Understanding the Role of Metallocenium Ion-Pair Aggregates on the Rate of Olefin Insertion into the Metal–Carbon Bond

Diffusion Nuclear Magnetic Resonance Measurements on Cationic Gold (I) Complexes in Catalytic Conditions: Counterion and Solvent Effects

The amount of free ions, ion pairs, and higher aggregate of the possible species present in a solution during the gold(I)-catalyzed alkoxylation of unsaturated hydrocarbon, i.e., ISIP (inner sphere ion pair) [(NHC)AuX] and OSIP (outer sphere ion pairs) [(NHC)Au(TME)X] [NHC 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene; TME = tetramethylethylene (2,3-bis methyl-butene); X- = Cl-, BF4-, OTf-; and OTs- BArF4- (ArF = 3,5-(CF3)2C6H3)], has been determined. The 1H and 19F DOSY NMR measurements conducted in catalytic conditions indicate that the dissociation degree (α) of the equilibrium ion pair/free ions {[(NHC)Au(TME)X] [(NHC)Au(TME)]+ + X-} depends on the nature of the counterion (X-) when chloroform is the catalytic solvent: while the compounds containing OTs- and OTf- as the counterion gave a low α (which means a high number of ion pairs) of 0.13 and 0.24, respectively, the compounds containing BF4- and BArF4- showed higher α values of 0.36 and 0.32, respectively. These results experimentally confirm previous deductions based on catalytic and theoretical data: the lower the α value, the greater the catalytic activity because the anion that can activate methanol during a nucleophilic attack, although the lower propensity to activate methanol of BF4- and BArF4-, as suggested by the DFT calculations, cannot be completely overlooked. As for the effect of the solvent, α increases as the dielectric constant increases, as expected, and in particular, green solvents with high dielectric constants show a very high α (0.90, 0.84, 0.80, and 0.70 for propylene carbonate, γ-valerolactone, acetone, and methanol, respectively), thus confirming that the moderately high activity of NHC-Au-OTf in these solvents is due to the specific effect of polar functionalities (O-H, C=O, O-R) in activating methanol. Finally, the DOSY measurements conducted in p-Cymene show the formation of quadrupole species: under these conditions, the anion can better exercise its 'template' and 'activating' roles, giving the highest TOF.

Ammonium Tetrakis(pentafluorophenyl)borate: Preparation and Application in Olefin Coordination Polymerization as the Cocatalyst Compound

Metallocene catalysts have attracted much attention from academia and industry for their excellent catalytic activity in the field of olefin polymerization. Cocatalysts play a key role in metallocene catalytic systems, which can not only affect the overall catalytic activity, but also have an obvious influence on the structure and properties of the polymer. Although methylaluminoxane (MAO) is currently the most widely used cocatalyst, its price increases the production cost of polyolefin materials. Ammonium tetrakis(pentafluorophenyl)borate has shown excellent performance in polymerization, being one of the best substitutes for the traditional cocatalyst MAO. Compared with the main catalyst, whose composition and structure are relatively complex, the research on cocatalyst is very limited. This review mainly introduces the research history, preparation methods, and application progress in polymerization of ammonium tetrakis(pentafluorophenyl)borate, deepening our understanding of the role of cocatalyst in polymerization, with the hope of inspiring brand-new thinking on improving and enhancing the overall performance of catalyst systems.

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 CS-symmetric (Hf-CS-Met) and C2-symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu2(PhNMe2)Al]2(μ-H)}+[B(C6F5)4]- (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium. The worst catalytic performance is observed with MAO. Conversely, the best cocatalyst varies depending on the Hf catalyst used and the performance indicator of interest, highlighting the complexity and importance of selecting the right precatalyst/cocatalyst combination. AlHAl proved to be a suitable system for all catalysts tested and, in some cases, it provides the best performance in terms of productivity (e.g. with hafnocenes). Furthermore, it generally leads to high molecular weight polymers, also with catalysts enabling easy chain transfer to Al like Hf-PyAm. This suggests that AlHAl has a low tendency to form heterodinuclear adducts with the cationic active species, therefore preventing the formation of dormant sites and/or termination events by chain transfer to Al.

