Cationic rare earth metal alkyls as novel catalysts for olefin polymerization and copolymerization

Cationic rare earth metal alkyl species, generated by the treatment of mono(cyclopentadienyl) bis(alkyl) rare earth metal complexes with 1 equiv. of a borate compound such as [Ph₃C][B(C₆F₅)₄], act as an excellent catalyst for the polymerization and copolymerization of various olefins such as ethylene, 1-hexene, styrene, norbornene, dicyclopentadiene, and isoprene. These catalysts show unprecedented activity and regio- and stereo-selectivity and afford a series of new polymers which are difficult to be prepared previously.     

Problems and solutions for alkene polymerisation catalysts incorporating Schiff-bases; migratory insertion and radical mechanisms of catalyst deactivation

Steric blocking of an intramolecular 1,2-migratory insertion reaction of a zirconium salicylaldiminato complex leads to a long-lived catalyst for ethene polymerisation, but promotes a new radical catalyst decomposition mechanism in certain instances; kinetic and thermodynamic parameters for both pathways have been established.     

Cationic organoaluminum compounds as intramolecular hydroamination catalysts

Cationic dialkylaluminum and m-terphenylalkylaluminum compounds catalyze the intramolecular hydroamination of primary and secondary aminopentenes. The reaction rates are strongly dependent on the substrate and the catalyst substituents. The bulky species [Dipp1AlEt][CHB₁₁H₅I₆] (Dipp1 = 2,6-Dipp₂C₆H₃–, Dipp = 2,6-iPr₂C₆H₃–), 4, was the most active catalyst. Although the neutral species DcpAlEt₂ (Dcp = 2,6-(2,6-Cl₂C₆H₃)₂C₆H₃–), 7, and Dipp1AlEt₂, 8, showed some catalytic activity, they were more than 25 times less reactive than their cationic counterparts [DcpAlEt][CHB₁₁H₅Cl₆], 3, and 4. The cyclization of secondary benzylaminopentenes with [Et₂Al][CHB₁₁H₅I₆], 1, was strongly dependent on the substitution of the C-2 olefinic carbon.     

Cationic organoaluminum compounds as intramolecular hydroamination catalysts

Cationic dialkylaluminum and m-terphenylalkylaluminum compounds catalyze the intramolecular hydroamination of primary and secondary aminopentenes. The reaction rates are strongly dependent on the substrate and the catalyst substituents. The bulky species [Dipp1AlEt][CHB₁₁H₅I₆] (Dipp1 = 2,6-Dipp₂C₆H₃–, Dipp = 2,6-iPr₂C₆H₃–), 4, was the most active catalyst. Although the neutral species DcpAlEt₂ (Dcp = 2,6-(2,6-Cl₂C₆H₃)₂C₆H₃–), 7, and Dipp1AlEt₂, 8, showed some catalytic activity, they were more than 25 times less reactive than their cationic counterparts [DcpAlEt][CHB₁₁H₅Cl₆], 3, and 4. The cyclization of secondary benzylaminopentenes with [Et₂Al][CHB₁₁H₅I₆], 1, was strongly dependent on the substitution of the C-2 olefinic carbon.     

Bifunctional yttrium(III) and titanium(IV) NHC catalysts for lactide polymerisation

Lewis acidic yttrium(III) and titanium(IV) derivatives of anionic, metal-tethered carbenes apparently act as bifunctional catalysts for the polymerisation of D,L-lactide, using a combination of Lewis acid and base functionalities to initiate ring opening of the cyclic monomer; the alcohol- and amino-functionalised carbenes from which they derive provide models for the first insertion step, and also display metal-free polymerisation catalysis to generate polylactic acid.     

Cationic fac-tris(pyrazole) complexes as anion receptors

New receptors fac-[Re(CO)3(pz)3]BAr'4 (pz = 3,5-dimethylpyrazole or 3(5)-tert-butylpyrazole, Ar' = 3,5-(CF3)2C6H3), synthesized from [Re(OTf)(CO)5] and the pyrazoles, have been found to show a high affinity for chloride.     

