Zirconocene Complexes Research Articles

A Phosphorus-Centred, Zirconocene-Bridged Tetraradical: Synthesis, Structure and Application as Molecular Double Switch.

Biradicals are important intermediates in the formation and breaking of a chemical bond. Their use as molecular switches is of particular interest. Much less is known about tetraradicals, which can, for example, consist of two biradical(oid) units. Here we report the synthesis of the first persistent phosphorus-centred tetraradical bound to a transition metal fragment. Starting from a zirconocene complex, rac-(ebthi)ZrCl2 (rac-(ebthi)=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl), two cyclo-1,3-diphospha-pentane-1,3-diyls were successfully introduced, which finally led to the isolation of a deep green zirconcene-bridged bis(biradicaloid) complex (5) that can act as a double molecular switch. Under the influence of light (570 nm), this tetraradical forms a transannular bond in each of the two five-membered biradical units, leading to the formation of housane 5 h. Upon irradiation at 415 nm, the reverse reaction is observed, fully recovering tetraradical 5. Through single-crystal-to-single-crystal transformation, both stable species of the molecular switch could be structurally characterised using SCXRD. The switching under the influence of light and the activation of molecular hydrogen were analysed in solution using NMR and UV spectroscopy. It was found that the addition of one or two equivalents of molecular hydrogen can be switched on and off by light.

Aluminiumalkylinduzierte Isomerisierung von Gruppe IV meso Metallocen Komplexen

AbstractBei der Synthese von Gruppe IV Metallocen‐Präkatalysatoren für die Polymerisation von Propylen werden im Allgemeinen zwei verschiedene Isomere gebildet: Das racemische Isomer, das isotaktisches Polypropylen (iPP) produziert und das meso‐Isomer, das ataktisches Polypropylen (aPP) produziert. Aufgrund seiner vergleichbar schlechten physikalischen Eigenschaften ist die Anwendung von aPP sehr begrenzt. Um Mischungen beider Polymere zu vermeiden, was die mechanischen und thermischen Eigenschaften von iPP zu verschlechtern würde, müssen die meso Metallocenkomplexe mühsam von den racemischen Komplexen getrennt werden, was die Metallocen‐basierte Polymerisation von Propylen industriell weit weniger attraktiv macht als das Ziegler/Natta‐Verfahren. Um dieses Problem zu umgehen, haben wir ein Isomerisierungsprotokoll zur Umwandlung von meso Metallocenkomplexen in ihre racemischen Analoga entwickelt. Dieses Protokoll steigerte die Ausbeute an iPP um 400 %, wobei die hervorragenden physikalischen Eigenschaften des Polymers beibehalten wurden; es war sowohl auf Hafnocen‐, als auch auf Zirkonocenkomplexe, sowie auf verschiedene Methoden zur Aktivierung des Präkatalysators anwendbar. Durch gezielte Variation der Liganden wurden Methoxygruppen an den Indenylsubstituenten als die für eine Isomerisierung verantwortlichen Strukturmotive gefunden ‐ dieser experimentelle Nachweis wurde durch Dichtefunktionaltheorieberechnungen bestätigt. Flüssig‐Injektion‐Feld‐Desorption‐Ionisierung‐Massenspektrometrie, sowie 1H‐ und 29Si‐Kernresonanzstudien ermöglichten den Vorschlag eines Isomerisierungsmechanismus.

Aluminum Alkyl Induced Isomerization of Group IV meso Metallocene Complexes.

