facial-facial coordination of the [OSSO] ligand to the metal complex. These compounds were active in the ring-opening polymerization of l-lactide exerting an effective control over the polymerization reaction. Activities correlated with the electron-withdrawing character of the ortho and para groups of the OSSO ligands. Density functional theory (DFT) investigations indicated that substitution with increasingly electron-withdrawing groups favors the monomer coordination and determines an earlier, more reactive transition state.

Ansa-zirconocenes bearing 5-NR2-6-alkyl-4-hydrocarbyl-2-methylindenyl moieties: Synthesis, structure, stereoselective polymerization of propylene

A series of C2–symmetric SiMe2-bridged bis-indenyl zirconium complexes containing 2-methyl-4-aryl/isopropyl-5-amino/pyrrolyl-6-alkylindenyl fragments have been synthesized and unambiguously characterized. These complexes activated by MAO demonstrated high catalytic activity in homo- and heterogeneous polymerization of propylene outperforming the traditional Spaleck system. The degree of electronic interaction of aminogroup in pos. 5 with indenyl ring system was tested via computational investigations, and notable influence of aminogroup on electronic structure of the complexes is witnessed by C (indenyl)N bond order and a sum of NAC of the ZrCl2 fragment.

Synthesis and Characterization of Mixed Ligand Complexes of Zirconium(IV) with Sulphur, Nitrogen and Oxygen Donor Ligands

Synthesis and Characterization of Mixed Ligand Complexes of Zirconium(IV) with Sulphur, Nitrogen and Oxygen Donor Ligands

Making the cut: Monopyrrolidine-based complexes for the ROP of lactide

A series of monopyrrolidine metal complexes have been prepared with the aim of elucidating structure-to-activity relationships for the ROP of lactide. Complexes of Li(I), Mg(II), Zn(II) and Zr(IV) were synthesised and fully characterised. Structural comparisons are made to the bipyrrolidine analogues. These monopyrrolidine complexes were found to be active towards the ROP of lactide under both solution and melt conditions. The zirconium complexes were found to have reduced and opposing stereocontrol compared to the bipyrrolidine analogues. The zinc and magnesium complexes demonstrated good activity and control under immortal polymerisation conditions.

Reaction of Pentafulvene Titanium and Zirconium Complexes with Phosphorus Ylides: Stoichiometric Reactions and Catalytic Intramolecular Proton Shuttles

The reaction of penta- and benzofulvene complexes of group 4 (M = Ti, Zr) with different phosphorus ylides is reported. Employing the bis(η5:η1-pentafulvene)titanium complexes Ti1a and Ti1b, the reactions with the corresponding phosphorus ylides Y1–5 result in a spontaneous single C–H bond activation and metallocene ylide complexes Ti2a–e can be isolated in good yields. On the basis of these results, this reactivity pattern has been extended to the mono(η5:η1-pentafulvene) and -benzofulvene complexes Ti3, Zr1a,b, and Zr1a,b-benzo. Transferring this to the titanium complex Ti3, an additional equilibrium between the reaction product (η5-C5Me5)(η5-CpAd)Ti(Cl)CHPPh3 (Ti4a) and the corresponding “tuck-in” complex (η5:η1-C5Me4CH2)(η5-CpAd)TiCl (Ti4b) can be observed. In Ti4b, a methyl group of the Cp*-ligand is activated by the coordinated phosphorus ylide. Using this intramolecular C–H activation, Ti4b can be synthesized by catalytic amounts (3 mol %) of the phosphorus ylide Y1 in quantitative yield. Subsequen...

Photoluminescence of Seven-Coordinate Zirconium and Hafnium Complexes with 2,2'-Pyridylpyrrolide Ligands.

Luminescent seven-coordinated zirconium and hafnium complexes bearing three mono-anionic 2,2'-pyridylpyrrolide ligands and one chloride were synthesized. Solid-state structures and the dynamic behaviors in solution were probed by X-ray crystallography and variable temperature 1 H NMR experiments, respectively. Absorption spectroscopy and time-dependent density functional theory (TD-DFT) calculations supported a hybrid of ligand-to-metal charge transfer (LMCT)/ligand-to-ligand charge transfer (LLCT) for the visible light absorption band. The complexes (Me PMPMe )3 MCl (M=Zr, Hf, Me PMPMe =3,5-dimethyl-2-(2-pyridyl)pyrrolide) are emissive in solution at room temperature upon irradiation with visible light due to a combination of phosphorescence and fluorescence characterized by excited state lifetimes in the μs and low to sub-ns timescale, respectively. Electrochemical experiments revealed that the zirconium complex possesses a reversible redox event under highly reducing condition (-2.29 V vs. Fc+/0 ).

