Efficient Synthesis of Chiral ansa-Metallocenes by Amine Elimination. Synthesis, Structure, and Reactivity of rac-(EBI)Zr(NMe2)2

Dinuclear Group 4 Metal Complexes Bearing Anthracene-Bridged Bifunctional Amido-Ether Ligands: Remarkable Metal Effect and Cooperativity toward Ethylene/1-Octene Copolymerization

Two types of bifunctional amido-ether ligands (syn-L and anti-L) with the rigid anthracene skeleton were designed to support dinuclear group 4 metal complexes. All organic ligands and organometallic complexes (syn-M₂ and anti-M₂; M = Hf, Zr, and Ti) were fully characterized by ¹H and ¹³C NMR spectroscopies and elemental analyses. The anti-Hf₂ complex showed two confirmations at room temperature with C₂-symmetry or S₂-symmetry that can inter-exchange, as indicated by VT NMR, while only a C₂-symmetric isomer was observed for syn-Hf₂ complex at room temperature. However, for Zr and Ti analogues, both syn and anti complexes exhibited only one conformation at room temperature. The molecular structures of complexes syn-Hf₂, anti-Hf₂, and syn-Ti₂ in the solid state were further determined by single-crystal X-ray diffraction, revealing the distances between two metal centers in syn-M₂ from 7.138 Å (syn-Ti₂) to 7.321 Å (syn-Hf₂) but a much farther separation in anti-M₂ (8.807 Å in C₂-symmetric anti-Hf₂). The mononuclear complex (2-CH₃O-C₆H₄-N-C₁₄H₉)Zr(NMe₂)₃ (mono-Zr₁) was also prepared for control experiments. In the presence of alkyl aluminum (AlEt₃) as the alkylating agent and trityl borate ([Ph₃C][B(C₆F₅)₄]) as the co-catalyst, all metal complexes were tested for copolymerization of ethylene with 1-octene at high temperature (130 °C). The preliminary polymerization results revealed that the activity was highly dependent upon the nature of metal centers, and syn-Zr₂ showed the highest activity of 9600 kg(PE)·mol^-1 (Zr)·h^-1, which was about 17- and 2.2-fold higher than those of syn-Hf₂ and syn-Ti₂, respectively. Benefitting from both steric proximity and electronical interaction of two metal centers, syn-Zr₂ exhibited significant cooperativity in comparison to anti-Zr₂ and mono-Zr₁, with regard to activity and molecular weight and 1-octene incorporation of resultant copolymers.

Mono- and dinuclear zirconocene(iv) amide complexes for the catalytic dehydropolymerisation of phenylsilane

Mono- and dinuclear zirconocene amide complexes were tested as catalysts for the dehydropolymerisation of phenylsilane. The dinuclear complex is surprisingly stable, producing mixtures of polysilanes and cyclic oligomers.

Strategies for the Synthesis of Chiral Carbon-Bridged Group IV ansa-Metallocenes

This minireview provides a survey of the various synthetic approaches to chiral ansa-metallocenes of Ti, Zr, and Hf containing a carbon-based bridge. The individual strategies to install substitution patterns at either the cyclopentadienyl framework or the bridging unit are highlighted with focus on the progress made towards a direct preparation of single complex stereoisomers. The review further includes the discussion of potential problems such as the formation of undesired diastereomers, the threat of racemization of enantiopure material, and synthetic challenges originating from the synthesis, purification, and isolation of the target complexes. The review has been written with the goal in mind to facilitate the design and synthesis of new chiral ansa-metallocene derivatives for emerging research areas in asymmetric catalysis and organometallic chemistry.

Unexpected Precatalyst σ-Ligand Effects in Phenoxyimine Zr-Catalyzed Ethylene/1-Octene Copolymerizations

