Triptycene as a scaffold in metallocene catalyzed olefin polymerization.

A set of metallocene olefin polymerization catalysts bearing triptycene moieties in either position 4-5 (complexes Ty1-Ty5) or in position 5-6 (complexes Ty6-Ty8) of the basic dimethylsilyl-bridged bis(indenyl) system has been tested in propene polymerization and in ethene/1-hexene copolymerization. Comparison of the results with QSPR (quantitative structure-property relationship) predictions not parametrized for these exotic ligand variations demonstrates that trends can still be identified by extrapolation. Interestingly, Ty7, upon suitable activation, provides a highly isotactic polypropylene with an exceptional amount of 2,1 regio-errors (8%).

Gold-Catalysed Heck Reaction: Fact or Fiction? Correspondence on "Unlocking the Chain Walking Process in Gold Catalysis".

Two recent high-profile publications reported the formation of Heck-type arylated alkenes catalysed by MeDalPhosAuCl/AgOTf (J. Am. Chem.

Revisiting Stereoselective Propene Polymerization Mechanisms: Insights through the Activation Strain Model.

The stereoelectronic factors responsible for stereoselectivity in propene polymerization with several metallocene and post-metallocene transition metal catalysts have been revisited using a combined approach of DFT calculations, the Activation Strain Model, Natural Energy Decomposition Analysis and a molecular descriptor (%V). There are in most cases two different paths leading to the formation of stereoerrors (SE), and the classical model does not suffice to fully understand stereoregulation. Improving stereoselectivity requires raising the energies of both SE insertion transition states.

Cocatalyst effects in Hf-catalysed olefin polymerization: taking well-defined Al-alkyl borate salts into account.

Hafnium catalysts for olefin polymerization are often very sensitive to the nature of cocatalysts, especially if they contain "free" aluminium trialkyls. Herein, cocatalyst effects in Hf-catalysed propene polymerization are examined for four Hf catalysts belonging to the family of -symmetric (Hf-CS-Met) and -symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu(PhNMe)Al](μ-H)}[B(CF)] (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium.

Octahedral Zirconium Salan Catalysts for Olefin Polymerization: Substituent and Solvent Effects on Structure and Dynamics.

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a isomer (MM); formation of the polymerization active species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes in energy than MM.

A Hydrocarbon Soluble, Molecular and "Complete" Al-Cocatalyst for High Temperature Olefin Polymerization.

The dinuclear aluminum salt {[Bu(DMA)Al](-H)}[B(CF)] (; DMA = ,-dimethylaniline) is the prototype of a new class of molecular cocatalysts for catalytic olefin polymerization, its modular nature offering easy avenues for tailoring the activator to specific needs. We report here, as proof of concept, a first variant () bearing -hexadecyl-,-dimethylaniline (DMA) units, which enhances solubility in aliphatic hydrocarbons. The novel was used successfully as an activator/scavenger in ethylene/1-hexene copolymerization in a high-temperature solution process.

Tweaking the bridge in metallocene Zr(IV)/W(IV) bimetallic hydrides.

Zirconocene cations react with CpWH affording the bimetallic [CpZr(μ-H)(μ-η:η-CH)WHCp] bridging hydride 1 (Cp = cyclopentadienyl anion, CH) σ-bond metathesis. Complex 1 features an atypical out of plane Zr(μ-H)W moiety, where no intermetallic interaction is involved, and a fluxional core. Coordination geometry and bond distances of the bridging hydride interaction can be modulated upon reaction with Lewis bases and unsaturated substrates.

Unconventional Stereoerror Formation Mechanisms in Nonmetallocene Propene Polymerization Systems Revealed by DFT Calculations.

An unconventional mechanism for the stereoerror formation in propene polymerization catalyzed by -symmetric salalen-M systems (M = Zr, Hf) is suggested by DFT calculations. While propagation happens with the ligand in its conformation, a change of ligand wrapping mode from to is the main source of the lower stereoselectivities obtained with Zr and Hf. This is different for the Ti analogues, where the ligand - wrapping mode does not play a role.

