1. New Catalysts for Regio- and Stereospecific Polymerization

Compared to most transition metals which can show variable (different) oxidation states, rare earth metals usually adopt the +3 oxidation state as the most stable oxidation state, which is not easily changed to other oxidation states under normal conditions. This nature makes rare earth metals unique candidates for the formation of true "single-site" polymerization catalysts. Aiming towards the creation of new, high performance polymer materials, a part of our research program focuses on developing highly active and selective polymerization catalysts on the basis of the unique character of the rare earth metal complexes.

1-1. Development of Effective Catalyst for Syndiospecific Styrene-Ethylene Copolymerization

Syndiotactic polystyrene (sPS, see Scheme 1), is a very promising polymer because of its excellent resistance to heat and chemicals. A drawback that limits the application of sPS, however, is its brittleness. Since the discovery of sPS, extensive studies on the copolymerization of styrene with ethylene have been carried out to improve the toughness of this material. However, such attempts to obtain a styrene-ethylene copolymer having syndiotactic styrene-styrene sequences were not successful.   In striking contrast, our original catalyst system prepared from half-sandwich scandium complex 1 and [Ph3CB][(C6F5)4] exhibits excellent activity and selectivity for syndiospecific styrene-ethylene copolymerization. When the copolymerizations were carried out in the presence of ethylene and styrene by using this catalyst system, multi-block styrene-ethylene copolymers consisting of sPS sequences connected by polyethylene units were obtained. This is the first example of syndiospecific copolymerization of styrene with ethylene. Also A-B diblock styrene-ethylene copolymers with a sPS block could be synthesized by a sequential polymerization of styrene and ethylene in the presence of the same catalyst system.

Reference (1) J. Am. Chem. Soc. 2004, 126, 13190. <ACS e-reprint>

1-2. Development of a New Catalyst for Norbornene-Ethylene Copolymerization

Cyclic olefin copolymers (COCs) have recently attracted significant interest as one of the most important engineering plastics because of their many desirable properties such as thermal stability, good transparency, and chemical resistance. We recently found that cationic scandium species generated from half-sandwich scandium complex 1 and [Ph3CB][(C6F5)4] can act as an excellent catalyst system for alternating copolymerization of norbornene and ethylene, providing very transparent plastics (Scheme 2). This catalyst system is also effective for ethylene-dicyclopentadiene (DCPD, 3)-styrene terpolymerization (Scheme 3), constituting the first example of terpolymerization of ethylene, styrene, and a cyclic olefin.

References (1) Angew. Chem. Int. Ed. 2005, 44, 962. (2) Maclomolecules 2005, 38, 6767. <ACS e-reprint>

1-3. Development of High-Performance Catalysts for Isoprene Polymerization

Isoprene has a conjugated diene moiety, and its polymerization can provide various isomeric polymers (cis-1,4-; trans-1,4-; iso-, syndio-, or atactic 3,4-), depending upon how the C-C double bonds react. Therefore, it is critical to control the regio- and stereospecificities, in addition to molecular weight in order to obtain a high-quality polyisoprene having desirable properties.

We recently found that a catalyst system prepared from the binuclear half-sandwich yttrium complex 4, in which the two metal centers are bridged by two phosphido ligands, exhibits excellent activity and unprecedented isospecific 3,4-selectivity for isoprene polymerization, which affords for the first time a polyisoprene polymer with almost perfect isotactic 3,4-microstructure, high molecular weight, and unimodal narrow molecular weight distribution (Scheme 5). The obtained isotactic 3,4-polyisoprene is a new polymer and its applications will be explored.

Cis-1,4-polyisoprene is the main component of natural rubber (NR). One of the drawbacks of natural rubber, however, is that it contains allergic proteins and its molecular weight distribution is usually rather broad. On the other hand, the Cis-1,4-microstructure content of conventional synthetic polyisoprenes (isoprene rubber) is generally lower (up to 98%) than that of NR, and their physical properties are not as good as those of NR, despite just a 2% of difference.   We recently succeeded to synthesize a novel, non-metallocene yttrium complex 6 (Scheme 6) that exhibits excellent "livingness" and an extremely high cis-1,4 regio- and stereoselectivity for the polymerization of isoprene. This new catalytic system can provide an almost perfectly cis-1,4-regulated polyisoprene (nearly 100%) with a very narrow molecular weight distribution (<1.1) even at room or higher temperatures. The newly synthesized cis-1,4-polyisoprene is a very promising new polymer material suitable for a large number of applications, such as high-quality tires.

References (1) J. Am. Chem. Soc. 2005, 127, 14562. <ACS e-reprint> (2) Tetrahedron 2003, 59, 10525.

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2. New Organometallic Catalysts for Efficient Organic Synthesis

We also focus our efforts on developing new, efficient, and selective catalytic methods for organic synthesis using our rare earth complexes.

2-1. Selective Dimerization of Terminal Aromatic Alkynes

Transition-metal-catalyzed dimerization of terminal alkynes is an atom-economic and straightforward route to enynes, but it often gives a mixture of regio- and stereoisomers. By use of lanthanide catalyst 1with linked cyclopentadienyl-arylamide ligands, we have first realized the regio- and stereoselective head-to-head (Z)-dimerization of various aromatic terminal alkynes (Scheme 1). Aromatic C-Cl, C-Br and C-I survived the reaction conditions. The binuclear alkynide complex such as 2 has been confirmed to be a true catalyst, which can be recovered and reused , thus constituting a rare example of catalyst recycling in a homogeneous catalytic system.