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 mer-mer isomer (MM); formation of the polymerization active fac-fac 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 lower in energy than MM. Detailed in situ NMR studies of a fast (o-anthracenyl) and a slow (o-tBu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o-anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (ΔS > 0, dissociative) and the slow catalyst (ΔS < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho-substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.

Fluorescent organometallic dyads and triads: establishing spatial relationships

FRET pairs involving up to three different Bodipy dyes are utilized to provide information on the assembly/disassembly of organometallic complexes. Azolium salts tagged with chemically robust and photostable blue or green or red fluorescent Bodipy, respectively, were synthesized and the azolium salts used to prepare metal complexes [(NHC_blue)ML], [(NHC_green)ML] and [(NHC_red)ML] (ML = Pd(allyl)Cl, IrCl(cod), RhCl(cod), AuCl, Au(NTf₂), CuBr). The blue and the green Bodipy and the green and the red Bodipy, respectively, were designed to allow the formation of efficient FRET pairs with minimal cross-talk. Organometallic dyads formed from two subunits enable the transfer of excitation energy from the donor dye to the acceptor dye. The blue, green and red emission provide three information channels on the formation of complexes, which is demonstrated for alkyne or sulfur bridged digold species and for ion pairing of a red fluorescent cation and a green fluorescent anion. This approach is extended to probe an assembly of three different subunits. In such a triad, each component is tagged with either a blue, a green or a red Bodipy and the energy transfer blue →green → red proves the formation of the triad. The tagging of molecular components with robust fluorophores can be a general strategy in (organometallic) chemistry to establish connectivities for binuclear catalyst resting states and binuclear catalyst decomposition products in homogeneous catalysis.

Transition Metal-(μ-Cl)-Aluminum Bonding in α-Olefin and Diene Chemistry

Olefin and diene transformations, catalyzed by organoaluminum-activated metal complexes, are widely used in synthetic organic chemistry and form the basis of major petrochemical processes. However, the role of M−(μ-Cl)−Al bonding, being proven for certain >C=C< functionalization reactions, remains unclear and debated for essentially more important industrial processes such as oligomerization and polymerization of α-olefins and conjugated dienes. Numerous publications indirectly point at the significance of M−(μ-Cl)−Al bonding in Ziegler−Natta and related transformations, but only a few studies contain experimental or at least theoretical evidence of the involvement of M−(μ-Cl)−Al species into catalytic cycles. In the present review, we have compiled data on the formation of M−(μ-Cl)−Al complexes (M = Ti, Zr, V, Cr, Ni), their molecular structure, and reactivity towards olefins and dienes. The possible role of similar complexes in the functionalization, oligomerization and polymerization of α-olefins and dienes is discussed in the present review through the prism of the further development of Ziegler−Natta processes and beyond.

Effect of Borate Cocatalysts toward Activity and Comonomer Incorporation in Ethylene Copolymerization by Half-Titanocene Catalysts in Methylcyclohexane

Ethylene copolymerizations with 2-methyl-1-pentene, 1-dodecene (DD), vinylcyclohexane (VCH), [Me₂Si(C₅Me₄)(N ^t Bu)]TiCl₂ (1), Cp*TiMe₂(O-2,6- ⁱ Pr₂-4-RC₆H₂) [R = H (2), SiEt₃ (3)]-borate, and [A(H)]⁺[BAr₄]^- [Ar = C₆F₅; A(H)⁺ = N⁺(H)Me(n-C₁₈H₃₇)₂, N⁺(H)(CH₂CF₃)(n-C₁₈H₃₇)₂, HO⁺(n-C₁₄H₂₉)₂·O(n-C₁₄H₂₉)₂, HO⁺(n-C₁₆H₃₃)₂·O(n-C₁₆H₃₃)₂; Ar = C₁₀F₇, A(H)⁺ = HO⁺(n-C₁₄H₂₉)₂·O(n-C₁₄H₂₉)₂ (B5), N⁺(H)(CH₂CF₃)(n-C₁₈H₃₇)₂] catalyst systems conducted in methylcyclohexane (MCH) exhibited better comonomer incorporation than those conducted in toluene (in the presence of methylaluminoxane (MAO) or borate cocatalysts). The activity was affected by the borate cocatalyst and 1,3-B5 catalyst systems in MCH and showed the highest activity in the ethylene copolymerizations with VCH and DD.

