Reactions of bimetallic Zr,Al- hydride complexes with methylaluminoxane: NMR and DFT study

Solution Structure and Dynamics of Hf-Al and Hf-Zn Heterobimetallic Adducts Mimicking Relevant Intermediates in Chain Transfer Reactions

Cationic cyclometalated hafnocenes [CpPrCpCH2CH2CH2Hf][B(C6F5)4] (4Pr) and [CpiBuCpCH2CH(Me)CH2Hf][B(C6F5)4] (4aiBu and 4biBu) were synthesized from the corresponding [(CpPr)2HfMe][B(C6F5)4] (1Pr) and [(CpiBu)2HfMe][B(C6F5)4] (1iBu) complexes via C-H activation. 4aiBu, 4biBu, and 4Pr, mimicking a propagating M-polymeryl species (M = transition metal) with or without a β-methyl branch on the metalated chains, serve to investigate whether and how the nature of the last inserted olefin molecules changes the structure, stability, and reactivity of the corresponding heterobimetallic complexes, formed in the presence of aluminum- or zinc-alkyl chain transfer agents (CTAs), which are considered relevant intermediates in coordinative chain transfer polymerization (CCTP) and chain shuttling polymerization (CSP) technologies. NMR and DFT data indicate no major structural difference between the resulting heterobridged complexes, all characterized by the presence of multiple α-agostic interactions. On the contrary, thermodynamic and kinetic investigations, concerning the reversible formation and breaking of heterobimetallic adducts, demonstrate that isomer 4aiBu, in which the β-Me is oriented away from the reactive coordination site on Hf, but not 4biBu, having the β-Me pointing in the opposite direction, is capable of reacting with CTAs. Quantification of kinetic rate constants highlights that the formation process is rate limiting and that the nature of the last inserted α-olefin unit modulates transalkylation kinetics. The reaction of 4aiBu, 4biBu, and 4Pr with diisobutylaluminum hydride (DiBAlH) allows the interception and characterization of new heterobinuclear and heterotrinuclear species, featuring both hydride and alkyl bridging moieties, which represent structural models of elusive intermediates in CCTP and CSP processes, capturing the instant when an alkyl chain has just transferred from a transition metal to a main group metal, while the two metals remain engaged in a single heterobimetallic intermediate.

The Dimerization and Oligomerization of Alkenes Catalyzed with Transition Metal Complexes: Catalytic Systems and Reaction Mechanisms

Dimers and oligomers of alkenes represent a category of compounds that are in great demand in diverse industrial sectors. Among the developing synthetic methods, the catalysis of alkene dimerization and oligomerization using transition metal salts and complexes is of undoubted interest for practical applications. This approach demonstrates substantial potential, offering not only elevated reaction rates but also precise control over the chemo-, regio-, and stereoselectivity of the reactions. In this review, we discuss the data on catalytic systems for alkene dimerization and oligomerization. Our focus lies in the analysis of how the activity and chemoselectivity of these catalytic systems are influenced by various factors, such as the nature of the transition metal, the ligand environment, the activator, and the substrate structure. Notably, this review particularly discusses reaction mechanisms, encompassing metal complex activation, structural and dynamic features, and the reactivity of hydride intermediates, which serve as potential catalytically active centers in alkene dimerization and oligomerization.

Catalytic Properties of Zirconocene-Based Systems in 1-Hexene Oligomerization and Structure of Metal Hydride Reaction Centers

Despite large-scale investigations of homogeneous single-site metallocene catalysts and systems based on them, there are still unsolved problems related to the control of their activity and chemo- and stereoselectivity. A solution to these problems is required to develop efficient methods for the synthesis of practically useful products of alkene transformations, such as dimers, oligomers, and polymers. Here we studied the catalytic activity of structurally diverse zirconocenes (L2ZrCl2, L = Cp, C5Me5, Ind, L2 = Me2CCp2, Me2SiCp2, Me2C2Cp2, rac-Me2CInd2, rac-H4C2Ind2, BIPh(Ind)2, H4C2[THInd]2), and co-catalysts activating the system, namely HAlBui2, MMAO-12, and (Ph3C)[B(C6F5)4], at low activator/Zr ratios in a 1-hexene oligomerization reaction. The influence of catalyst structure and system composition on the alkene conversion, the type of products, and the reaction stereoselectivity were investigated. The composition of hydride intermediates formed in the L2ZrCl2–HAlBui2–activator system (L2 = ansa-Me2CCp2, Ind) was studied by NMR spectroscopy. Participation of the bis-zirconium hydride complex as the precursor of catalytically active sites of the alkene dimerization reaction was shown.

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.

Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers

1-Hexene transformations in the catalytic systems L₂MCl₂–XAlBuⁱ₂ (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H₄C₂[THInd]₂, M = Zr; X = H, Bu ⁱ) and [Cp₂ZrH₂]₂-ClAlR₂ activated by MMAO-12, B(C₆F₅)₃, or (Ph₃C)[B(C₆F₅)₄] in chlorinated solvents (CH₂Cl₂, CHCl₃, o-Cl₂C₆H₄, ClCH₂CH₂Cl) were studied. The systems [Cp₂ZrH₂]₂-MMAO-12, [Cp₂ZrH₂]₂-ClAlBuⁱ₂-MMAO-12, or Cp₂ZrCl₂-HAlBuⁱ₂-MMAO-12 (B(C₆F₅)₃) in CH₂Cl₂ showed the highest activity and selectivity towards the formation of vinylidene head-to-tail alkene dimers. The use of chloroform as a solvent provides further in situ dimer dimerization to give a tetramer yield of up to 89%. A study of the reaction of [Cp₂ZrH₂]₂ or Cp₂ZrCl₂ with organoaluminum compounds and MMAO-12 by NMR spectroscopy confirmed the formation of Zr,Zr-hydride clusters as key intermediates of the alkene dimerization. The probable structure of the Zr,Zr-hydride clusters and ways of their generation in the catalytic systems were analyzed using a quantum chemical approach (DFT).

