Recent Advances in Rare Earth Complexes Bearing Allyl Ligands and Their Reactivity towards Conjugated Dienes and Styrene Polymerization

Rare-Earth Metallocenes for Polymerization of Olefins and Conjugated Dienes: From Fundamental Studies to Olefin Block Copolymers

The various steps in the mechanism of olefin polymerizations mediated by neutral rare-earth metallocene complexes are discussed. The complexes are either trivalent hydride and alkyl rare-earth compounds or divalent metallocenes that are activated by the monomer via an oxidation step. The stereospecific polymerizations of conjugated dienes based on the association of a cationic metallocene complex and an alkylaluminum and the polymerization mechanism based on monomer insertion into an aluminum-carbon bond are also discussed. The exploitation of metallocene complexes for the copolymerization of olefins with conjugated dienes is the subject of a third part of this review. The synthesis of new elastomers called ethylene butadiene rubber (EBR) is highlighted. Finally, the use of rare-earth metallocenes in macromolecular engineering is detailed. This includes the synthesis of functional polyolefins and block copolymers including thermoplastic elastomers.

Regio- and Diastereoselective Annulation of α,β-Unsaturated Aldimines with Alkenes via Allylic C(sp3)-H Activation by Rare-Earth Catalysts

The [3 + 2] or [4 + 2] annulation of α,β-unsaturated aldimines with alkenes via β'- or γ-allylic C(sp3)-H activation is, in principle, an atom-efficient route for the synthesis of five- or six-membered-ring cycloalkylamines, which are important structural motifs in numerous natural products, bioactive molecules, and pharmaceuticals. However, such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. We report herein for the first time the regio- and diastereoselective [3 + 2] and [4 + 2] annulations of α,β-unsaturated imines with alkenes via allylic C(sp3)-H activation by half-sandwich rare-earth catalysts having different metal ion sizes. The reaction of α-methyl-substituted α,β-unsaturated aldimines with alkenes by a C5Me4SiMe3-ligated scandium catalyst took place in a trans-diastereoselective [3 + 2] annulation fashion via C(sp3)-H activation at the α-methyl group (β'-position), exclusively affording alkylidene-functionalized cyclopentylamines with excellent trans-diastereoselectivity. In contrast, the reaction of β-methyl-substituted α,β-unsaturated aldimines with alkenes by a C5Me5-ligated cerium catalyst proceeded in a cis-diastereoselective [4 + 2] annulation fashion via γ-allylic C(sp3)-H activation, selectively yielding multisubstituted 2-cyclohexenylamines with excellent cis-diastereoselectivity. The mechanistic details of these transformations have been elucidated by deuterium-labeling experiments, kinetic isotope effect studies, and the isolation and transformations of key reaction intermediates. This work offers an efficient and selective protocol for the synthesis of a new family of cycloalkylamine derivatives, featuring 100% atom efficiency, high regio- and diastereoselectivity, broad substrate scope, and an unprecedented reaction mechanism.

Divalent Lanthanide Tetraisobutylaluminates: Reactivity and Living Isoprene Polymerization

Lanthanide (Ln) tetraisobutylaluminates constitute key components in commercial 1,3-diene polymerization catalysts, and likewise are the homogeneous rare-earth-metal catalysts of prime industrial importance. Discrete divalent rare-earth-metal complexes [Ln(AliBu₄ )₂ ] (Ln=Sm, Eu, Yb) reported here display the first structurally characterized homoleptic metal tetraisobutylaluminates. Treatment of [Ln(AliBu₄ )₂ ] with C₂ Cl₆ gives access to Sm^II /Sm^III mixed-valence cluster [Sm₆ Cl₈ (AliBu₄ )₆ ] and the Yb^II cluster [Yb₄ Cl₄ (AliBu₄ )₄ ], respectively. Reaction with B(C₆ F₅ )₃ leads to hydride abstraction and formation of arene-coordinated hydroborates such as [Sm{HB(C₆ F₅ )₃ }₂ (toluene)₂ ]. Complexes [Ln(AliBu₄ )₂ ] engage in single-component isoprene polymerization, affording high cis-1,4 polyisoprenes with narrow molecular weight distributions. Binary [Yb(AliBu₄ )₂ ]/[HNPhMe₂ ][B(C₆ F₅ )₄ ] fabricates polyisoprene in a perfectly living manner. The catalytically active species are scrutinized by NMR spectroscopy.