Partial Double Metal-Carbon Bonding Model in Transition Metal Methyl Compounds

The chemical bond between a transition metal and a methyl group (M-CH₃) is typically defined as a single covalent bond, which is of fundamental significance and general interest in understanding the structural properties and reactivity of transition metal alkyl compounds. Herein, we demonstrate that the M-CH₃ bonding involves varying σ and π components and thus should be best described in terms of the partial double M═CH₃ bond. The often-neglected π bonding stems from an occupied π-symmetric orbital of the methyl group comprising all three C-H σ bonds (but one C-H' contributes more than the other two) and a vacant low-lying metal d(π) orbital, and is associated with the intramolecular C-H'···M agostic effect (i.e., an acute M-C-H' angle and a short H'···M distance), whose origin is still controversial. We quantify the geometric and energetic impacts of the π interaction involved in the M-CH₃ bond by explicitly computing the intramolecular π_CH' → d_M interaction with the ab initio valence bond (VB) theory. Our computations of the ligand-free [TiCH₃]³⁺ and a series of metallocene catalysts provide a direct proof for the presence of the π bonding in M-CH₃ bonds, which is the cause for the agostic effect. The partial double M═CH₃ bonding model is not only validated by a range of bonding analyses including VB self-consistent field (VBSCF)-based energy decomposition and quantum theory of atoms in molecules (QTAIM) but also authenticated by the specific activity of double M═CH₃ bonds in the C-H activation and olefin insertion. More importantly, the σ bond gradually switches from a classical covalent bond to a novel charge-shift bond with the π bonding becoming increasingly significant. We anticipate that the recognition of the π interaction between electrophilic metal centers and C-H bonds can benefit the understanding of the nature of metal-carbon bonds in transition metal ethyl, alkyl, and carbene compounds.

Highly efficient vinylic addition polymerization of 5-vinyl-2-norbornene using benzylic palladium complexes as precatalysts

A very small amount of a palladium benzylic complex as initiator brings about the vinylic addition polymerization of the reluctant 5-vinyl-2-norbornene.

Preparation of Pyridylamido Hafnium Complexes for Coordinative Chain Transfer Polymerization

The pyridylamido hafnium complex (I) discovered at Dow is a flagship catalyst among postmetallocenes, which are used in the polyolefin industry for PO-chain growth from a chain transfer agent, dialkylzinc. In the present work, with the aim to block a possible deactivation process in prototype compound I, the corresponding derivatives were prepared. A series of pyridylamido Hf complexes were prepared by replacing the 2,6-diisopropylphenylamido part in I with various 2,6-R2C6H3N-moieties (R = cycloheptyl, cyclohexyl, cyclopentyl, 3-pentyl, ethyl, or Ph) or by replacing 2-iPrC6H4C(H)- in I with the simple PhC(H)-moiety. The isopropyl substituent in the 2-iPrC6H4C(H)-moiety influences not only the geometry of the structures (revealed by X-ray crystallography), but also catalytic performance. In the complexes bearing the 2-iPrC6H4C(H)-moiety, the chelation framework forms a plane; however, this framework is distorted in the complexes containing the PhC(H)-moiety. The ability to incorporate α-olefin decreased upon replacing 2-iPrC6H4C(H)-with the PhC(H)-moiety. The complexes carrying the 2,6-di(cycloheptyl)phenylamido or 2,6-di(cyclohexyl)phenylamido moiety (replacing the 2,6-diisopropylphenylamido part in I) showed somewhat higher activity with greater longevity than did prototype catalyst I.

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

The established model cluster (AlOMe)16(AlMe3)6 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-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A-C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.