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.

Iron-catalysed hydroalumination of internal alkynes

Although research on iron-catalysed reactions has recently achieved significant progress, the activity and selectivity of iron catalysts are generally inferior to those of noble-metal catalysts. The development of new iron-catalysed reactions, especially those in which iron catalysts exhibit superior activity or selectivity to other catalysts, is the key to promote iron catalysis. Herein, we report the first protocol for iron-catalysed hydroalumination of internal alkynes. Specifically, in the presence of iron catalysts bearing 2,9-diaryl-1,10-phenanthroline ligands, internal alkynes were stereo- and regioselectively hydroaluminated with the commercially available reagent diisobutylaluminum hydride. Compared with other metal-catalysed alkyne hydroalumination reactions reported in the literature, the iron-catalysed protocol has the following advantages: unusual amino-group-directed regioselectivity, a wide substrate scope, good functional group tolerance, high selectivity, and mild reaction conditions. The alkenylaluminum products prepared in this way could undergo a diverse array of transformations, and were used for the synthesis of bioactive compounds. The current study expands the scope of iron catalysis, provides a new efficient access to alkenylaluminum, discloses the origin of the superiority of iron catalysts, and thus may inspire further studies in related fields.

Zirconocene dichlorides as catalysts in alkene carbo- and cyclometalation by AlEt3: intermediate structures and dynamics

A series of zirconocenes L₂ZrCl₂ (23 examples) were studied as catalysts in the reaction of alkenes with AlEt₃. The catalyst activity and reaction chemoselectivity were found to depend on the ligand structure in the complex and the nature of the solvent. The alkyl exchange in the triethylaluminum dimer was studied by NMR spectroscopy; a solvent effect on the alkyl exchange parameters was established. In the reaction between L₂ZrCl₂ and AlEt₃, the formation of intermediates L₂ZrEtCl, L₂Zr(μ-Cl)CH₂CH₂AlEt₂, L₂Zr(μ-H)CH₂CH₂AlEt₂, and L₂Zr(μ-Cl)CH₂CH(AlEt₂)₂ was shown; the ratio of the intermediates depends on the ligand structure in the initial complex and the solvent. The exchange in the ethanediyl bridge ZrCH₂CH₂Al, proceeding via zirconocenecyclopropane structures, was demonstrated for the first time for five-membered bimetallic complexes L₂Zr(μ-Cl)CH₂CH₂AlEt₂ with ansa-ligands (L₂ = Me₂SiCp₂, H₂CInd₂, Me₂CInd₂, Me₂SiInd₂, and C₂H₄Ind₂). Five-membered bimetallic complexes were shown to participate in the formation of cyclic organoaluminum products - 3-substituted alumolanes.

Photoluminescence and mechanoluminescence of solid-state zirconocene dichlorides

Spectral-luminescence properties of 23 samples of zirconium complexes were studied. Mechanoluminescence spectra of 10 complexes were obtained. The solid-state component of the mechanoluminescence spectrum, that is the luminescence of the crystal itself, coincided with the photoluminescence spectra of these complexes, which indicated identical emission from the same excited states in mechanoluminescence and photoluminescence, despite the different ways of excitation. The luminescence maximum was red shifted as substituents appeared in the ligand, in particular in the presence of a bridging group connecting π-ligands (ansa-complexes) and also for a substituted bis-indenyl complex rac-Me₂ Si(2-Me-4-Ph-5-OMe-6-Bu^t -Ind)₂ ZrCl₂ ). It was found that mechanical destruction of the rac-isomer of complex Mе₂ С(2-Me-4-Bu^t -C₅ H₂ )₂ ZrCl₂ , unlike that of the meso-isomer, was accompanied by a more than a 10-fold increase in intensity and by a shift in the mechanoluminescence spectrum to longer wavelengths.

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.

Ligand exchange processes in zirconocene dichloride-trimethylaluminum bimetallic systems and their catalytic properties in reaction with alkenes

Ligand exchange processes in the systems L₂ZrCl₂-AlMe₃ (L₂ZrCl₂: Cp₂ZrCl₂, (CpMe)₂ZrCl₂, (C₅Me₅)₂ZrCl₂, Me₂SiCp₂ZrCl₂, Me₂Si(C₅Me₄)₂ZrCl₂, rac-Me₂C(2-Me-4-Bu^t-Cp)₂ZrCl₂, meso-Me₂C(2-Me-4-Bu^t-Cp)₂ZrCl₂, rac-Me₂C(3-Bu^t-Cp)₂ZrCl₂, Ind₂ZrCl₂, rac-H₂C(Ind)₂ZrCl₂, rac-Me₂C(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, rac-C₂H₄(Ind)₂ZrCl₂, rac-C₂H₄(THInd)₂ZrCl₂, rac-Me₂Si(THInd)₂ZrCl₂) and Cp₂ZrMeCl_2-n-AlMe₃ (n = 1, 2) were studied by NMR spectroscopy with the goal to establish the structures and dynamic features of probable intermediates in the zirconocene-catalyzed reactions of alkenes with AlMe₃. The effect of solvent, the organoaluminum compound concentration and the addition of (ClAlMe₂)₂ on the activation parameters of the alkyl exchange in the trimethylaluminum dimer was studied as well. The constants and activation parameters of the methyl group exchange in the monoalkyl-substituted ansa-complexes L₂ZrMeCl (L₂ = rac-Me₂C(2-Me-4-Bu^t-Cp)₂, rac-Me₂C(3-Bu^t-Cp)₂, rac-H₂CInd₂, rac-Me₂CInd₂, rac-Me₂SiInd₂, rac-H₄C₂Ind₂) were established for the first time. The catalytic activity and chemoselectivity of zirconocenes in the reaction of alkenes with AlMe₃ were evaluated and compared with the exchange and equilibrium constants of ligand exchange processes. A mechanism of the reaction was proposed.

Cobalt-bis(imino)pyridine complexes as catalysts for hydroalumination-isomerisation of internal olefins

The insertion of α- and internal octenes (hydroalumination) and chain walking isomerisation at di-n-octylaluminium hydride [Al(Oct)2H], catalysed by bis(imino)pyridine-Co complexes has been investigated by NMR spectroscopy. The Co-based catalysts promote efficient hydroalumination of 1-octene. Internal olefins are partially hydroaluminated, with isomerisation to the primary alkyls, but the catalyst responsible appears to deactivate rapidly. The reaction between the Co precatalysts and [Al(Oct)2H] generates a Co-hydride species, likely to be a hydride bridged dinuclear Co and Al complex. This species is reactive towards α-olefins but inert towards internal olefins. In contrast to hydroalumination, the catalysts promote efficient hydroboration, where insertion and isomerisation of internal octenes goes to completion. The differences between the systems may be partially ascribed to formation of an active mononuclear Co catalyst in the borane system versus a less active Co/Al dinuclear complex in hydroalumination.

Intramolecular mobility of η5-ligands in chiral zirconocene complexes and the enantioselectivity of alkene functionalization by organoaluminum compounds

Conformational behavior of chiral zirconocenes affects their activity and enantioselectivity.

Aza-nickelacycle key intermediate in nickel(0)-catalyzed transformation reactions

Oxidative cyclization of alkynes and imines with nickel(0) is a key step in multicomponent coupling and cycloaddition reactions to give nitrogen-containing organic compounds.

Insertion and isomerisation of internal olefins at alkylaluminium hydride: catalysis with zirconocene dichloride

The insertion of internal olefins and chain walking isomerisation at di-n-octylaluminium hydride [Al(Oct)₂H], promoted by zirconocene dichloride [Cp₂ZrCl₂] has been studied.