Dichloro(2,2'-bipyridine)copper/MAO: An Active and Stereospecific Catalyst for 1,3-Diene Polymerization

Dichloro(2,2'-bipyridine)copper was synthesized by reacting copper dichloride with bypyridine, and its behavior, in combination with methylaluminoxane (MAO), in the polymerization of butadiene, isoprene, 2,3-dimethyl-1,3 butadiene, and 3-methyl-1,3-pentadiene was examined. The purpose of this study is to find catalytic systems that are more sustainable than those currently used for the polymerization of butadiene and isoprene (e.g., Co and Ni), but that are comparable in terms of catalytic activity and selectivity. Predominantly, syndiotactic 1,2 polybutadiene, crystalline syndiotactic 3,4 polyisoprene, crystalline syndiotactic 1,2 poly(3-methyl-1,3-pentadiene), and crystalline cis-1,4 poly(2,3-dimethyl-1,3-butadiene) were obtained in a manner similar to that observed with the analogous iron complex. As far as we know, the investigated catalytic system represents the first example of a copper-based catalyst in the field of stereospecific polymerization. Given the great availability of copper, its extremely low toxicity (and therefore high sustainability), and the similarity of its behavior to that of iron, the result obtained seems to us of considerable interest and worthy of further investigation.

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

A method to explore head‐to‐head ϕ back‐bonding from uranium f‐orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single‐crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N‐((dimethyl)prop‐2‐enylsilyl)‐2,6‐diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI₂(L1)₂ (1), which was synthesized by using the mono‐deprotonated ligand [K][N‐((dimethyl)prop‐2‐enyl)silyl)‐2,6‐diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2]₂ (2) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligand [K]₂[N‐((dimethyl)propenidesilyl)‐2,6‐diisopropylanilide] (L2). This allyl‐based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back‐bonding. The bonding between U^IV and the allyl fragments is best described as ligand‐to‐metal electron donation from a two carbon fragment‐localized electron density into empty f‐orbitals.

Organometallic complexes of neodymium: an overview of synthetic methodologies based on coordinating elements

Organometallic complexes of neodymium have unique coordinating ability to form both micro and macromolecules as well as metal-based polymers. These complexes have been reported in different fields and play a tremendous role in luminescence, catalytic, biological and magnetic applications. So, the current study will comprise all possible routes for the synthesis of organometallic complexes of neodymium. Neodymium complexes have been synthesized of single, double, triple and tetra linkages with H, C, N, O as well as S, B, and X. The detailed synthetic routes have been classified into four categories but in brief, neodymium forms complexes by reacting metal chloride, nitrate or oxide (hydrated or dehydrated) as precursor along with appropriate ligand. Most applied solvents for neodymium complexes were Toluene and THF. These complexes required a range of temperature based on the nature of complexes as well as linkages. The authors have surveyed the research work published through 2011–2020 and provide a comprehensive overview to understand the synthetic routes of organometallic complexes of neodymium.

Evidence of coordinative chain transfer polymerization of isoprene using iron iminopyridine/ZnEt2 catalytic systems

Herein, the first example of a reversible iron-mediated chain transfer polymerisation of isoprene with ZnEt2 as the chain transfer agent is reported.

Neodymium catalysts for polymerization of dienes, vinyl monomers, and ε-caprolactone

This review discusses various neodymium catalysts for stereospecific polymerization of dienes, vinyl monomers, and ε-caprolactone.

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

Lutetium and yttrium complexes supported by an anilido-oxazoline ligand for polymerization of 1,3-conjugated dienes and ε-caprolactone

Lutetium and yttrium complexes supported by an anilido-oxazoline ligand exhibit highcis-1,4 stereoselectivity for 1,3-conjugated diene polymerization and high activity for ring-opening polymerization of ε-caprolactone.

C-H-Bond Activation and Isoprene Polymerization Studies Applying Pentamethylcyclopentadienyl-Supported Rare-Earth-Metal Bis(Tetramethylaluminate) and Dimethyl Complexes

As previously shown for lutetium and yttrium, 1,2,3,4,5-pentamethylcyclopentadienyl (C₅Me₅ = Cp*)-bearing rare-earth metal dimethyl half-sandwich complexes [Cp*LnMe₂]₃ are now also accessible for holmium, dysprosium, and terbium via tetramethylaluminato cleavage of [Cp*Ln(AlMe₄)₂] with diethyl ether (Ho, Dy) and tert-butyl methyl ether (TBME) (Tb). C–H-bond activation and ligand redistribution reactions are observed in case of terbium and are dominant for the next larger-sized gadolinium, as evidenced by the formation of mixed methyl/methylidene clusters [(Cp*Ln)₅(CH₂)(Me)₈] and metallocene dimers [Cp*₂Ln(AlMe₄)]₂ (Ln = Tb, Gd). Applying TBME as a “cleaving” reagent can result in both TBME deprotonation and ether cleavage, as shown for the formation of the 24-membered macrocycle [(Cp*Gd)₂(Me)(CH₂OtBu)₂(AlMe₄)]₄ or monolanthanum complex [Cp*La(AlMe₄){Me₃Al(CH₂)OtBu}] and monoyttrium complex [Cp*Y(AlMe₄)(Me₃AlOtBu)], respectively. Complexes [Cp*Ln(AlMe₄)₂] (Ln = Ho, Dy, Tb, Gd) and [Cp*LnMe₂]₃ (Ln = Ho, Dy) are applied in isoprene and 1,3-butadiene polymerization, upon activation with borates [Ph₃C][B(C₆F₅)₄] and [PhNHMe₂][B(C₆F₅)₄], as well as borane B(C₆F₅)₃. The trans-directing effect of AlMe₃ in the binary systems [Cp*Ln(AlMe₄)₂]/borate is revealed and further corroborated by the fabrication of high-cis-1,4 polybutadiene (97%) with “aluminum-free” [Cp*DyMe₂]₃/[Ph₃C][B(C₆F₅)₄]. The formation of multimetallic active species is supported by the polymerization activity of pre-isolated cluster [(Cp*Ho)₃Me₄(CH₂)(thf)₂].