Effective copolymerization of ethylene with α,ω-alkenols and homopolymerization of α,ω-alkenols catalyzed by aminophenolate zirconium complex

A zirconium complex of diamine-bis(phenolate) ligand, [(tBu2O2NN’)ZrCl]2(μ-O) where (tBu2O2NN’) = Me2N(CH2)2N(CH2–2-O−-3,5-tBu2-C6H2)2, activated with (iBu)3Al/Ph3CB(C6F5)4, was for the first time used in copolymerization of ethylene with unsaturated alcohols (CH2 = CH(CH2)nCH2OH, where n = 7, 8, 3). The hydroxyl groups of comonomers were protected with R3-xAlClx (where x = 0 or 1, R = iBu, Et). In contrast to the formerly reported catalysts, the activity of this catalyst is much higher in ethylene/alkenols copolymerization than in ethylene homopolymerization and its lifetime is long. Moreover, the copolymers with high polar comonomer contents (up to 16.4 mol%, 52.3 wt%) were produced. The effects of copolymerization conditions such as comonomer/protecting agent molar ratio, comonomer concentration, ethylene pressure and reaction time on the catalyst activity and comonomer incorporation were studied. In addition, polyalkenols were synthesized by the coordination polymerization of α,ω-alkenols with the use of [(tBu2O2NN’)ZrCl]2(μ-O) and rac-Et(Ind)2ZrCl2 for reference. DSC, TG, GPC, SEM, 1H and 13C NMR, and FTIR methods were used to characterize the (co)polymerization products.

Titanium and zirconium complexes bearing new tridentate [OSO] bisphenolato-based ligands: synthesis, characterization and catalytic properties for alkene polymerization.

A number of new sulfur-bridged tridentate [OSO] bisphenolato-based ligand precursors S(2-CH2-4-tBu-6-R-C6H2OH)2 [R = CMe3 (H2L1), CMe2Ph (H2L2), CMePh2 (H2L3), CPh3 (H2L4), and C(p-Tol)3 (H2L5)] were synthesized by reactions of Na2S·9H2O with 2 eq. of the corresponding 2-(bromomethyl)-4-(tert-butyl)-6-R-phenol. Their neutral titanium complexes [S(2-CH2-4-tBu-6-R-C6H2O)2]TiCl2 [R = CMe3 (1), CMe2Ph (2), CMePh2 (3), CPh3 (4), and C(p-Tol)3 (5)] were synthesized in high yields by direct HCl-elimination reactions of TiCl4 with the corresponding ligand precursors in toluene. Ionic titanium complexes [NHEt3][S(2-CH2-4-tBu-6-R-C6H2O)2TiCl3] [R = CMe3 (6), CMePh2 (7)] and [NH2Et2][S(2-CH2-4-tBu-6-R-C6H2O)2TiCl3] [R = CMe3 (8) and CMePh2 (9)] were obtained in high yields from the reactions of TiCl4 with the corresponding ligand precursors in the presence of 2 eq. of triethylamine or diethylamine. Neutral zirconium complexes [S(2-CH2-4-tBu-6-R-C6H2O)2]ZrCl2(THF) [R = CMe2Ph (10·THF), and CMePh2 (11·THF)] were synthesized by reactions of ZrCl4 with 1 eq. of the dilithium salt of the corresponding ligand precursors Li2L in THF. The new titanium and zirconium complexes were characterized by 1H and 13C NMR, IR and elemental analyses. The molecular structures of complexes 4, 6 and 10·THF were determined by single-crystal X-ray diffraction analysis. The X-ray crystallography analysis reveals that titanium complex 4 has a five-coordinating environment surrounding the central metal atom, while the titanium complex 6 and the THF-solvated zirconium complex 10·THF possess a six-coordinating pseudo-octahedral environment around the central metal atom. Upon activation with MAO or AliBu3/Ph3CB(C6F5)4, all these titanium and zirconium complexes exhibit moderate to high catalytic activities for ethylene polymerization and ethylene/1-hexene copolymerization with moderate to high comonomer incorporation, and the ionic titanium complexes 6, 7, 8 and 9 show lower catalytic activity than their corresponding neutral complexes under similar conditions.

Salen complexes of zirconium and hafnium: synthesis, structural characterization and polymerization studies

Salen complexes of zirconium and hafnium were synthesized and used as effective catalysts for the polymerization of lactide and ε-CL and homopolymerization, copolymerization and coupling of epoxides with CO2.