Jun Okuda

Rare-Earth Metal Complexes Supported by 1,ω-Dithiaalkanediyl-Bridged Bis(phenolato) Ligands : Synthesis, Structure, and Heteroselective Ring-Opening Polymerization of rac-Lactide

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.

Half‐Sandwich Alkyl and Hydrido Complexes of Yttrium: Convenient Synthesis and Polymerization Catalysis of Polar Monomers

Direct preparation of alkyl half-sandwich complexes of the heavier rare earth elements (such as 1) is possible by alkane elimination from the corresponding cyclopentadiene derivative and the trialkyl complex [Y(CH2SiMe3)3(thf)2]. Hydrogenation of 1 gives the highly fluxional dimeric hydride complex 2. Both complexes catalyze the polymerization of the polar monomers tert-butyl acrylate and acrylonitrile.

Rare earth metal complexes that contain linked amido-cyclopentadienyl ligands: ansa-metallocene mimics and “constrained geometry” catalysts

A survey of group 3 metal complexes that contain a linked amido-cyclopentadienyl ligand is given. Originally designed as ansa-metallocene analogues for the development of single-component olefin polymerization catalysts, variations in the metals and ligand substituents have allowed the synthetic access to new rare earth metal complexes including olefin polymerization initiators, divalent complexes and heterobimetallic metallocenes. The linked amido-cyclopentadienyl ligands have made half-sandwich complexes of group 3 metals accessible in a more systematic manner and provided a better understanding of the electronic and steric constraints of chelating ligands for the larger rare earth metal centres.

Rare-Earth Metal Alkyl and Hydride Complexes Stabilized by a Cyclen-Derived [NNNN] Macrocyclic Ancillary Ligand

A trinuclear rare-earth metal hydride complex was synthesized from the dialkyl complex supported by a monoanionic [NNNN] macrocycle and shown to catalyze the hydrosilylation of olefins efficiently.

Copolymerization of ethene with styrene using different methylalumoxane activated half-sandwich complexes

Ethene was copolymerized with styrene using five different methylalumoxane (MAO) activated half-sandwich complexes of the general formula Me 2 Si(Cp)(N-R)MCl 2 , varying the substituents on the cyclopentadienyl ring and the substituent on the amide (Cp = tetramethylcyclopentadiene CBT, 1-indenyl IBT, 3-trimethylsilyl-1-indenyl SIBT, or fluorenyl FBZ, R = tert-butyl (complexes CBT,IBT, SIBT, FBZ ) or benzyl CAT), as well as the metal center ( M = Ti, except FBZ: M = Zr). Polymerization behavior was analyzed with respect to catalyst activity and polymerization kinetics, styrene incorporation, copolymer microstructure, and molecular weight. All complexes produced random poly(ethene-co-styrene) without any regioregular or stereoregular microstructure. Complex CBT showed the highest catalytic activity, the fluorenyl-substituted complex FBZ produced the highest molecular weight polymer, and complexes SIBT and CAT promoted high styrene incorporation. Cp-substitution pattern influenced deactivation of the catalytic system with bulky substituents of the Cp-ring slowing down deactivation at the expense of styrene incorporation. Moreover, deactivation was accelerated with increasing styrene concentration.

Initiators for the stereoselective ring-opening polymerization of meso-lactide

Poly(lactides) (PLAs), or poly(lactic acid)s, are among the first commercial biodegradable polymers that have the potential to become commodity plastics. meso-Lactide, a by-product of L-lactide production, will become more easily available. Highly stereoregular poly(meso-lactides) should be crystalline and thus interesting as polymeric material. In this short review, initiators capable of inducing the stereoselective ring-opening polymerization of meso-lactide to ideally give syndiotactic or heterotactic PLAs will be discussed. Mechanistic discussions with regard to understanding the reactivity differences between the various lactide monomers are included.

