A Bulky Pd(II) α-Diimine Catalyst Supported on Sulfated Zirconia for the Polymerization of Ethylene and Copolymerization of Ethylene and Methyl Acrylate

Alkene Isomerization Using a Heterogeneous Nickel-Hydride Catalyst

Transition metal-catalyzed alkene isomerization is an enabling technology used to install an alkene distal to its original site. Due to their well-defined structure, homogeneous catalysts can be fine-tuned to optimize reactivity, stereoselectivity, and positional selectivity, but they often suffer from instability and nonrecyclability. Heterogeneous catalysts are generally highly robust but continue to lack active-site specificity and are challenging to rationally improve through structural modification. Known single-site heterogeneous catalysts for alkene isomerization utilize precious metals and bespoke, expensive, and synthetically intense supports. Additionally, they generally have mediocre reactivity, inspiring us to develop a heterogeneous catalyst with an active site made from readily available compounds made of Earth-abundant elements. Previous work demonstrated that a very active homogeneous catalyst is formed upon protonation of Ni[P(OEt)3]4 by H2SO4, generating a [Ni-H]+ active site. This catalyst is incredibly active, but also decomposes readily, which severely limits its utility. Herein we show that by using a solid acid (sulfated zirconia, SZO300), not only is this decomposition prevented, but high activity is maintained, improved selectivity is achieved, and a broader scope of functional groups is tolerated. Preliminary mechanistic experiments suggest that the catalytic reaction likely goes through an intermolecular, two-electron pathway. A detailed kinetic study comparing the state-of-the-art Ni and Pd isomerization catalysts reveals that the highest activity and selectivity is seen with the Ni/SZO300 system. The reactivity of Ni/SZO300, is not limited to alkene isomerization; it is also a competent catalyst for hydroalkenylation, hydroboration, and hydrosilylation, demonstrating the broad application of this heterogeneous catalyst.

A co-anchoring strategy for the synthesis of polar bimodal polyethylene

Since polar groups can poison the metal centers in catalysts, the incorporation of polar comonomers usually comes at the expense of catalytic activity and polymer molecular weight. In this contribution, we demonstrate polar bimodal polyethylene as a potential solution to this trade-off. The more-polar/more-branched low-molecular-weight fraction provides polarity and processability, while the less-polar/less-branched high-molecular-weight fraction provides mechanical and melt properties. To achieve high miscibility between these two fractions, three synthetic routes are investigated: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy (CAS) to heterogenize different homogeneous catalysts on one solid support. The CAS route is the only viable strategy for the synthesis of polar bimodal polyethylene with good molecular level entanglement and minimal phase separation. This produces polyolefin materials with excellent mechanical properties, surface/dyeing properties, gas barrier properties, as well as extrudability and 3D-printability.

Effects of surface acidity on the structure of organometallics supported on oxide surfaces

Well-defined organometallics supported on high surface area oxides are promising heterogeneous catalysts. An important design factor in these materials is how the metal interacts with the functionalities on an oxide support, commonly anionic X-type ligands derived from the reaction of an organometallic M-R with an -OH site on the oxide. The metal can either form a covalent M-O bond or form an electrostatic M⁺⋯^-O ion-pair, which impacts how well-defined organometallics will interact with substrates in catalytic reactions. A less common reaction pathway involves the reaction of a Lewis site on the oxide with the organometallic, resulting in abstraction to form an ion-pair, which is relevant to industrial olefin polymerization catalysts. This Feature Article views the spectrum of reactivity between an organometallic and an oxide through the prism of Brønsted and/or Lewis acidity of surface sites and draws analogies to the molecular frame where Lewis and Brønsted acids are known to form reactive ion-pairs. Applications of the well-defined sites developed in this article are also discussed.

Ethylene Polymerization through Neutral Nickel Complexes Bearing Cyclic Imides

The catalyst synthesis of salicylaldimine Ni(II) complexes with bulky imide moieties, ethylene polymerization, and characterization of synthesized polyethylenes are described in this paper. These Ni(II) complexes are designed to bear 2-aminobiphenyl and 4-tritylaniline. Results confirmed relatively high activity (up to $4\times {10}^4$  g PE mol Ni-1 h-1) of these catalysts in ethylene polymerization. Moreover, Ni(II) complexes demonstrated enhanced thermal stability, maintaining activity level up to 80°C. The generated polyethylenes possess moderate branching density and high melting temperatures. Less bulky 2-aminobiphenyl group resulted in higher branch content, while in Ni(II) complexes bearing 4-tritylaniline, more linear structure was observed. These semicrystalline polyethylenes showed mechanical properties similar to thermoplastics.