The synthesis of group IV metallocene precatalysts for the polymerization of propylene generally yields two different isomers: The racemic isomer that produces isotactic polypropylene (iPP) and the meso isomer that produces atactic polypropylene (aPP). Due to its poor physical properties, aPP has very limited applications. To avoid obtaining blends of both polymers and thus diminish the mechanical and thermal properties of iPP, the meso metallocene complexes need to be separated from the racemic ones tediously-rendering the metallocene-based polymerization of propylene industrially far less attractive than the Ziegler/Natta process. To overcome this issue, we established an isomerization protocol to convert meso metallocene complexes into their racemic counterparts. This protocol increased the yield of iPP by 400 % while maintaining the polymer's excellent physical properties and was applicable to both hafnocene and zirconocene complexes, as well as different precatalyst activation methods. Through targeted variation of the ligand frameworks, methoxy groups at the indenyl moieties were found to be the structural motifs responsible for an isomerization to take place-this experimental evidence was confirmed by density functional theory calculations. Liquid injection field desorption ionization mass spectrometry, as well as 1H and 29Si nuclear magnetic resonance studies, allowed the proposal of an isomerization mechanism.

Ethylene Polymerization over Metal-Organic Framework-Supported Zirconocene Complexes.

Metallocene immobilization onto a solid support helps to overcome the drawbacks of homogeneous metallocene complexes in the catalytic olefin polymerization. In this study, valuable insights have been obtained into the effects of pore size, linker composition, and surface groups of metal-organic frameworks (MOFs) on their role as support materials for metallocene-based ethylene polymerization catalysis. Three distinct Zn-based metal-organic frameworks (MOFs), namely, MOF-5, IRMOF-3, and ZIF-8, with different linkers have been activated with methylaluminoxane (MAO) and zirconocene complexes, followed by materials characterization and testing for ethylene polymerization. Characterization has been performed by multiple analytical tools, including X-ray diffraction (XRD), scanning electron microscopy (SEM), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and CO Fourier transform infrared (FT-IR) spectroscopy. It was found that the interactions between MOFs, MAO, and the zirconocene complex not only lead to both catalyst activation and deactivation but also result in the creation of multiple active sites. By alteration of the MOF support, it is possible to obtain polyethylene with different properties. Notably, ultrahigh molecular weight polyethylene (UHMWPE, M W = 5.34 × 106) was obtained using IRMOF-3 as support. This study reveals the potential of MOF materials as tunable porous supports for metallocene catalysts active in ethylene polymerization.

Biopotential insights and structural chemistry of some zirconocene incorporated heterocyclic β-diketones and flexible N-protected α/β- amino acids

The interaction of zirconocene dichloride with the heterocyclic β-diketones (LH) and flexible N-protected α/ β-amino acids (AH) in the presence of triethylamine in 1:1:1:2 molar ratio in refluxing dry THF has been resulted in the formation of zirconocene complexes of the general formula (where R= -CH3, -CH2CH3, p-ClC6H4-, -C6H5 and CHR’ = -CH2CH2and R’ = CH(CH3)2). These newly generated zirconocene incorporated organic-inorganic hybrid products have been characterized by FTIR, NMR (1H and 13C) and mass spectrometry. Some of these complexes and their corresponding ligands have been tested for their antimicrobial activity to examine the biopotential activity-structure relationship.

Luminescent Zirconocene Complexes with Pendant Phosphine Chalcogenide Donor Groups

Neutral zirconocene complexes of the type [(η5-C5Me5){η5-C5H4CMe2P(═E)Ph2}ZrCl2] (E = not present, O, S, or Se) with a cyclopentadienyl-attached phosphine and related phosphine oxide, sulfide, or selenide moiety were prepared and converted to their cationic counterparts by chloride abstraction with K[B(C6F5)4] to give compounds of the general formula [(η5-C5Me5){η5-C5H4CMe2P(═E)Ph2-κE}ZrCl]+[B(C6F5)4]−. Alternatively, the chloride abstraction was accomplished by the reaction with ZnCl2 to give the same cationic species with a [Zn2Cl6]2– counteranion. The ionic complexes, unlike the neutral counterparts, exhibited strongly enhanced luminescence properties originating in triplet ligand-to-metal (3LMCT) excited states. Luminescence quantum yields up to 0.95 were determined in the solid state with luminescence lifetimes up to 50.6 μs. In addition to structural characterization of the prepared complexes by X-ray diffraction methods, their electrochemistry was investigated by cyclic voltammetry, and their photophysical properties were studied with quantum-chemical calculations.