A High-Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure-activity correlations. The upcycling conditions involve activation of the catalysts (at 0.

Hafnium vs. Zirconium, the Perpetual Battle for Supremacy in Catalytic Olefin Polymerization: A Simple Matter of Electrophilicity?

The performance of -symmetric -hafnocene catalysts for isotactic polypropylene typically deteriorates at increasing temperature much faster than that of their zirconium analogues. Herein, we analyze in detail a set of five Hf/Zr metallocene pairs-including some of the latest generation catalysts-at medium- to high-polymerization temperature. Quantitative structure-activity relationship (QSAR) models for stereoselectivity, the ratio allyl/vinyl chain ends, and 2,1/3,1 misinsertions in the polymer indicate a strong dependence of polymerization performance on electrophilicity of the catalyst, which is a function of the ligand framework and the metal center.

-Zirconocene Catalysts for Isotactic-Selective Propene Polymerization at High Temperature: A Long Story Finds a Happy Ending.

Absolute rigidity is rare in the "soft" world of organometallics. Here we introduce two cyclopenta[]triptycyl -zirconocene catalysts for isotactic-selective propene polymerization, designed by means of an integrated high-throughput experimentation/quantitative structure-activity relationship modeling approach. An ultrarigid ligand precisely wrapped around the Zr center enforces an enzyme-like lock and key fit, effectively hampering undesired reactive events, even at high temperature.

On the limits of tuning comonomer affinity of 'Spaleck-type'ansa-zirconocenes in ethene/1-hexene copolymerization: a high-throughput experimentation/QSAR approach.

For a set of 40 silicon-bridged C-symmetric ansa-zirconocenes, reactivity ratios in ethene/1-hexene copolymerization were experimentally determined by means of an accurate high-throughput experimentation (HTE) approach, and used to develop quantitative structure-activity relationship (QSAR) models for comonomer affinity using chemically meaningful descriptors. These QSAR models rely almost exclusively on steric descriptors, with the single most important descriptor being the 'openness' of the open quadrants. Catalysts with an unobstructed main insertion pathway, i.

An Integrated High Throughput Experimentation/Predictive QSAR Modeling Approach to -Zirconocene Catalysts for Isotactic Polypropylene.

Compared to heterogenous Ziegler-Natta systems (ZNS), -metallocene catalysts for the industrial production of isotactic polypropylene feature a higher cost-to-performance balance. In particular, the -symmetric (indenyl) -zirconocenes disclosed in the 1990s are complex to prepare, less stereo- and/or regioselective than ZNS, and lose performance at practical application temperatures. The golden era of these complexes, though, was before High Throughput Experimentation (HTE) could contribute significantly to their evolution.

Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications.

The established model cluster (AlOMe)(AlMe) for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di--butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites - to release neutral Al fragments (i.e.

H activation by zirconaziridinium ions: σ-bond metathesis versus frustrated Lewis pair reactivity.

Zirconaziridinium ions [CpZr(η-CHNR)] can potentially activate H by two routes: σ-bond metathesis or FLP reactivity. We show here that Zr-C hydrogenolysis by σ-bond metathesis precedes and enables subsequent heterolytic H cleavage through FLP pathways. DFT calculations show the involvement of transition states with approximately linear NHH and bent ZrHH arrangements without any direct Zr-amine interaction.

Do Gold(III) Complexes Form Hydrogen Bonds? An Exploration of Au Dicarboranyl Chemistry.

The reaction of 1,1'-Li [(2,2'-C B H ) ] with the cyclometallated gold(III) complex (C^N)AuCl afforded the first examples of gold(III) dicarboranyl complexes. The reactivity of these complexes is subject to the trans-influence exerted by the dicarboranyl ligand, which is substantially weaker than that of non-carboranyl anionic C-ligands. In line with this, displacement of coordinated pyridine by chloride is only possible under forcing conditions.

Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage.

The coordinatively unsaturated gold(iii) chelate complex [(C^N-CH)Au(CF)] ( ) reacts with main group hydrides H-BPin and H-SiEt in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(CHBu )-6-(CHBu )pyridine anion; Pin = OCMeCMeO). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuCF}(μ-H)] ( ) and (C^N-CH)AuH(CF) (), together with spectroscopically detected [EtSi-OEt]. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature (-20 °C).