Highly Active Iminopyridyl Iron-Based Catalysts for the Polymerization of Isoprene

A series of iminopyridyl-based ligands, 6-[(Ar)N=C(R)]-2-C₆H₅N [(Ar = 2,6-Me₂-C₆H₃, R = Me (L1); Ar = 2,6-ⁱPr₂-C₆H₃, R = Me (L2); Ar = 2,6-Me₂-C₆H₃, R = H (L3); Ar = 2,6-ⁱPr₂-C₆H₃, R = H (L4); Ar = 3,5-(CF₃)₂-C₆H₃, R = Me (L5); Ar = C₆F₅, R = Me (L6)], and their corresponding iron (II) complexes were developed to investigate their application in the controlled coordinative polymerization of isoprene. The modulation of steric and electronic properties within this family of ligands/pre-catalysts has shown to influence the stereo-selectivity and activity of the polymerization of isoprene after activation. Upon activation with various co-catalysts such as AlⁱBu₃/[Ph₃C][B(C₆F₅)₄], AlEt₃/[Ph₃C][B(C₆F₅)₄] or MAO, the resulting catalysts produced polyisoprenes with an excellent conversion (>99% of 500–5000 equiv.) within less than 1 h (TOF > 500 h⁻¹) and having a variety of stereo-/regio-regularities. The presence of electron-donating and withdrawing groups drastically impacted the activity and the stereoselectivity of the catalysts during the course of the polymerization of isoprene. When activated with AlⁱBu₃/[Ph₃C][B(C₆F₅)₄], the complexes {6-[(2,6-Me₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C1) and {6-[(2,6-ⁱPr₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C2) exhibited moderate trans-1,4 selectivity (>67%) while the iron-based systems bearing related aldiminopyridyl ligands {6-[(2,6-Me₂-C₆H₃)N=C(H)]-2-C₆H₅N}FeCl₂ (C3) and {6-[(2,6-ⁱPr₂-C₆H₃)N=C(H)]-2-C₆H₅N}FeCl₂ (C4) were found to afford significant cis-1,4 selectivity at low temperature (>86% at −40 °C). On the other hand, the ternary {6-[(3,5-(CF₃)₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C5) or {6-[(C₆F₅)N=C(Me)]-2-C₆H₅N}FeCl₂ (C6)/AlⁱBu₃/[Ph₃C][B(C₆F₅)₄] catalytic combinations showed exceptional activity for the polymerization of isoprene (TOF > 1,000,000 h⁻¹), albeit providing less stereoselectivity.

Preparation of Butadiene-Isoprene Copolymer with High Vinyl Contents by Al(OPhCH₃)(i-Bu)₂/MoO₂Cl₂∙TNPP

In this study, a butadiene-isoprene coordination polymerization was initiated by a binary molybdenum (Mo)-based catalytic system consisting of modified MoO₂Cl₂ as the primary catalyst, triethyl aluminum substituted by m-cresol as the co-catalyst and tris(nonyl phenyl) phosphate (TNPP) as the ligand. The effects of the amount of catalyst and type of co-catalyst were investigated in detail. Experimental results indicated that when the butadiene-isoprene coordination polymerization was initiated by the binary Mo-based catalytic system, the monomer conversion could reach 90%. The resulting butadiene units were primarily based on 1,2-structures, and the reactivity ratios of butadiene and isoprene were 1.13 and 0.31, respectively. The reaction in the catalytic system was attributed to the non-ideal and non-constant ratio copolymerization. When the addition of isoprene monomers was relatively low, the isoprene units on the butadiene-isoprene copolymers were primarily based on the 1,2- and 3,4-structures. Moreover, the orientation of active centers to 1,2- and 3,4-structures gradually decreased with an increase in the addition of isoprene monomers, which resulted in the generation of high vinyl butadiene-isoprene copolymers.

Pentadienyl migration and abstraction in yttrium aluminabenzene complexes including a single-component catalyst for isoprene polymerization

The pentadienyl ligand in A engages in a donor-promoted intramolecular metal(Y)-to-metal(Al) migration and is easily abstracted to afford B.

Catalysts for the Controlled Polymerization of Conjugated Dienes

Since its first discovery at the beginning of the 1960s […]