Catalytic Systems Based on Cp2ZrX2 (X = Cl, H), Organoaluminum Compounds and Perfluorophenylboranes: Role of Zr,Zr- and Zr,Al-Hydride Intermediates in Alkene Dimerization and Oligomerization

The activity and chemoselectivity of the Cp2ZrCl2-XAlBui2 (X = H, Bui) and [Cp2ZrH2]2-ClAlEt2 catalytic systems activated by (Ph3C)[B(C6F5)4] or B(C6F5)3 were studied in reactions with 1-hexene. The activation of the systems by B(C6F5)3 resulted in the selective formation of head-to-tail alkene dimers in up to 93% yields. NMR studies of the reactions of Zr complexes with organoaluminum compounds (OACs) and boron activators showed the formation of Zr,Zr- and Zr,Al-hydride intermediates, for which diffusion coefficients, hydrodynamic radii, and volumes were estimated using the diffusion ordered spectroscopy DOSY. Bis-zirconium hydride clusters of type x[Cp2ZrH2∙Cp2ZrHCl∙ClAlR2]∙yRnAl(C6F5)3−n were found to be the key intermediates of alkene dimerization, whereas cationic Zr,Al-hydrides led to the formation of oligomers.

Experimental and Theoretical Study of Zirconocene-Catalyzed Oligomerization of 1-Octene

Zirconocene-catalyzed coordination oligomerization of higher α-olefins is of theoretical and practical interest. In this paper, we present the results of experimental and theoretical study of α-olefin oligomerization, catalyzed by (η⁵-C₅H₅)]₂ZrX₂1/1′ and O[SiMe₂(η⁵-C₅H₄)]₂ZrX₂2/2′ (X = Cl, Me) with the activation by modified methylalymoxane MMAO-12 or by perfluoroalkyl borate [PhNMe₂H][B(C₆F₅)₄] (NB^F) in the presence and in the absence of organoaluminium compounds, Al(CH₂CHMe₂)₃ (TIBA) and/or Et₂AlCl. Under the conditions providing a conventional mononuclear reaction mechanism, 1′ catalyzed dimerization with low selectivity, while 2′ initiated the formation of oligomers in equal mass ratio. The presence of TIBA and especially Et₂AlCl resulted in an increase of the selectivity of dimerization. Quantum chemical simulations of the main and side processes performed at the M-06x/ DGDZVP level of the density functional theory (DFT) allowed to explain experimental results involving traditional mononuclear and novel Zr-Al₁ and Zr-Al₂ mechanistic concepts.

Fair Look at Coordination Oligomerization of Higher α-Olefins

Coordination catalysis is a highly efficient alternative to more traditional acid catalysis in the oligomerization of α-olefins. The distinct advantage of transition metal-based catalysts is the structural homogeneity of the oligomers. Given the great diversity of the catalysts and option of varying the reaction conditions, a wide spectrum of processes can be implemented. In recent years, both methylenealkanes (vinylidene dimers of α-olefins) and structurally uniform oligomers with the desired degrees of polymerization have become available for later use in the synthesis of amphiphilic organic compounds and polymers, high-quality oils or lubricants, and other prospective materials. In the present review, we discussed the selective dimerization and oligomerization of α-olefins, catalyzed by metallocene and post-metallocene complexes, and explored the prospects for the further applications of the coordination α-olefin dimers and oligomers.

Bimetallic Zr,Zr-Hydride Complexes in Zirconocene Catalyzed Alkene Dimerization

Being valuable precursors in the production of adhesives, lubricants, and other high-performance synthetic compounds, alkene dimers and oligomers can be obtained using homogeneous zirconocene catalytic systems. Further advances in such systems require precise control of their activity and chemoselectivity, increasing both the purity and yield of the products. This relies on the process mechanism usually built around the consideration of the hydride complexes as active intermediates in the alkene di- and oligomerization; however, the majority of studies lack the direct evidence of their involvement. Parallel studies on a well-known Cp2ZrCl2-AlR3 or HAlBui2 and a novel [Cp2ZrH2]2-ClAlR2 (R = Me, Et, Bui) systems activated by methylaluminoxane (MMAO-12) have shown a deep similarity both in the catalytic performance and intermediate composition. As a result of the NMR studies, among all the intermediates considered, we proved that new Zr,Zr- hydride complexes having the type x[Cp2ZrH2∙Cp2ZrHCl∙ClAlR2]∙yMAO appear to be specifically responsible for the alkene dimerization with high yield.

Zirconocene-Catalyzed Dimerization of α-Olefins: DFT Modeling of the Zr-Al Binuclear Reaction Mechanism

Zirconocene-mediated selective dimerization of α-olefins usually occurs when precatalyst (η⁵-C₅H₅)₂ZrCl₂ is activated by minimal excess of methylalumoxane (MAO). In this paper, we present the results of density functional theory (DFT) simulation of the initiation, propagation, and termination stages of dimerization and oligomerization of propylene within the framework of Zr-Al binuclear mechanism at M-06x/DGDZVP level of theory. The results of the analysis of the reaction profiles allow to explain experimental facts such as oligomerization of α-olefins at high MAO/(η⁵-C₅H₅)₂ZrCl₂ ratios and increase of the selectivity of dimerization in the presence of R₂AlCl. The results of DFT simulations confirm the crucial role of the presence of chloride in the selectivity of dimerization. The molecular hydrogen was found in silico and proven experimentally as an effective agent that increases the rate and selectivity of dimerization.