A general strategy for heterogenizing olefin polymerization catalysts and the synthesis of polyolefins and composites

The heterogenization of homogeneous metal complexes on solid supports presents an efficient strategy for bridging homogeneous catalysts with industrially-preferred heterogeneous catalysts; however, a series of drawbacks restrict their implementation in olefin polymerization, particularly for copolymerization with polar comonomers. In this contribution, we report an ionic anchoring strategy that is highly versatile, generally applicable to different systems, and enables strong catalyst-support interactions while tolerating various polar functional groups. In addition to greatly enhanced polymerization properties, the supported catalysts achieved higher comonomer incorporation than their unsupported counterparts. This strategy enabled efficient polymerization at high temperatures at large scale and great control over product morphology, and the facile synthesis of polyolefin composites. More importantly, the dispersion of different fillers in the polyolefin matrix produced great material properties even at low composite loadings. It is expected that this strategy will find applications in different catalytic systems and the synthesis of advanced engineering materials.

Multivariate Linear Regression Models to Predict Monomer Poisoning Effect in Ethylene/Polar Monomer Copolymerization Catalyzed by Late Transition Metals

This study combined density functional theory (DFT) calculations and multivariate linear regression (MLR) to analyze the monomer poisoning effect in ethylene/polar monomer copolymerization catalyzed by the Brookhart-type catalysts. The calculation results showed that the poisoning effect of polar monomers with relatively electron-deficient functional groups is weaker, such as ethers, and halogens. On the contrary, polar monomers with electron-rich functional groups (carbonyl, carboxyl, and acyl groups) exert a stronger poisoning effect. In addition, three descriptors that significantly affect the poisoning effect have been proposed on the basis of the multiple linear regression model, viz., the chemical shift of the vinyl carbon atom and heteroatom of polar monomer as well as the metal-X distance in the σ-coordination structure. It is expected that these models could guide the development of efficient catalytic copolymerization system in this field.

Hydrogen-Bonding-Induced Heterogenization of Nickel and Palladium Catalysts for Copolymerization of Ethylene with Polar Monomers

The practical synthesis of polar-functionalized polyolefins using transition-metal-catalyzed copolymerization of olefins with polar monomers is a challenge; the use of heterogeneous catalysts is little explored. Herein, we report the synthesis of heterogeneous naphthoquinone-based nickel (Ni/SiO₂ ) and palladium (Pd/SiO₂ ) catalysts through hydrogen bonding interactions of the ligands with the silica surface. Ni/SiO₂ exhibits high activities (up to 2.65×10⁶  g mol^-1  h^-1 ) during the copolymerization of ethylene with 5-hexene-1-yl-acetate, affording high-molecular-weight (M_n up to 630 000) polar-functionalized semicrystalline polyethylene (comonomer incorporation up to 2.8 mol %), along with great morphology control. The resulting copolymers possess improved surface properties and great mechanical properties. Pd/SiO₂ can mediate ethylene copolymerization with polar monomers with moderate activity to produce high-molecular-weight copolymers with tunable comonomer incorporation.

Ethylene Polymerization Activity of (R3P)Ni(codH)+ (cod = 1,5-cylcooctadiene) Sites Supported on Sulfated Zirconium Oxide

PAr₃ containing o-OMe, o-Me, or o-Et substituents reacts with Brønsted sites on sulfated zirconium oxide (SZO) to form [HPAr₃][SZO]. The phosphonium sites on this material react with bis(cyclooctadiene)nickel [Ni(cod)₂] to form [Ni(PAr₃)(codH)][SZO] that are active in ethylene polymerization reactions. Selective poisoning studies with pyridine show that ∼90% of the Ni(PAr₃)(codH)⁺ sites in this material are active in polymerization reactions.

Molecular and supported Ti(iii)-alkyls: efficient ethylene polymerization driven by the π-character of metal-carbon bonds and back donation from a singly occupied molecular orbital