C 1-Symmetric {cyclopentadienyl/indenyl}-metallocene catalysts: synthesis, structure, isospecific polymerization of propylene and stereocontrol mechanism.

A series of new Me2Si-bridged cyclopentadiene/indene proligands {Me2Si(R2',5'2-R3',4'2-Cp)(R2,R4,R5,R6-Ind)H2} (1a-j) with various substitutions both on the indene and cyclopentadiene moieties was prepared. The corresponding C1-symmetric group 4 ansa-metallocene complexes (M = Zr, Hf), namely, {Me2Si(Me4Cp)(Ind)}ZrCl2 (2a-Zr), {Me2Si(Me4Cp)(2-Me,4-Ph-Ind)}MCl2 (2b-M), {Me2Si(Me4Cp)(2-Me,4-Ph,6-tBu-Ind)}ZrCl2 (2c-Zr), {Me2Si(Me4Cp)(2-Me,4-Ph,5-OMe,6-tBu-Ind)}MCl2 (2d-M), {Me2Si(Me4Cp)(2-R',4-(3',5'-tBu2,4'-OMe-C6H2),5-OMe,6-tBu-Ind)}ZrCl2, R' = Me (2e-Zr), R' = Et (2f-Zr), {Me2Si(2,5-Ph2-3,4-Me2-Cp)(2-Me,4-(3',5'-tBu2,4'-OMe-C6H2),5-OMe,6-tBu-Ind)}ZrCl2 (2g-Zr), {Me2Si(Me4Cp)(2-Me,4-(3',6'-tBu2-carbazol-4'-yl)-Ind)}ZrCl2 (2h-Zr), {Me2Si(2,5-Me2,3,4-iPr2-Cp)(2-Me,4-Ph-Ind)}ZrCl2 (2i-Zr), {Me2Si(2,5-Me2,3,4-iPr2-Cp)(2-Me,4-Ph,6-tBu-Ind)}ZrCl2 (2j-Zr) and {Me2Si(Me4Cp)(2-Me-4,5-[a]anthracene-Ind)}MCl2 (2k-Zr) were synthesized and characterized by NMR spectroscopy and mass spectrometry. The solid-state molecular structures of 2b-Zr, 2d-Zr, 2e-Zr, 2f-Zr, 2j-Zr and 2k-Zr were determined by X-ray crystallography. The zirconocene complexes, once activated with MAO in toluene solution, exhibited propylene polymerization activities at 60 °C up to 161 000 kg(PP) mol(Zr)-1 h-1, affording highly isotactic polypropylenes (iPP) with [m]4 up to 96.5% and Tm up to 157 °C. Also, metallocene complexes 2b-e-Zr were supported on SiO2-MAO and evaluated in slurry bulk propylene polymerization at 70 °C, producing iPPs with [m]4 = 91.7-96.6 mol% and low regiodefects (0.2-0.3 mol%) content, with productivities up to 636 000 kg(PP) mol(Zr)-1 h-1. DFT calculations allowed rationalizing a polymerization reaction mechanism occurring through "chain-stationary" enchainment with preference for 1,2-insertions.

SYNTHESIS, SPECTRAL CHARACTERIZATION AND ANTIMICROBIAL STUDIES OF BIS(CYCLOPENTADIENYL)- ZIRCONIUM(IV) DERIVATIVES WITH QUINAZOLINONE SCHIFF BASES

