FI catalysts for olefin polymerization--a comprehensive treatment.

ion using borate reagents [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4], respectively, results in highly active polymerization catalysts (Table 4). Scandium bis(alkyl) complex 51Sc shows excellent activity for the syndiospecific styrene homopolymerization (activity, 1.36 × 104 (kg PS)/ (mol Sc h); Mw/Mn ) 1.37), and complexes 51, 53, and 54 proved suitable for the coand terpolymerization of a series of monomers.204,206,210-225 8.2.3. Constraint Geometry Complexes Incorporation of the cyclopentadienyl ancillary ligand into a chelate array of (pendant) donor functionalities gives access to prominent Cp derivatives. Since the original introduction by the Bercaw group, the linked amido-cyclopentadienyl (Cp) ligand has advanced to be one of the most versatile ligands for group 4 metal polymerization catalysts.235 Catalysts based on this type of ligand provide a constrained ligand environment but are anticipated to be more active toward sterically demanding monomers than metallocenes. Aminecyclopentadiene proligands react with Ln(CH2SiMe3)3(thf)x (Jthf) according to an alkane-elimination reaction in a similar manner as shown in Scheme 30. Because of the dianionic nature of the resulting ligand, only one [CH2SiMe3] ligand is retained in the resulting compounds, which allows for further derivatization (Chart 11). In the presence of Ph3SiH or H2, complexes 66-73 form dimeric hydrido complexes,176,177,204,236 showing high potential in the catalytic hydrosilylation of olefins.226,237,238 Catalytic activities and stereoselectivities are hereby influenced by the length of the linker between the cyclopentadienyl and the amido-functionality and the substituents at the amidonitrogen.237 Remarkable catalytic activity was observed for complexes 68cyclohexyl. Upon activation with equimolar amounts of [Ph3C][B(C6F5)4], such compounds polymerized ethylene and isoprene, regiospecifically yielding 3,4-polyisoprene with isotactic-rich stereo microstructures and relatively narrow molecular weight distribution (Mw/Mn ) 1.8).222 Complex 67Y was found to initiate the polymerization of the polar monomers tert-butyl acrylate and acrylonitrile, however, yielding atactic polymeric products (see Table 5).204 8.2.4. Complexes with Neutral Nitrogenand Oxygen-Based Ligands While early work in organorare-earth metal chemistry was dominated by complexes supported by cyclopentadienyl-type ligands of varying substitution and modification, the limitations inherent to these ligand sets triggered the development of alternative ancillary ligands. Particularly in the past 15 years, advanced ligand design gave access to a wide variety of rare-earth metal complexes supported by noncyclopentadienyl ligand environments. Because of the Lewis acidic nature of the rare-earth metal ions, ligands based on the hard donor elements oxygen and nitrogen are most commonly used, while some notable exceptions have been reported. To avoid ligand redistribution, multidentate ligands are generally favored. Since rare-earth metal cations are invariable in the +3 oxidation state (except Eu(II), Sm(II), Yb(II), and Ce(IV)), neutral, monoanionic, or dianionic ligand sets are the most desirable. Table 4. Further Applications of Half-Sandwich Complexes (Cp)Ln(CH2SiMe3)2(donor)x compound further application ref 50 [CH2SiMe3] exchange reactions 205-209 formation of mono(cations) alternating ethylene-norbornene copolymerization

Homoleptic Rare-Earth Metal Complexes Containing Ln−C σ-Bonds†

Publisher Summary The chapter discusses methods for quantifying Lewis acidity; a survey of the chemistry of X 2 BAr F , XB(Ar F ) 2 and B(Ar F ) 3 compounds and their adducts; polyfunctional perfluoroaryl boranes; and applications that do not involve a-olefin polymerization by single-site catalysts via a coordination polymerization mechanism. The chapter highlights the chemistry of perfluoroaryl boranes. The related compound of perfluoroaryl boranes, that is, B(C 6 F 5 ) 3, is an effective co-catalyst for olefin- polymerization processes has led to the proverbial explosion of research activity in its chemistry and that of its derivatives. The unique properties of B(C 6 F 5 ) 3 , that is, thermal and hydrolytic stability coupled with strong Lewis acidity have led to extensive applications in widely varying areas of chemistry. The chapter focuses on applications as Lewis acids, but emerging applications such as anion transport additives in lithium batteries suggest an even broader utility for this remarkable class of compounds. The chapter discusses that as B(C 6 F 5 ) 3 and its derivatives become more broadly available, this should continue to be an active area of research.