Hybrid catalysts: the synthesis, structure and ethene polymerisation activity of (salicylaldiminato)(pyrrolylaldiminato) titanium complexes

The mono(salicylaldiminato) complexes Ti{3-tBu-2-(O)C6H3CH=N(R)}Cl3(THF)(where R = C6H5, C6F5) react with the metallated pyrrolylaldiminato ligand, K[2-(C6H5NCH)C4H3N], to afford the first examples of hybrid salicylaldiminato-ligated octahedral titanium complexes; the pre-catalysts give from very high to extremely high ethene polymerisation productivities when activated with MAO.     

From zirconium to titanium: the effect of the metal in propylene polymerisation using fluxional unbridged bicyclic catalysts

Catalytic systems based on unbridged substituted indenyl systems are becoming of interest in the production of elastomeric polypropylene. A full understanding of the structural features necessary to control this kind of behaviour has not yet been achieved, since relatively slight changes in the molecular architecture can lead to polymers with remarkably different properties. We report here our recent findings regarding the study of bicyclic zirconium and titanium complexes as fluxional catalysts in propylene polymerisation. Most of them have been synthesised according to a synthetic procedure that allowed us to prepare a series of complexes in which the ring fused to the cyclopentadienyl moiety is saturated and of different sizes, thus introducing a flexibility parameter that can be finely tuned. The results obtained show that the stereoselectivity induced by this class of catalysts strongly depends both on the structure of the ligand and on the nature of metal atom (Zr vs. Ti). The titanium-based catalysts yield polypropylenes with new and interesting microstructures, in particular when an higher stability is achieved through a careful choice of the substitution pattern of the ligands.     

Novel ruthenium(II)2 carboxylates as catalysts for alkene metathesis

The reactions of [Ru-(=CHR)Cl2(PCy3)2] (1: R = Ph; 1a: R = -CH=CPh2) with silver salts of carboxylic acids afforded new dimeric complexes of the general formula [Ru2(=CHR)2-(R'CO2)2(mu-R'CO2)2(PCy3)2(mu-H2O)] (2: R = Ph, R' = CF3; 3: R = Ph, R' = C2F5; 4: R = -CH=CPh2, R' = CF3; 5: R = Ph, R' = C6F5; 6: R = -CH=CPh2, R' = C6F5; 7: R = -CH=CPh2, R'=CCl3) in good yields. With R' = CF3, C2F5 or CCl3 these complexes are active catalysts for metathesis of acyclic alkenes, including unsaturated fatty acid esters, as well as for ring closing metathesis. The reactivity of these complexes with bases and weak donor solvents has been studied and their half-life times in several media were determined.     

Reactivity of secondary metallocene alkyls and the question of dormant sites in catalytic alkene polymerization

The reactivity of [rac-(C2H4(1-indenyl)2)Zr(n-butyl)][MeB(C6F5)3] (4), [rac-(C2H4(1-indenyl)2)Zr(sec-butyl)][MeB(C6F5)3] (5), and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3] with propene, ethene, and hydrogen was studied by low-temperature (<-40 degrees C) 1H and 13C NMR spectroscopy in toluene solutions. In contrast with previous suggestions that 2 degrees zirconium alkyl species such as 5 are dormant sites, these measurements demonstrate reactivity of 2 degrees zirconium alkyls with propene and ethene comparable to the 1 degrees zirconium alkyl species 4 and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3]. Because 2,1-insertion of propene is an infrequent event, these results preclude significant accumulation of catalyst in the form of 2 degrees zirconium alkyls for this metallocene and counterion. The reactivity of 5 with hydrogen is at least 2 orders of magnitude faster than other 1 degrees zirconium alkyls. Such high reactivity accounts for the puzzlingly high fraction of butyl end groups in prior hydrooligomerization studies and implies that catalyst responsivity to H2 as a molecular weight control agent correlates with the regioselectivity of the catalyst.     