Zirconium(IV) complexes of types [(C5H5)2Zr(L)] and [(C5H5)Zr(LÂ’)Cl] (LH2 represents Schiff bases derived from quinazolinone and salycylaldehye or o-hydroxyacetophenone LÂ’H represents Schiff bases derived from quinazolinone and 4-fluorobenzaldehyde/ 4-fluoro acetophenone /1-(2-fluoro-4-methylphenyl)-2-methylpropan-1-one / 1-(2-fluoro-4-methylphenyl) propan-1-one ) have been synthesized by the reactions of bis(cyclopentadienyl)zirconium(IV) dichloride with appropriate ligands in tetrahydrofuran in the presence of triethylamine. All these complexes are soluble in THF, PhNO2, DMF and DMSO. The complexes were characterized by elemental analyses, electrical conductance, magnetic susceptibility, IR, 1H NMR, XRD and SEM spectral techniques. Low molar conductance values indicate non-electrolytic nature of the complexes. The spectral data indicate 5-coordinate geometry for the complexes. XRD pattern evince that the complexes have monoclinic crystal system and particle sizes were found in the range 46.40-49.30 nm (nano-size). In vitro antifungal activity of synthesized compounds was evaluated against fungi Aspergillus niger, Aspergillus fumigatus, Candida albicans and In vitro antibacterial activity was determined by screening the compounds against gram negative (P. aeruginosa, E.coli) and gram positive (S. aureus and B. subtilis) bacterial strains using minimum inhibition concentration method (MIC) by serial dilution technique. The zirconocene(IV) complexes have higher antimicrobial impact than the parent Schiff bases.

Density functional theory and CCSD(T) evaluation of ionization potentials, redox potentials, and bond energies related to zirconocene polymerization catalysts.

Quantum-mechanical-based computational design of molecular catalysts requires accurate and fast electronic structure calculations to determine and predict properties of transition-metal complexes. For Zr-based molecular complexes related to polyethylene catalysis, previous evaluation of density functional theory (DFT) and wavefunction methods only examined oxides and halides or select reaction barrier heights. In this work, we evaluate the performance of DFT against experimental redox potentials and bond dissociation enthalpies (BDEs) for zirconocene complexes directly relevant to ethylene polymerization catalysis. We also examined the ability of DFT to compute the fourth atomic ionization potential of zirconium and the effect the basis set selection has on the ionization potential computed with CCSD(T). Generally, the atomic ionization potential and redox potentials are very well reproduced by DFT, but we discovered relatively large deviations of DFT-calculated BDEs compared to experiment. However, evaluation of BDEs with CCSD(T) suggests that experimental values should be revisited, and our CCSD(T) values should be taken as most accurate.

Catalytic reductive ring opening of epoxides enabled by zirconocene and photoredox catalysis

Catalytic reductive ring opening of epoxides enabled by zirconocene and photoredox catalysis

Natural Amino Acid L‐Phenylalanine Coordinated Zirconocene Complex as Bifunctional Catalyst for the Synthesis of 1,5‐Benzothiazepines

AbstractTaken the synthesis of 1,5‐benzothiazepines via the condensation between o‐aminothiophenol with ynones as a paradigm, novel mode of IVB metal based bifunctional catalysis in Lewis acidic‐Brønsted basic (LA‐BB) mode, as well as the effective evidence for how the two catalytic sites interact with substrates has been realized and provided respectively. Specifically, the bifunctional catalyst Cp2Zr[OCOCH(NH2)CH2C6H5]2 was formed through in situ coordination between Cp2ZrCl2 and natural amino acid L‐phenylalanine, which was elucidated by 1H NMR experiment and ESI‐MS analysis. Further control experiment reveals that the excellent catalytic efficiency can attribute to coexistence of Zr (IV) center and Brønsted basic NH2 group contained amino acid ligand. More importantly, with the aid of in situ ESI‐MS, the first catalyst‐substrate adduct for IVB metallocene (IV) catalyst was detected, indicating the Lewis acidic catalytic mode of Zr (IV) center on O atom of ynone. Meanwhile, DFT studies indicated that the true catalytic species presumably be zirconocene (IV) bearing both a bidentate and mono‐dentate carboxylate ligand.

A uniformly anchored zirconocene complex on magnetic reduced graphene oxide (rGO@Fe3O4/ZrCp2Cl x (x = 0, 1, 2)) as a novel and reusable nanocatalyst for synthesis of N-arylacetamides and reductive-acetylation of nitroarenes.