The Chemistry of Perfluoroaryl Boranes

Monomeric diolate amido yttrium complexes [Y{diolate}{N(SiHMe2)2}(thf)2] can be prepared in good yield by treating [Y{N(SiHMe2)2}3(thf)2] with either 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diol (H2(Biphen)), 3,3′-bis(2,4,6-triisopropylphenyl)-2,2′-dihydroxy-1,1′-dinaphthyl (H2(Trip2BINO)) or 3,3′-bis(2,6-diisopropylphenyl)-2,2′-dihydroxy-1,1′-dinaphthyl (H2(Dip2BINO)) in racemic and enantiopure form. The racemic complex [Y(biphen){N(SiHMe2)2}(thf)2] dimerizes upon heating to give the heterochiral complex (R,S)-[Y(biphen){N(SiHMe2)2}(thf)]2. The corresponding dimeric heterochiral lanthanum complex was the sole product in the reaction of H2(Biphen) with [La{N(SiHMe2)2}3(thf)2]. Single-crystal X-ray diffraction of both dimeric complexes revealed that the two Ln(biphen){N(SiHMe2)2}(thf) fragments are connected through bridging phenolate groups of the biphenolate ligands. The two different phenolate groups undergo an intramolecular exchange process in solution leading to their equivalence on the NMR timescale. All complexes were active catalysts for the hydroamination/cyclization of aminoalkynes and aminoalkenes at elevated temperature, with [Y((R)-dip2bino){N(SiHMe2)2}(thf)2] being the most active one giving enantioselectivities of up to 57 % ee. Kinetic resolution of 2-aminohex-5-ene proceeded with this catalyst with 6.4:1 trans selectivity to give 2,5-dimethylpyrrolidine with a krel of 2.6.

Synthesis and characterization of new biphenolate and binaphtholate rare-Earth-metal amido complexes: catalysts for asymmetric olefin hydroamination/cyclization.

Monomeric yttrium and lutetium bis(phenolato) complexes [Ln(OSSO){N(SiHMe 2) 2}(THF)] (Ln = Y, Lu) were prepared from the reaction of silylamido complexes [Ln{N(SiHMe 2) 2} 3(THF) 2] with 1 equiv of tetradentate 1,omega-dithiaalkanediyl-bridged bis(phenol) (OSSO)H 2 1- 9 in moderate to high yields. In contrast to the rigid configuration of scandium analogues, the yttrium complexes 2b and 3b and the lutetium complex 3c that contain a C 2 bridge between the two sulfur donors of the ligand are symmetric in solution. The monomeric nature of these complexes was indicated by an X-ray diffraction study of the yttrium complex 6b. The yttrium center in 6b is coordinated to the tetradentate [OSSO]-type ligand, one silylamido group and one THF ligand with the two oxygen donors of the [OSSO]-type ligand located trans. Corresponding bis(phenolato) silylamido complexes of larger rare-earth metals could not be obtained from similar reactions: Reaction of [La{N(SiHMe 2) 2} 3(THF) 2] with 1,2-xylylene-linked bis(phenol) gave a dinuclear lanthanum complex 6d of the formula [La 2(OSSO) 3] with two inequivalent eight-coordinate metal centers. The yttrium and lutetium complexes efficiently initiated the ring-opening polymerization (ROP) of lactides in THF. The heteroselectivity during the ROP of rac-lactide was enhanced when the steric demand of the bis(phenolato) ligand was increased, either by extending the bridge length or by introducing bulky ortho-substituents in the phenoxy units. A C 3 bridge within the ligand backbone is essential to allow configurational interconversion of the active site between Lambda and Delta configuration during polymerization, allowing accommodation of both enantiomers of the monomer in an alternating fashion.

Rare-earth metal complexes supported by 1,omega-dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, structure, and heteroselective ring-opening polymerization of rac-lactide.

a and Masood Parvez

Papers

Organometallic complexes of scandium and yttrium supported by a bulky salicylaldimine ligand

Reaction of Bis(pentafluorophenyl)borane with Methylidyne Complexes: Synthesis and Characterization of a Cationic Tungsten(VI) Borylalkylidyne Hydride