Design and understanding of heterogeneous alkene metathesis catalysts

This review outlines the molecular insights provided by surface organometallic chemistry in the area of alkene metathesis, and particularly in the development and the understanding of heterogeneous catalysts.     

Protic metal-containing ionic liquids as catalysts: Cooperative effects between anion and cation

HCo(CO)₄ is known to be the active species in the cobalt-catalyzed hydroformylation reaction. Although it is known that the anion [Co(CO)₄]⁻ is catalytically inactive, some cobalt carbonyl-containing ionic liquids are surprisingly able to catalyze hydroformylation reactions. However, only ionic liquids with protic cations demonstrate activity, whilst aprotic cations such as BMIM⁺ result in a completely inactive compound. The four applied cobalt-containing ionic liquids differ only by the cation component. Their different performance in catalytic activity allows the presumption of cooperative effects between the cation and the anion. These fundamental influences of the cation on the hydroformylation kinetics give hints for the reaction mechanism of biphasic hydroformylation reactions as well as on the reaction pathways of the conventional hydroformylation reaction under different reaction conditions.     

Kinetic and mechanistic aspects of metallocene polymerisation catalysts

The nature of the counteranion is an essential component of metallocene polymerisation catalysts. Detailed mechanistic investigations show how the anion is able to determine the activity and, in many cases, also the stereoselectivity of the catalyst. This review summarises recent advances in mechanistic understanding of well defined metallocene catalysts based on ion pairs [L₂ZrR⁺ ?X⁻] and describes recent insights in ion mobility and kinetics of alkene polymerisation processes. The interplay of ligand structure and nature of the counteranions demonstrates a fascinating versatility and subtlety that continually challenge our ability to rationalise and predict catalyst performance.     

Cationic zinc enolates as highly active catalysts for acrylate polymerization

Unprecedented cationic zinc enolates have been generated by a novel activation route involving the amido to imino ligand transformation with B(C6F5)3, structurally characterized, and utilized as highly active catalysts for the production of high molecular weight polyacrylates at ambient temperature.     

Monoalkyl,chiral-at-metal 'constrained geometry' complexes as efficient alpha-olefin and methyl methacrylate polymerisation catalysts

A new class of monoalkyl or monochloro constrained geometry group 4 complexes has been synthesized; upon activation with aluminium activators they serve as efficient catalysts for olefin polymerisation and for polymerisation of methyl methacrylate.     

The catalysis of alkene isomerisation,oligomerisation, and polymerisation by cationic nitrosylrhodium complexes

The complex [Rh(NO)(NCMe)₄][BF₄]₂ catalyses the isomerisation of terminal to internal alkenes, the oligomerisation of branched alkenes such as 2-methylpropene, and the stereospecific polymerisation of buta-1,3-diene to trans-1,4-polybutadiene.     

Chromium imine and amine complexes as homogeneous catalysts for the trimerisation and polymerisation of ethylene

Cr(III) complexes of tridendate imine and amine ligands with N, P, O, S donor atoms 1 and 2 have been prepared and tested as catalysts in the oligomerisation and polymerisation of ethylene giving excellent selectivity towards 1-hexene and polymerisation to polyethylene when activated with cocatalysts. X-ray structure analyses of the precatalysts 1a-c, 1i, and 2b are investigated. The metal-ligand binding in 1a and 1b is nearly the same, which leads to similar catalytic activities of these precatalysts.     

Reactions of strained hydrocarbons with alkene and alkyne metathesis catalysts

Here we describe the metathesis reactions of a strained eight-membered ring that contains both alkene and alkyne functionality. We find that the alkyne metathesis catalyst produces polymer through a ring-opening alkyne metathesis reaction that is driven by the strain release from the monomer. The strained monomer provides unusual reactivity with ruthenium-based alkene metathesis catalysts. We isolate a discrete trimeric species a Dewar benzene derivative that is locked in this form through an unsaturated cyclophane strap.