Copolymerization of Ethylene with 1-Hexene and 1-Octene Catalyzed by Fluorenyl N-Heterocyclic Carbene Ligated Rare-Earth Metal Precursors

Bis-[C(sp3)-chelating] Ti2 catalysts supported by arylene-1,4-diyl-2,3-X2 bridges for olefin copolymerisation: X substituents impose conformational cooperative effects

The synthesis, spectroscopic characterisation and catalytic olefin polymerisation behaviour of a class of binuclear titanium bis(benzyl) complexes bearing bis-(pyridine-2-phenolate-6-methine)-[μ-(arylene-1,4-diyl-2,3-X2)] ligands [X2 = -C4H4- (1), F2 (2), H2 (3)], and mononuclear analogues, are described. These bimetallic catalyst frameworks are designed to exhibit a degree of conformational flexibility, which is regulated by steric effects and crucially permits tuning of intermetallic distances and geometry, yet their shape-persistent nature can also confer favourable entropic terms. Complexes 1-3 are characterised as two diastereomers [meso (RS) and rac (RR,SS)] in ratios of 1.32, 1.18 and 1.13 respectively, according to 1H NMR spectroscopy. In contrast to 3, [1H,1H]-ROESY experiments for 1 and 2 revealed that the X2 substituents can impose preferred conformations with syn orientations of Ti2 centres and benzyl groups, thus implying that the activated catalysts would present binding sites with the same direction of access. For ethylene-(1-octene) copolymerisation reactions, in conjunction with [Ph3C][B(C6F5)4], catalyst 1 displayed superior efficiencies and produced polymers with higher Mw values and enhanced comonomer incorporation ratios (up to 41%), when compared with the mononuclear 5m (22%). These results are indicative of enhanced comonomer enchainment and cooperative reactivity by the Ti2 sites.

N-Heterocyclic Carbene-Coordinated M(II) (M = Yb, Sm, Ca) Bisamides: Expanding the Limits of Intermolecular Alkene Hydrophosphination

A series of NHC-stabilized amido compounds (NHC)_nM[N(SiMe₃)₂]₂ (M = Yb(II), Sm(II), Ca(II); n = 1, 2) showed remarkable catalytic efficiency in addition of PhPH₂ and PH₃ to alkenes under mild conditions and low catalyst loading. The effect of σ-donor capacity of NHCs on catalytic activity in hydrophosphination of styrene with PhPH₂ and PH₃ was revealed. For the series of three-coordinate complexes 1-4M, a tendency to increase the catalytic activity with growth of σ-donating strength of the carbene ligand was clearly demonstrated. The complex (NHC)₂Sm[N(SiMe₃)₂]₂ (NHC = 1,3-diisopropyl-2H-imidazole-2-ylidene) (5Sm) proved to be the most efficient catalyst, which enabled hardly realizable transformations such as PhPH₂ addition across internal C═C bonds of norbornene and cis- and trans-stilbenes, providing the highest reaction rate for addition of PH₃ to styrene. Excellent regio- and chemoselectivities of alkylation of PH₃ with styrenes allow for a selective and good-yield synthesis of desired organophosphines─either primary, secondary, or tertiary. Stepwise alkylation of PH₃ with various substituted styrenes can be efficiently applied as an approach to nonsymmetric secondary phosphines. The rate equation of the addition of styrene to PH₃ promoted by 5Sm was found: rate = k[styrene]¹[5Sm]¹.

Transformation of the sp2 Carbanion to Carbene with Subsequent 1,1-Migratory Insertion and Nucleophilic Substitution in Rare-Earth Metal Chemistry

The development of Fischer-type electrophilic carbene chemistry with early transition metals has been a great challenge due to the fact that such metals in their high oxidation states lack the d electrons to stabilize the electrophilic carbene. Herein, we disclose the first experimental and theoretical findings of in situ transformation of an sp² carbanion to a Fischer-type electrophilic carbene with rare-earth metals in their high oxidation state with a d⁰ electron via electron transfer. The carbene may undergo 1,1-migratory insertion into an adjacent RE-C(sp³) bond, and an unprecedented ring opening of the indole ring of the ligand occurs when the carbenes undergo nucleophilic substitution with a special organolithium reagent o-Me₂NC₆H₄CH₂Li. The key to success is the uniquely tailored novel ligand systems featuring a suitable conjugate building block (^-C═C-C═N) bearing an sp² carbanion connected to the rare-earth metal center.

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.

Bis(oxazoline)-derived N-heterocyclic carbene ligated rare-earth metal complexes: synthesis, structure, and polymerization performance

Bis(oxazoline)-derived NHC supported rare-earth complexes have been synthesized for the first time and showed good activity for 1-hexene (co)polymerization.

Copolymerization of propylene with styrene and ethylene by a THF-containing half-sandwich scandium catalyst: efficient synthesis of polyolefins with a controllable styrene content

A novel family of multi-block propylene–styrene copolymers with syndiotactic styrene–styrene blocks and random propylene–ethylene–styrene terpolymers with a broad composition range were obtained by using (C₅Me₄SiMe₃)Sc(CH₂SiMe₃)₂(THF).

The terpolymerization of ethylene and propylene with isoprene via THF-containing half-sandwich scandium catalysts: a new kind of ethylene–propylene–diene rubber and its functionalization

A novel family of ethylene–propylene–isoprene terpolymers was prepared by the terpolymerization of ethylene and propylene with isoprene catalyzed by the half-sandwich scandium complexs.

Highly 3,4-selective living polymerization of 2-phenyl-1,3-butadiene with amidino N-heterocyclic carbene ligated rare-earth metal bis(alkyl) complexes

2-Phenyl-1,3-butadiene was polymerized using an amidino N-heterocyclic carbene lutetium bis(alkyl) precursor in high 3,4-selectivity (96.9%) and a living mode to afford a new plastic polydiene with a high T_g of 56.1 °C.

Bis(phenolate) N-heterocyclic carbene rare earth metal complexes: synthesis, characterization and applications in the polymerization of n-hexyl isocyanate

Polyhexyl isocyanantes catalyzed by N-heterocyclic carbene rare earth metal complexes show high molecular weight with narrow molecular weight distribution.