Rare earth metal complexes supported by 1,ω-dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, characterization and ring-opening polymerization catalysis of L-lactide

Monomeric rare earth metal bis(phenolato) complexes [(Ls,s)Ln{N(SiHMe2)2}(THF)] (1a–4c) were isolated from the reaction of silylamido complexes [Ln{N(SiHMe2)2}3(THF)x] (Ln = Sc, x = 1; Ln = Y, Lu, x = 2) and one equivalent of tetradentate 1,ω-dithiaalkanediyl-bridged bis(phenol)s etbmpH2, ptbmpH2, edtbpH2 and pdtbpH2 in moderate to high yields. In contrast to the unsymmetrical scandium complexes 1a and 3a, the scandium complex 2a, the yttrium complexes 1b and 4b as well as the lutetium complexes 1c–4c show Cs or C2 symmetry due to the relatively fast dissociation of THF on the NMR time scale at room temperature. The monomeric structures of the complexes 2a and 4b were confirmed by X-ray diffraction studies. The six-coordinate central metal with the tetradentate ligand, the silylamido group, and one THF, adopts a C1 symmetrical configuration with trans(O,O) or cis(O,O) orientation of the two oxygen donors of the ligand. Distorted octahedral and trigonal prismatic coordination geometries are found for 2a and 4b. Substitution reaction with 2,2,6,6-tetramethyl-3,5-heptanedione afforded the corresponding complexes 6b, 6c, 7c and 8b with dimeric structures and with trityl alcohol the alkoxide complex [(etbmp)Y(OCPh3)(THF)] (9). All new complexes efficiently initiated the ring-opening polymerization of L-lactide in THF. High molecular weight poly(L-lactide)s with narrow molecular weight distributions (Mw/Mn 1.15–1.41) were obtained using complexes 1a–4c. Dimeric β-diketonato complexes were only active in the presence of THF or excess isopropanol.

Bis(allyl)calcium

In contrast to the widely employed organomagnesium and organolithium compounds, 2] the s and p complexes of the heavier alkaline earth metals calcium, strontium, and barium have not achieved their full potential. Whereas the chemistry of cyclopentadienyl (Cp) calcium compounds is fairly well developed, that of Cp-free alkyl, alkynyl, aryl, benzyl, indenyl, and fluorenyl calcium compounds has just begun unfolding. Allyl calcium compounds of unknown structure were first mentioned in a patent as polymerization initiators for vinylic monomers. The first structurally characterized calcium–allyl bond was reported by Hanusa et al. for [Ca{h-C3(SiMe3)2H3}2(thf)2] (1), the reactivity of which is suppressed by bulky ligands and stabilization through negative hyperconjugation of the silyl moieties.

Efficient cyclic carbonate synthesis catalyzed by zinc cluster systems under mild conditions

An efficient catalytic system of a zinc cluster and tetrabutylammonium iodide (TBAI) was developed for cyclic carbonate synthesis from epoxides and carbon dioxide (CO2) without the use of any organic solvents under very mild conditions (25 °C, 1 atm), even in the presence of impurities such as water and air. Electrophilicity of the central Zn(II) ion, the number of trifluoromethyl groups, nucleophilicity and leaving ability of the anion of alkylammonium salts, and various reaction parameters have a great effect on the catalytic activity of the bifunctional catalyst. Therefore, this solvent-free process represents an environmentally friendly example for the catalytic conversion of CO2 into value-added chemicals and also has the potential to contribute towards decreasing atmospheric CO2 emission from the burning of fossil fuels.

Ethylene polymerization catalysts based on nickel(II) 1,4-diazadiene complexes: the influence of the 1,4-diazadiene backbone substituents on structure and reactivity

Abstract Thermally sensitive dialkyl nickel complexes {DAD(H,H)}Ni(CH2SiMe3)2 and {DAD(Me,Me)}Ni(CH2SiMe3)2 [DAD(X,X)=2,6-iPr2C6H4–NC(X)–C(X)N–C6H4iPr2-2,6] were synthesized and were characterized by X-ray diffraction studies on single crystals. The substituents X on the backbone of the α-diimine ligand significantly influence the conformation of the 2,6-diisopropylphenyl substituents. This effect is thought to be of crucial importance for the polymerization of ethylene when {DAD(X,X)}NiBr2/MAO is used as catalyst. The influence of the catalyst structure, pressure, and temperature on the polymerization activity, molar mass, glass transition temperature, melting temperature and branching of the polymers has been studied. The dialkyl complex {DAD(H,H)}Ni(CH2SiMe3)2 underwent rapid reductive carbonylation giving the dicarbonyl complex {DAD(H,H)}Ni(CO)2 along with both bis(trimethylsilyl)ethane and α,α′-bis(trimethylsilyl)acetone. The dicarbonyl {DAD(H,H)}Ni(CO)2 was characterized by X-ray crystallography.