This catalyst system can also be used for the regio- and stereoselective polymerization of aromatic terminal diynes such as 5 to afford π-conjugated polyenynes. The sequential coopolymerization of aromatic terminal diynes with caprolactone could also be achieved , which affords the corresponding block copolymers (Scheme 2). Such π-conjugated polymers have recently attracted much interest because they are of potential use as light-emitting materials for organic electro-luminescent (EL) devices.

We recently found that carbazole-substituted aromatic enynes, which are easily prepared by catalytic dimerization of the corresponding terminal alkynes by use of organolanthanide catalysts, can show unique optical properties (Scheme 3). For example, the (E)-enyne compound 7 shows blue fluorescence in solution, but can emit white electroluminescence in solid state, as a result of combination of the blue emission from an isolated molecule with the longer-wavelength emissions (green and orange-red) from eximers. Such an EL device can emit almost pure white light with CIE coordinates of (0.32, 0.33). This is the purest white emission ever reported for a single-emitting-component EL device. The quality of the white emission remained almost unchanged under varying driving voltages, demonstrating an advantageous potential of a single-emitting-component white organic light-emitting device (WOLED).

References (1) J. Am. Chem. Soc. 2003, 125, 1184. <ACS e-reprint> (2) J. Mol. Catal. A: Chem. 2004, 213, 101. (3) J. Am. Chem Soc. 2006, 128, 5592.

2-2. Catalytic Addition of Terminal Alkynes to Carbodiimides.

The catalytic addition of terminal alkynes to carbodiimides has been achieved for the first time by use of half-sandwich yttrium complex 9 as a catalyst, which offers a straightforward, atom-economical route to N,N'-disubstituted propiolamidines, a new family of amidines that were difficult to access by other means.

Reference (1) J. Am. Chem. Soc. 2005, 127, 16788. <ACS e-reprint>

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3. Synthesis of Rare Earth Metal Hydride Clusters and Studies on Synergistic Reactivity of These Complexes

Metal hydride complexes are fundamental components in a wide range of stoichiometric and catalytic reactions. Their importance in modern inorganic and organic chemistries cannot be overemphasized. For rare earth metals, well defined, discrete, molecular metal hydride complexes have long been limited to mono(hydrido) complexes of general type "L2MH" or "(L)(L')MH". By use of C5Me4SiMe3 as an ancillary ligand, we have recently succeeded in the isolation and structural characterization of a new type of rare earth hydride complex, which consists of the "LMH2" units and forms a stable tetranuclear structure via hydride bridges. Such polyhydrido complexes showed rich reaction chemistry toward a variety of substrates, some of which are unique to the rare earth metal polyhydrides. Shown below are some typical examples..

3-1. Synthesis of Polynuclear Rare-Earth Metal Polyhydride Complexes

The reaction of the half-sandwich rare earth metal bis(alkyl) complexes 1with H2 or PhSiH3 readily gives the corresponding tetranuclear octahydrido complexes 2 (Scheme 1). The polyhydrido clusters 2 are soluble and thermally stable in common organic solvents such as hexane, toluene, and THF. No decomposition or ligand redistribution or change of the cluster framework was observed in THF-d8 or toluene-d8, as monitored by 1H NMR.

References (1) Organometallics 2003, 22, 1171. <ACS e-reprint> (5) Organometallics 2005, 24, 4362. <ACS e-reprint>

3-2. Studies on Unique Reactivities of Rare-Earth Metal Polyhydride Clusters - Synergistic Activation Effect -

These hydride clusters shown above exhibit high reactivities towards C-C, C-N, and C-O multiple bonds (Scheme 2) . Some of these reactions were found to be unique to the polyhydride rare earth metal clusters. For example, the reaction of complex 2 with benzonitrile afforded tetranuclear benzylimido complex 3 through complete reduction of the C-N triple bond to a C-N single bond. In the reaction of 2 with γ-butylolactone, the C-O double bonds of butylolactone were completely reduced to a single bond, giving rise to a tetranuclear mixed alkoxo/hydride complex 4.   The reaction of 2 with the diyne 5 afforded tetranuclear complex 6, which consists formally of a [(Cp'YH)4]4+ unit and a butene-tetraanion unit such like 7 or 8. An X-ray crystallographic analysis revealed that butene-tetraanion unit in 6 is bonded to two Y metals in an "inverse sandwich" fashion. Complex 6 also exhibited unique reactivity towards CO2 affording the methylene diolate complex 9 through complete reduction of both two C-O double bonds in CO2.

In addition to the above novel (stoichiometric) reactions, our recent studies revealed that some of these polynuclear complexes can catalyze alternative copolymerization of cyclohexene oxide with CO2 and trimerization of aromatic nitriles (Scheme 3).

Reference (1) J. Am. Chem. Soc. 2004, 126, 1312. <ACS e-reprint> (2) J. Am. Chem. Soc. 2004, 126, 8080. <ACS e-reprint>(3) Maclomolecules 2005, 38, 6767. <ACS e-reprint> (4) Angew. Chem. Int. Ed. 2005, 44, 959.

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