Recent decades have witnessed intense research efforts aimed at developing new homogeneous olefin polymerization catalysts, with a primary focus on metal-Cl or metal-hydrocarbyl precursors. Curiously, metal-NR₂ precursors have received far less attention. In this contribution, the Zr-amido complex FI₂ZrX₂ (FI = 2,4-di- tert-butyl-6-((isobutylimino)methyl)phenolate, X = NMe₂) is found to exhibit high ethylene polymerization activity and relatively high 1-octene coenchainment selectivity (up to 7.2 mol%) after sequential activation with trimethylaluminum, then Ph₃C⁺B(C₆F₅)₄^-. In sharp contrast, catalysts with traditional hydrocarbyl ligands such as benzyl and methyl give low 1-octene incorporation (0-1.0 mol%). This unexpected selectivity persists under scaled/industrial operating conditions and was previously inaccessible with traditional metal-Cl or -hydrocarbyl precursors. NMR, X-ray diffraction, and catalytic control experiments indicate that in this case an FI ligand is abstracted from FI₂Zr(NMe₂)₂ by trimethylaluminum in the activation process to yield a catalytically active cationic mono-FIZr species. Heretofore this process was believed to serve only as a major catalyst deactivation pathway to be avoided. This work demonstrates the importance of investigating diverse precatalyst monodentate σ-ligands in developing new catalyst systems, especially for group 4 olefin polymerization catalysts.

Chemoselective, Stereospecific, and Living Polymerization of Polar Divinyl Monomers by Chiral Zirconocenium Catalysts

This contribution reports the first chemoselective, stereospecific, and living polymerization of polar divinyl monomers, enabled by chiral ansa-zirconocenium catalysts through an enantiomorphic-site controlled coordination-addition polymerization mechanism. Silyl-bridged-ansa-zirconocenium ester enolate 2 has been synthesized and structurally characterized, but it exhibits low to negligible activity and stereospecificity in the polymerization of polar divinyl monomers including vinyl methacrylate (VMA), allyl methacrylate (AMA), 4-vinylbenzyl methacrylate (VBMA), and N,N-diallyl acrylamide (DAA). In contrast, ethylene-bridged-ansa-zirconocenium ester enolate 1 is highly active and stereospecific in the polymerization of such monomers including AMA, VBMA, and DAA. The polymerization by 1 is perfectly chemoselective for all four polar divinyl monomers, proceeding exclusively through conjugate addition across the methacrylic C═C bond, while leaving the pendant C═C bonds intact. The polymerization of DAA is most stereospecific and controlled, producing essentially stereoperfect isotactic PDAA with [mmmm] > 99%, M(n) matching the theoretical value (thus a quantitative initiation efficiency), and a narrow molecular weight distribution (Đ = 1.06-1.16). The stereospecificity is slightly lower for the AMA polymerization but still leading to highly isotactic poly(allyl methacrylate) (PAMA) with 95-97% [mm]. The polymerization of VBMA is further less stereospecific, affording PVBMA with 90-94% [mm], while the polymerization VMA is least stereospecific. Several lines of evidence from both homo- and block copolymerization results have demonstrated living characteristics of the AMA polymerization by 1. Mechanistic studies of this polymerization have yielded a monometallic coordination-addition polymerization mechanism involving the eight-membered chelating intermediate. Post-functionalization of isotactic polymers bearing the pendant vinyl group on every repeating unit via the thiol-ene "click" reaction achieves a full conversion of all the pendant double bonds to the corresponding thioether bonds. Photocuring of such isotactic polymers is also successful, producing an elastic material readily characterizable by dynamic mechanical analysis.

Cationic Group-IV pincer-type complexes for polymerization and hydroamination catalysis

Neutral Zr(IV) and Hf(IV) dimethyl complexes stabilized by unsymmetrical dianionic {N,C,N'} pincer ligands have been prepared from their corresponding bis-amido complexes upon treatment with AlMe3. Their structure consists of a central σ-bonded aryl donor group (C) capable of forming robust M-C bonds with the metal center, enforced by the synergic effect of both the coordination of peripheral donor groups (N) and the chelating rigid structure of the {N,C,N} ligand framework. Such a combination translates into systems having a unique balance between stability and reactivity. These Zr(IV) and Hf(IV) dimethyl complexes were converted in situ into cationic species [M(IV){N(-),C(-),N}Me][B(C6F5)4] which are active catalysts for the room temperature (r.t.) intramolecular hydroamination/cyclization of primary and secondary aminoalkenes as well as for the high temperature ethylene-1-octene copolymerizations.