In this study, a crafted zirconocene complex on rGO@Fe3O4 as a novel magnetic nanocatalyst was synthesized and then characterized using FT-IR, SEM, EDX, VSM, ICP-OES, TGA, BET and MS analyses. Next, catalytic activity of the prepared nanocomposite rGO@Fe3O4/ZrCp2Clx (x = 0, 1, 2) towards successful reduction of aromatic nitro compounds to arylamines using N2H4·H2O (80%) was investigated. The examined nanocatalyst also showed perfect catalytic activity for reductive-acetylation of aromatic nitro compounds to the corresponding N-arylacetamides without isolation of the prepared in situ amines using the N2H4·H2O/Ac2O system. Furthermore, acetylation of the commercially available arylamines to the corresponding N-arylacetamides was carried out by acetic anhydride in the presence of the rGO@Fe3O4/ZrCp2Clx (x = 0, 1, 2) nanocomposite. All reactions were carried out in refluxing EtOH as a green solvent to afford the products in high yields. The obtained results exhibited that the nanocomposite of rGO@Fe3O4/ZrCp2Clx (x = 0, 1, 2) showed a great catalytic activity in comparison to rGO and rGO@Fe3O4 as the parent constituents. Recovery and reusability of rGO@Fe3O4/ZrCp2Clx (x = 0, 1, 2) were also examined for 8 consecutive cycles without significant loss of the catalytic activity. This establishes the sustainable anchoring of the zirconocene complex on the surface and mesopores of the rGO@Fe3O4 nanohybrid system.

Synthesis of zirconocene complexes and their use in slurry-phase polymerisation of ethylene.

A new family of zirconocene complexes of the type (3-RInd#)2ZrX2 (where Ind# = C6Me5H and R = Me, Et and Ph) have been synthesised and fully characterised. Six new crystal structures have been reported (meso-(3-EtInd#)2ZrBr2, rac-(3-EtInd#)2ZrCl2, rac-(3-EtInd#)2Zr(CH2Ph)2, meso-(3-EtInd#)2Zr(CH2Ph)2, meso-(3-MeInd#)2ZrBr2 and meso-(3-MeInd#)2Zr(CH2Ph)2). The complexes were studied for slurry-phase ethylene polymerisation when immobilised on solid polymethylaluminoxane (sMAO). Variation in the initiation group was found to have greater influence over polymerisation activity for meso-catalysts than rac-catalysts, with meso-alkyl catalysts showing higher polymerisation activities than meso-halide. Below 70 °C, polymerisation activity follows the order sMAO-meso-(3-EtInd#)2Zr(CH2Ph)2, sMAO-meso-(3-EtInd#)2ZrCl2 and sMAO-meso-(3-EtInd#)2ZrBr2 (activities of 657, 561, and 452 kgPE molM−1 h−1 bar−1, respectively). sMAO-meso-(3-EtInd#)2ZrBr2 produces HDPE with the highest molecular weight, followed by sMAO-meso-(3-EtInd#)2ZrCl2 and sMAO-meso-(3-EtInd#)2Zr(CH2Ph)2 (Mw of 503, 406, and 345 kg mol−1, respectively, at 50 °C). sMAO-meso-(3-MeInd#)2ZrBr2 produced HDPE with almost identical molecular weights to sMAO-meso-(3-EtInd#)2ZrCl2 (395 kg mol−1 at 50 °C).

Synthesis, characterisation and ethylene polymerisation performance of silyl bridged peralkylated bis(indenyl) zirconocenes