While Ti(iii) alkyl species are the proposed active sites in Ziegler–Natta ethylene polymerization catalysts, the corresponding well-defined homogeneous catalysts are not known. We report that well-defined neutral β-diiminato Ti(iii) alkyl species, namely [Ti(nacnac)(CH₂^tBu)₂] and its alumina-grafted derivative [(Al_sO)Ti(nacnac)(CH₂^tBu)], are active towards ethylene polymerization at moderate pressures and temperatures and possess an electron configuration well-adapted to insertion of ethylene. Advanced EPR spectroscopy showed that ethylene insertion into a Ti(iii)–C bond takes place during polymerization from Ti(nacnac)(CH₂^tBu)₂. A combination of pulsed EPR spectroscopy and DFT calculations, based on a crystal structure of [Ti(nacnac)(CH₂^tBu)₂], enabled us to reveal details about the structure and electronic configurations of both molecular and surface-grafted species. For both compounds, the α-agostic C–H interaction, which involves the singly occupied molecular orbital, indicates a π character of the metal–carbon bond; this π character is enhanced upon ethylene coordination, leading to a nearly barrier-less C₂H₄ insertion into Ti(iii)–C bonds after this first step. During coordination, back donation from the SOMO to the π*(C₂H₄) occurs, leading to stabilization of π-ethylene complexes and to a significant lowering of the overall energy of the C₂H₄ insertion transition state. In d¹ alkyl complexes, ethylene insertion follows an original “augmented” Cossee–Arlman mechanism that involves the delocalization of unpaired electrons between the SOMO, π*(C₂H₄) and σ*(Ti–C) in the transition state, which further favors ethylene insertion. All these factors facilitate ethylene polymerization on Ti(iii) neutral alkyl species and make d¹ alkyl complexes potentially more effective polymerization catalysts than their d⁰ analogues.

Ti(iii) alkyl species polymerize ethylene via an original mechanism, which involves back donation to the π*(C₂H₄) and a delocalization of the unpaired electron in the transition state of C₂H₄ insertion into the partially alkylidenic Ti(iii)–C bond.

Ligand–metal secondary interactions in phosphine–sulfonate palladium and nickel catalyzed ethylene (co)polymerization

Ligand secondary interactions and Lewis acid modulation are simultaneously achieved in palladium and nickel catalyzed ethylene polymerization and copolymerization.

Synthesis of characteristic polyisoprenes using rationally designed iminopyridyl metal (Fe and Co) precatalysts: investigation of co-catalysts and steric influence on their catalytic activity

A series of iminopyridyl ligand-based iron and cobalt complexes are utilized as catalysts in isoprene polymerization, in which the microstructure of resultant polyisoprenes was investigated.

Origin of the 29Si NMR chemical shift in R3Si–X and relationship to the formation of silylium (R3Si+) ions

The origin in deshielding of ²⁹Si NMR chemical shifts in R₃Si–X, where X = H, OMe, Cl, OTf, [CH₆B₁₁X₆], toluene, and O_X (O_X = surface oxygen), as well as ⁱPr₃Si⁺ and Mes₃Si⁺ were studied using DFT methods.

Al(ORF)3 (RF = C(CF3)3) activated silica: a well-defined weakly coordinating surface anion

Weakly Coordinating Anions (WCAs) containing electron deficient delocalized anionic fragments that are reasonably inert allow for the isolation of strong electrophiles. Perfluorinated borates, perfluorinated aluminum alkoxides, and halogenated carborane anions are a few families of WCAs that are commonly used in synthesis. Application of similar design strategies to oxide surfaces is challenging. This paper describes the reaction of Al(OR^F)₃*PhF (R^F = C(CF₃)₃) with silica partially dehydroxylated at 700 °C (SiO_2-700) to form the bridging silanol [triple bond, length as m-dash]Si-OH⋯Al(OR^F)₃ (1). DFT calculations using small clusters to model 1 show that the gas phase acidity (GPA) of the bridging silanol is 43.2 kcal mol^-1 lower than the GPA of H₂SO₄, but higher than the strongest carborane acids, suggesting that deprotonated 1 would be a WCA. Reactions of 1 with NOct₃ show that 1 forms weaker ion-pairs than classical WCAs, but stronger ion-pairs than carborane or borate anions. Though 1 forms stronger ion-pairs than these state-of-the-art WCAs, 1 reacts with alkylsilanes to form silylium type surface species. To the best of our knowledge, this is the first example of a silylium supported on derivatized silica.

Emerging Palladium and Nickel Catalysts for Copolymerization of Olefins with Polar Monomers

Transition-metal-catalyzed copolymerization of olefins with polar monomers represents a challenge because of the large variety of substrate-induced side reactions. However, this approach also holds the potential for the direct synthesis of polar functionalized polyolefins with unique properties. After decades of research, only a few catalyst systems have been found to be suitable for this reaction. Some major advances in catalyst development have been made in the past five years. This Minireview summarizes some of the recent progress in the extensively studied Brookhart and Drent catalyst systems, as well as emerging alternative palladium and nickel catalysts.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

Here we will summarize some of the recent advances in α-diimine ligand developments, as well as some new and challenging monomers that this class of catalysts can address through these ligand improvements.

Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: sterically controlled monomer enchainment

Aminopyridine iron(ii) chloride complexes coordinating a type of flexible ligand are reported as precursors for controlled polymerization of isoprenes.