The group 4 metallocene compounds have been becoming an important class of catalysts for the synthesis of organic molecules and polymers. The known organometallic chemistry of group 4 metals (Ti, Zr and Hf) has been largely developed by the use of cyclopentadienyl (or indenyl) and substituted cyclopentadienyl (or indenyl) ligand. The broader tunable feature of eletronic and steric factors in the cyclopentadienyl type ligands allows greater versatility in the design of catalytic systems that are able to give regio- and stereo-regular polyolefins. As part of our efforts in pursuing this matter, a new group 4 organometallic compounds were synthesized and the full characterizations including the molecular structure analysis were carried out. The catalytic activity of these complexes in olefin polymerization has been also examined.
To study catalytic properties for styrene polymerization, the half-sandwich type titanium compound $(p-MePhC_5Me_4)TiCl_3$ (3) was synthesized by the reaction of the silylated Cp ligand $(p-MePhC_5Me_4)SiMe_3$ (2) with $TiCl_4$ in dichloromethane and the catalytic behavior of the compound 3 for styrene polymerization in the presence of excess amount of methylaluminoxane (MAO) has been carried out. This catalyst system produces a mixture of syndiotactic and atactic polystyrenes at 50℃. Treatment of 3 with stoichiometric amount of water gave oxo-bridged dinuclear titanium complex [$(p-MePhC_5Me_4)TiCl_2]_2$ ($μ_2$-O) (4) which was characterized by various spectroscopic means and single crystal X-ray diffraction study. We have also synthesized homoleptic titanium alkoxide compound bis (2,6-pyridinedimethoxy)titanium (5) and heteroleptic complex (2,6-pyridinedicarboxylate)(2,6-pyridinedimethoxy) titanium (6). Both titanium alkoxide compounds activated with MAO produce only atactic polystyrene, which differ from catalytic behavior of the general titanium alkoxide catalyst system. The molecular structure of 6 has been determined by X-ray crystallography. The coordination geometry about the titanium atom in 6 is the distorted octahedral consisted of two nitrogens and four oxygens.
CGC compound is usefully employed as catalyst precursor in commercial polymerization processes due to its high activity for either ethylene polymerization or ethylene/a-olefin copolymerization. An very efficient synthetic manner to constrained geometry catalysts $Me_2Si(Ind)(^{t}BuN)TiCl_2$ (9) was devised and the characterization of 9 and ligand precursors was carried out by NMR spectroscopy.
The synthesis, characterization and olefin polymerization properties of group 4 compounds ${1,2-(CH_2)_{10}(C_5H_3)}_2MCl_2$(M = Ti: 12, Zr: 13, Hf: 14) containing annulated twelve-membered ring cyclopentadienyl ligand have been studied. The reactions of lithiated annulated cyclopentadiene with $MCl_4(thf)_2$ (M = Ti, Zr) or $HfCl_4$ in tetrahydrofuran gave new non-bridged metallocene products in reasonable yields. The molecular structures of 13 and 14 have been characterized by single crystal X-ray diffraction studies. Both compounds are isostructural and adopt distorted tetrahedron around metal center. The steric crowding of the annulated twelve-membered ring in 13 and 14 is greater than that of seven-membered ring in ${2-Me-4,5-(CH_2)_5Cp}_2ZrCl_2$. The catalytic behavior of the new metallocenes in propylene and ethylene polymerization has been performed in the presence of excess amount of methylaluminoxane. Relatively low activities in olefin polymerization and the observation of atactic feature in polypropylene indicate that the use of the annulated twelve membered cyclopentadienyl ligand, a form of the elongated tetrahydroindenyl ligand is less favorable in electronic and steric points of view compared to the use of indenyl ligand.
Another non-bridged zirconocene complex $[1-(3-indenyl)indenyl]_2ZrCl_2$ (18) was obtained by the reaction of mono-lithium salt of 1-(3-indenyl)indenyl ligand (1,3'-biindene) with $ZrCl_4$ in diethyl ether or toluene. At low temperature, the zirconocene catalyst 18 with MAO produces isotactic-atactic stereoblock polypropylene by reversible isomerization of the non-bridged metallocene catalyst between chiral and achiral geometries during chain growth. In order to get some indirect insight into the ligand in 18, the corresponding ferrocene compound, $[1-(3-indenyl)indenyl]_2$Fe(19) was synthesied and characterized. We have also prepared zirconocene complexes, ${1,3-(i-Pr)_2Ind}_2ZrCl_2$ (23) and $C_2H_4{3-(i-Pr)Ind}_2ZrCl_2$ (24), containg isopropyl substituted indenyl derivative. The structural characterizations of 23 and only meso-diastereomer (24m) of the ethylene bridged ansa-metallocene 24 have been achieved by single crystal X-ray diffraction studies.
Silatranes of type $X\overline{Si(OCH_2CH_2)_3N}$ are cyclic silicon ethers containing a hypervalent silicon atom with a transannular bonding interaction between the silicon and the bridgehead nitrogen atom. The nature of substituents X affects the properties of silatranes such as the biological activities including antitumor properties and the strength of the transannular N-Si bonding. Our interests in silatrane chemistry have concentrated around developing new synthetic route to novel silatrane derivatives, although the reactivity of silatranes is limited in narrow range and largely depends on the type of X. As part of our synthetic efforts in diversifying the group X, the oxidative addition reactivity of 1-hydrosilatrane with $Co_2(CO)_8$ has been investigated. A novel silatranyl-oxymethylidyne cobalt cluster $[Co_3(CO)_9{μ_3-CO\overline{Si(OCH_2CH_2)_3N}}]$ (25) was prepared and characterized by various means including a single crystal X-ray diffraction study. The crystal structure contains two independent molecules in the asymmetric unit. The two molecules are similar and each molecule has a silatranyloxymethylidyne group triply bridging $Co_3(CO)_9$ fragment. A new fullerene derivative $[(C_{60}){Co_3(CO)_7-{μ_3-CO\overline{Si(OCH_2CH_2)_3N}}_2]ㆍ2(C_6H_5 CH_3)$ (26) was obtained by treatment of 25 with $C_{60} and characterized by the elemental analysis and the solid FT-IR spectroscopy.