Six new peralkylated bis(indenyl) zirconocene complexes have been synthesised and fully characterised; rac-­Me2SB(3-­EtI*)ZrX2 and meso-­Me2SB(3­-EtI*)ZrX2 (X = Cl, Br and CH2Ph). Solution-phase ethylene polymerisation using rac-Me2SB(3-­EtI*)ZrCl2/methylaluminoxane (MAO) and slurry-phase ethylene polymerisation using rac-Me2SB(3­-EtI*)ZrCl2 immobilised on solid polymethylaluminoxane (sMAO), MAO modified layered double hydroxide (LDHMAO) and MAO modified silica (SSMAO) were studied. Polymerisation conditions were optimised for sMAO-rac-Me2SB(3-­EtI*)ZrCl2 and applied to the other catalysts. A 75:25 mixture of rac-­ and meso-Me2SB(3­-EtI*)Zr(CH2Ph)2 supported on sMAO showed the highest polymerisation activities (4832 kgPEmolZr−1 h−1 bar−1 at 60 °C), producing high density polyethylenes (HDPE) with modest molecular weights (Mw of 336 kg mol−1 at 60 °C) and molecular weight distributions (Mw/Mn) less than 3.2. sMAO-rac-Me2SB(3-­EtI*)ZrCl2 was found to be responsive to H2 in a high-throughput screening system, producing polyethylenes with low molecular weights (Mw of 41 kg mol−1 with 1.6% H2). In the same high-throughput system, slurry-phase ethylene polymerisation in the presence of 1-hexene comonomer was shown to increase the polymerisation activity of sMAO-rac-Me2SB(3-­EtI*)ZrCl2) compared to polymerisation in the same system without 1-hexene.

Kinetic Analysis as an Optimization Tool for Catalytic Esterification with a Moisture-Tolerant Zirconium Complex.

This work describes the use of kinetics as a tool for rational optimization of an esterification process with down to equimolar ratios of reagents using a recyclable commercially available zirconocene complex in catalytic amounts. In contrast to previously reported group IV metal-catalyzed esterification protocols, the work presented herein circumvents the use of water scavengers and perfluorooctane sulfonate (PFOS) ligands. Insights into the operating mechanism are presented.

Actinide 2-metallabiphenylenes that satisfy Hückel's rule.

Aromaticity and antiaromaticity, as defined by Hückel's rule, are key ideas in organic chemistry, and are both exemplified in biphenylene1-3-a molecule that consists of two benzene rings joined by a four-membered ring at its core. Biphenylene analogues in which one of the benzene rings has been replaced by a different (4n+2) π-electron system have so far been associated only with organic compounds4,5. In addition, efforts to prepare a zirconabiphenylene compound resulted in the isolation of a bis(alkyne) zirconocene complex instead6. Here we report the synthesis and characterization of, to our knowledge, the first 2-metallabiphenylene compounds. Single-crystal X-ray diffraction studies reveal that these complexes have nearly planar, 11-membered metallatricycles with metrical parameters that compare well with those reported for biphenylene. Nuclear magnetic resonance spectroscopy, in addition to nucleus-independent chemical shift calculations, provides evidence that these complexes contain an antiaromatic cyclobutadiene ring and an aromatic benzene ring. Furthermore, spectroscopic evidence, Kohn-Sham molecular orbital compositions and natural bond orbital calculations suggest covalency and delocalization of the uranium f2 electrons with the carbon-containing ligand.

Ethylene-Bridged Multi-Substituted Indenyl-Fluorenyl Zirconocene and Hafnocene Complexes: Synthesis, Structure and Catalytic Behavior for Propylene Selective Oligomerization

Ethylene-Bridged Multi-Substituted Indenyl-Fluorenyl Zirconocene and Hafnocene Complexes: Synthesis, Structure and Catalytic Behavior for Propylene Selective Oligomerization

Bis(alkynyl)borane: A New Class of Acyclic Boron-Containing π Ligands in η5-Coordination Mode.

Reactions between Rosenthal's zirconocene synthon, [Cp2Zr(py)Me3SiC≡CSiMe3] (py = pyridine), and two different types of bis(alkynyl) boranes, (Me3Si)2NB(C≡CSiMe3)2 and MesB(C≡CSiMe3)2 (Mes = 2,4,6-trimethylphenyl), both resulting in the formation of η5-coordinated bis(alkynyl)borane zirconocene complexes. X-ray diffraction analysis revealed their six-membered cyclic structures featuring a boat conformation with considerable Zr-B interactions. The Zr-B bonding strength and conformational lability of the ZrC4B ring are dependent on the π basicity of the exocyclic B substituent. These compounds represent the first examples of η5-coordinated acyclic boron-containing π ligands.

