Our research efforts at West Virginia University focus on the synthesis of Group 4 (Ti, Zr, Hf) organometallic complexes and systematic investigations of their chemical reactivity. We are particularly interested in taking advantage of the availability of one or more vacant orbitals at the electrophilic metal center to activate unsaturated organic substrates by migratory insertion into specific metal-carbon and metal-hydride bonds, by selective C,C-coupling reactions, or by C-H bond activation. The characterization of these compounds is accomplished by an arsenal of spectroscopic (IR, NMR, EPR) and X-ray crystallographic methods.

The electrophilicity and reactivity of early transition metal metallocenes of the general form, (C₅R₅)₂ML₂, can be altered either by the introduction of a short interannular bridge between the two cyclopentadienyl rings or by the replacement of the two linked or unlinked cyclopentadienyl rings with a bifunctional monocyclopentadienyl ligand containing an appended anionic N-functionality. The attractiveness of this latter ligand system lies in its ability to enhance the metal's Lewis acidity by reducing the formal electon count and the steric congestion at the metal.

The corresponding 14-electron ansa-monocyclopentadienylamido group 4 metal complexes, [(C₅R₄)SiMe₂(NR')]MCl₂ (R = H, Me; R' = alkyl, aryl), when activated with methylalumoxane, have been shown independently by Dow Chemical and Exxon to provide highly active Ziegler-Natta catalysts for the copolymerization of ethylene and 1-alkenes. Recent efforts in our laboratories have led to the development of efficient synthetic procedures for the complexation of bifunctional ansa-monocyclopentadienylamido-type ligands to an electrophilic group 4 metal center. For example, the neat amine elimination reaction of M(NMe₂)₄ (M = Ti, Zr, Hf) with (C₅H₅)SiMe₂(N(H)-t-Bu) provides an effective method for the preparation of [(C₅H₄)SiMe₂(N-t-Bu)]M(NMe₂)₂, which is readily converted to [(C₅H₄)SiMe₂(N-t-Bu)]MCl₂ with SiMe₃Cl.

This general amine elimination strategy has been employed for the preparation of a variety of related complexes, including [(C₉H₆)SiMe₂(N-t-Bu)]MCl₂ (M = Ti, Zr, Hf), [(C₁₃H₈)SiMe₂(NR)]ZrCl₂ (R = i-Pr, t-Bu), and [(C₅Me₄)SiMe₂(NR)]ZrCl₂ (R = i-Pr, t-Bu) (see Organometallics, 15, 1572 (1996); Inorganic Chemistry, 43, 3976, (2004)).

Early transition metal butadiene complexes represent a versatile class of organometallic reagents that have led to the development of alternative stoichiometric reagents for stereoselective organic synthesis and the generation of novel homogeneous Ziegler-Natta polymerization catalysts. For example, the (⁴-butadiene) metallocene complexes (C₅H₅)₂M(C₄H₆) (M = Zr, Hf) when treated with B(C₆F₅)₃ afford the highly reactive zwitterionic species, (C₅H₅)₂M(+)(³-C₃H₄CH₂B(-)(C₆F₅)₃, which are capable of catalyzing the polymerization of ethylene and propylene. The corresponding Group 4 butadiene compounds containing a bifunctional ansa-monocyclopentadienyldimethylsilylamido ligand are readily obtained from the metathetical reaction of [(C₅H₄)SiMe₂(N-t-Bu)]MCl₂ (M = Ti, Zr) with [Mg(C₄H₆)(THF)₂]_n. The solution NMR data and molecular structure of [(C₅H₄)SiMe₂(N-t-Bu)]Ti(C₄H₆) indicate that the butadiene moiety adopts a symmetrical prone structure consistent with a Ti(II) pi-diene bonding representation. Alternatively, the solution and solid state structures for [(C₅H₄)SiMe₂(N-t-Bu)]Zr(C₄H₆) are markedly different. Whereas the NMR data supports a zirconacyclopentene structure in solution, the solid state structure of this compound is actually tetranuclear with the four electrophilic Zr(IV) centers linked by four unsymmetrically bridging butadiene groups via an unusual (1, 2, 3-³-Zr-(4-µ₂)-Zr) bonding interaction (see Organometallics, 20, 2623 (2001)).