Isocyanide insertion into Au-H bonds: first gold iminoformyl complexes.

Isocyanides insert into gold(iii)-hydrogen bonds to give the first examples of gold iminoformyl complexes. The reaction is initiated by catalytic amounts of radicals; DFT calculations indicate that this is an equilibrium reaction driven forward by isocyanide in sufficient excess to trap the Au(ii) intermediate.

Reductive Elimination Leading to C-C Bond Formation in Gold(III) Complexes: A Mechanistic and Computational Study.

The factors affecting the rates of reductive C-C cross-coupling reactions in gold(III) aryls were studied by using complexes that allow easy access to a series of electronically modified aryl ligands, as well as to gold methyl and vinyl complexes, by using the pincer compounds [(C^N^C)AuR] (R=C F , CH=CMe , Me and p-C H X, where X=OMe, F, H, tBu, Cl, CF , or NO ) as starting materials (C^N^C=2,6-(4'-tBuC H ) pyridine dianion). Protodeauration followed by addition of one equivalent SMe leads to the quantitative generation of the thioether complexes [(C^N-CH)AuR(SMe )] . Upon addition of a second SMe pyridine is displaced, which triggers the reductive aryl-R elimination.

E-Z isomerization in Suzuki cross-couplings of haloenones: ligand effects and evidence for a separate catalytic cycle.

Suzuki cross-coupling of haloalkenes is generally assumed to occur with retention of the alkene stereochemistry. While studying Suzuki cross-couplings on E-1,2-dichlorovinyl phenyl ketone, we were surprised to observe extensive isomerization. More surprisingly, the ligand employed strongly influenced the degree of isomerization: DPEphos and Xantphos led to 96% isomerized cross-coupled product whereas reactions in the absence of a phosphine ligand, or reactions employing t-BuXantphos, gave 94% retention of stereochemistry.

Gold(III) Alkyne Complexes: Bonding and Reaction Pathways.

The synthesis and characterization of hitherto hypothetical Au π-alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of Pt and their drastically more reactive Au congeners.

Formation of Gold(III) Alkyls from Gold Alkoxide Complexes.

The gold(III) methoxide complex (CNC)AuOMe () reacts with tris(-tolyl)phosphine in benzene at room temperature under O abstraction to give the methylgold product (CNC)AuMe () together with O=P(-tol) ((CNC) = [2,6-(CH Bu-4)pyridine]). Calculations show that this reaction is energetically favorable (Δ = -32.3 kcal mol).

Insight into Oxide-Bridged Heterobimetallic Al/Zr Olefin Polymerization Catalysts.

Reaction of (TBBP)AlMe⋅THF with [Cp* Zr(Me)OH] gave [(TBBP)Al(THF)-O-Zr(Me)Cp* ] (TBBP=3,3',5,5'-tetra-tBu-2,2'-biphenolato). Reaction of [DIPPnacnacAl(Me)-O-Zr(Me)Cp ] with [PhMe NH] [B(C F ) ] gave a cationic Al/Zr complex that could be structurally characterized as its THF adduct [(DIPPnacnac)Al(Me)-O-Zr(THF)Cp ] [B(C F ) ] (DIPPnacnac=HC[(Me)C=N(2,6-iPr -C H )] ). The first complex polymerizes ethene in the presence of an alkylaluminum scavenger but in the absence of methylalumoxane (MAO).

Stereo- and Regioselective Alkyne Hydrometallation with Gold(III) Hydrides.

The hydroauration of internal and terminal alkynes by gold(III) hydride complexes [(C^N^C)AuH] was found to be mediated by radicals and proceeds by an unexpected binuclear outer-sphere mechanism to cleanly form trans-insertion products. Radical precursors such as azobisisobutyronitrile lead to a drastic rate enhancement. DFT calculations support the proposed radical mechanism, with very low activation barriers, and rule out mononuclear mechanistic alternatives.