Trichlorido(tetra-hydro-furan){(1,2,3,3a,7a-η)-1-[2-(1-trimethyl-silyl-1H-imidazol-2-yl-κN)-1-methyl-prop-yl]inden-yl}zirconium(IV)

The title compound, [ZrCl(3)(C(19)H(25)N(2)Si)(C(4)H(8)O)], was prepared from bis-(N,N-dimethyl-amido-κN)(2-{2-[(1,2,3,3a,7a-η)-inden-yl]-2-methyl-prop-yl}-1H-imidazolido-κN(1))zirconium(IV) [(C(16)H(16)N(2))Zr(NMe(2))] by reaction with excess Me(3)SiCl in tetra-hydro-furan (THF) at elevated temperature. The crystal studied contained a minor non-merohedral twin contaminant [6.3 (4)%] which was taken into account during the refinement. The coordination polyhedron of the Zr(IV) atom is a distorted octa-hedron [assuming that the five-membered ring of the indenyl group (Cp) occupies one coordination site], with the Cp group and a THF O atom at the apical positions and the three Cl and ligating N atoms at the equatorial positions. The Zr, Si and the methyl-ene C atoms deviate noticeably from the imidazole ring plane [by -0.197 (5), -0.207 (5) and 0.119 (6) Å, respectively]. The THF ligand adopts an envelope conformation.

Novel dinuclear dimethylamido-3,5-dimethylpyrazolato and tetranuclear dimethylamido-3,5-dimethylpyrazolato-polyoxo zirconium(IV) complexes. Synthesis and structural characterisation

The dinuclear dimethylamido-tris(3,5-dimethylpyrazolato)-zirconium(IV) complex [Zr(3,5-Me2Pz)3(NMe2)]2 1 is prepared by treatment of [Zr(NMe2)4] with 3 equivalents of 3,5-dimethylpyrazole (3,5-Me2PzH) with elimination of dimethylamine. When [Zr(NMe2)4] reacted with 2 equivalents of 3,5-Me2PzH, the bis(dimethylamido)-bis(3,5-dimethylpyrazolato)zirconium(IV) compound [Zr(3,5-Me2Pz)2(NMe2)2]2 2 is obtained. Hydrolysis of [Zr(3,5-Me2Pz)3(NMe2)]2 in wet toluene affords the tetranuclear oxo compound [Zr4(eta2-3,5-Me2Pz)4(NMe2)2(mu3-O)2(mu2-3,5-Me2Pz)4(mu2-NMe2)2] . All synthesised compounds are characterised by NMR spectroscopic and analytical methods. Single crystal X-ray diffraction analysis has established the molecular structures of 1 and 4.

Amide−Silyl Ligand Exchanges and Equilibria among Group 4 Amide and Silyl Complexes

M(NMe2)4 (M = Zr, 1a; Hf, 1b) and the silyl anion (SiButPh2)- (2) in Li(THF)2SiButPh2 (2-Li) were found to undergo a ligand exchange to give [M(NMe2)3(SiButPh2)2]- (M = Zr, 3a; Hf, 3b) and [M(NMe2)5]- (M = Zr, 4a; Hf, 4b) in THF. The reaction is reversible, leading to equilibria: 2 1a (or 1b) + 2 2 <--> 3a (or 3b) + 4a (or 4b). In toluene, the reaction of 1a with 2 yields [(Me2N)3Zr(SiButPh2)2]-[Zr(NMe2)5Li2(THF)4]+ (5) as an ionic pair. The silyl anion 2 selectively attacks the -N(SiMe3)2 ligand in (Me2N)3Zr-N(SiMe3)2 (6a) to give 3a and [N(SiMe3)2]- (7) in reversible reaction: 6a + 2 2 <--> 3a + 7. The following equilibria have also been observed and studied: 2M(NMe2)4 (1a; 1b) + [Si(SiMe3)3]- (8) <--> (Me2N)3M-Si(SiMe3)3 (M = Zr, 9a; Hf, 9b) + [M(NMe2)5]- (M = Zr, 4a; Hf, 4b); 6a (or 6b) + 8 <--> 9a (or 9b) + [N(SiMe3)2]- (7). The current study represents rare, direct observations of reversible amide-silyl exchanges and their equilibria. Crystal structures of 5, (Me2N)3Hf-Si(SiMe3)3 (9b), and [Hf(NMe2)4]2 (dimer of 1b), as well as the preparation of (Me2N)3M-N(SiMe3)2 (6a; 6b) are also reported.