Synthesis and characterization of alkenyl and alkyl substituted group 4 metallocene dichloride complexes: Applications in ethylene polymerization

The alkenyl substituted zirconocene complexes, [Zr(η5-C5H4{CMeRCH2CH2CH = CMe2})2Cl2] (R = Me (1), Ph (2) were prepared from the reaction of the lithium derivative Li(C5H4{CMeRCH2CH2CH = CMe2}) (R = Me, Ph) with zirconium tetrachloride. The bulky alkyl substituted cyclopentadienyl ligand precursor was prepared as its fulvene derivative (C5H4) = CMeCH2CHMe2 (3) via the reaction of cyclopentadiene with 4-methyl-2-pentanone and then converted to Li(C5H4{CMe2CH2CHMe2}) (4) via nucleophilic attack of LiMe. The reaction of 4 with [TiCl4(THF)2] and ZrCl4 gave the metallocene complexes [M(η5-C5H4{CMe2CH2CHMe2})2Cl2] (M = Ti (5), Zr (6)), respectively. The ansa-ligand precursor, SiMe2(C5HMe4)(C5H4{CMe2CH2CHMe2}) (7), was prepared via the reaction of 4 with SiMe2(C5HMe4)Cl and subsequently converted to its dilithium derivate, Li2(Me2Si(C5Me4)(C5H3{CMe2CH2CHMe2})) (8), using n-butyllithium. The ansa-zirconocene complex, [Zr{Me2Si(η5-C5Me4)(η5-C5H3{CMe2CH2CHMe2})}Cl2] (9), was synthesized by the reaction of 8 with zirconium tetrachloride. Compounds 1–9 were characterized by 1H and 13C{1H} NMR spectroscopy. In addition, the molecular structures of 5, 6 and 9 were determined by single crystal X-ray diffraction studies. The metallocene complexes have been tested as catalyst (with co-catalyst MAO) in the polymerization of ethylene where the substituents on the cyclopentadienyl ligand have a direct influence on the catalytic activity.

Ethylene-bridged Indenyl-fluorenyl Metallocene Complexes for Efficient Preparation of Allyl-terminated Propylene Oligomers and Polymers via Selective β-Methyl Transfer

Four C1-symmetric ansa-metallocene complexes, C2H4(Ind)(2,7-tBu2-Flu)ZrCl2 (4), C2H4(3-Bn-Ind)(2,7-tBu2-Flu)ZrCl2 (5), C2H4(3-Bn-Ind)(3,6-tBu2-Flu)ZrCl2 (6), and C2H4(3-Bn-Ind)(2,7-tBu2-Flu)HfCl2 (7), were synthesized and characterized. The structures of complexes 4, 5, and 7 were further determined via X-ray diffraction studies. Upon activation with modified methylaluminoxane (MMAO) or AliBu3/[Ph3C][B(C6F5)4] (TIBA/TrB), most of these complexes showed high efficiency in catalyzing propylene oligomerization/polymerization to afford products dominantly with allyl terminals via selective β-methyl transfer (β-Me transfer). The introduction of 3-benzyl group on the indenyl ring of the complexes was found to be crucial in enabling highly selective β-Me transfer during the polymerization process, leading to selectivities up to 89% obtained by zirconocene complexes 5 and 6, and up to 91% obtained by hafnocene complex 7. Detailed chain-end analysis by 1H-NMR, 13C-NMR, and MALDI-TOF mass spectroscopy revealed that the allyl chain-ends of the polymeric products resulted from a selective β-Me transfer process after two successively primary insertions of the monomer. Further studies concerning the dependence of chain release selectivity as well as the molecular weight of products on monomer concentration suggested that both β-Me transfer (major) and β-hydrogen transfer (β-H transfer) (minor) mediated by 5/MMAO and 6/MMAO systems may mainly operate in a bimolecular pathway.