The stoichiometric reactions of [(C₅H₄)SiMe₂(N-t-Bu)]M(C₄H₆) with B(C₆F₅)₃ proceed with the exclusive formation of the zwitterionic metal betaine complexes, [(C₅H₄)SiMe₂(N-t-Bu)]M(+)(C₄H₆)B(-)(C₆F₅)₃, M = Ti, Zr. X-ray structural analyses of these novel compounds indicate that the pi-allyl unit within the unsymmetrically bridging cis-butadiene adopts the transzoid Z-configuration.

The structure of the Ti zwitterion is stabilized by a pair of Ti^...H-CB(-) agostic interactions, whereas the structure of the Zr analogue features a dative Zr^...F-C(ortho) interacton. Variable temperature ¹⁹F NMR data show that upon cooling the single ortho-F resonance observed at room temperature for the three freely rotating perfluorophenyl substituents separates into six distinct singlets, consistent with coordination of an ortho-F to the electrophilic Zr center. The free energy barrier for disruption of this dative interaction is estimated to be 9.8 kcal/mol (see Organometallics, 23, 4495 (2004)).

Complexes that feature a bifunctional monocyclopentadienyldimethylsilylamido ligand generally display reactivity patterns divergent from their metallocene analogs. For example, carboxylation of [(C₅Me₄)SiMe₂(N-t-Bu)]ZrMe₂ proceeds with CO₂ insertion into the Zr-C(Me) bonds affording the diacetate derivative, which is susceptible to electrophilic attack by an additional equivalent of CO₂ and subsequent elimination of OCN-t-Bu (see Organometallics, 15, 7 (1996)). Isocyanides, CNR, undergo migratory insertion into the two Zr-C(Me) bonds of [(C₅Me₄)SiMe₂(N-t-Bu)]ZrMe₂ to give the corresponding bis(iminoacyl) derivatives. Subsequent thermolysis leads to C,C-coupling and the formation of the corresponding enediamidate complexes, [(C₅Me₄)SiMe₂(N-t-Bu)]Zr[N(R)C(Me)=C(Me)N(R)] (see Organometallics, 16, 3548 (1997)).

Electrophilic early transition metal complexes featuring multifunctional ligands provide a new generation of "non-metallocene" catalysts for olefin polymerization. The incorporation of a chelating diamido ligand has led to the preparation of four- and five-coordinate precursors, such as [ArN(CH₂)₃NAr]TiCl₂ (McConville) and [(MesNCH₂CH₂)₂NMe]ZrMe₂ (Schrock), which are capable of promoting the living polymerization of -olefins. Amine elimination and toluene elimination strategies based upon M(NR₂)₄ and M(benzyl)₄, where R = Me, Et, and M = Ti, Zr, Hf, provide suitable alternatives to conventional metathesis reactions for the preparation of group 4 metal diamide and dibenzyl complexes featuring multifunctional ligands. We have recently begun to explore the corresponding amine elimination and toluene elimination reactions that employ a M(NR₂)₂Cl₂ or a M(benzyl)₂Cl₂ equivalent for the preparation of the four- and five-coordinate diamido metal dichloride complexes. The five-coordinate Ti dichloride complexes [(MesNCH₂CH₂)₂NR]TiCl₂ (R = H, SiMe₃) are conveniently prepared from the amine elimination reaction of the triamines (MesNHCH₂CH₂)₂NR with Ti(NEt₂)₂Cl₂. The corresponding five coordinate Zr and Hf homologues [(MesNCH₂CH₂)₂NR]MCl₂ are synthesized via the toluene elimination reactions of these triamines with M(benzyl)₂Cl₂(Et₂O)₂ (M = Zr, Hf). The thermally unstable and photosensitive M(benzyl)₂Cl₂(Et₂O)₂ species may be prepared in situ from the reaction of M(benzyl)₄ with 2 equiv of [NEt₃H]Cl in diethyl ether at O ^oC in the dark. The X-ray structural and solution NMR data reveal that these electrophilic group 4 metal complexes adopt the facial conformation with a chloride ligand lying trans to the central amino N atom of the tridentate diamido amine ligand (see Organometallics, 24, 5383 (2005)).

A list of publications based upon work performed in my research group or resulting from X-ray structural analyses performed in collaboration with others is provided below.

The X-ray diffraction facilities in the Department of Chemistry consist of a Siemens P4 diffractometer with a SMART 1K CCD detector system.