| Chapter 1. A review of recent literature on advances in intermolecular carbon-hydrogen (C-H) bond activation promoted by early-metal complexes bearing multiple metal-heteroatom bonds (MX, M = Groups 4-6, X = NR, O, CR2, and CR) is presented. These examples provide the background and context through which the work detailed in Chapters 2 through 4 can be viewed.;Chapter 2. Monomeric imidozirconocene complexes of the type Cp2(L)Zr=NCMe3 (Cp = cyclopentadienyl (eta5-C 5H5), L = Lewis base) have previously been shown to activate the sp2-hybridized C-H bonds of benzene, but not the sp 3-hybridized C-H bonds of saturated hydrocarbons. The racemic ethylenebis(tetrahydro)indenyl (ebthi) zirconium amide complex rac-(ebthi)Zr(Me)(NHCMe 3), however, cleanly and quantitatively activates a wide range of n-alkene, alkene, and arene C-H bonds. This system furnishes the first general method for the preparation of complexes containing zirconium-hydrocarbyl bonds via a C-H bond activation mechanism.;Chapter 3. In contrast to earlier results obtained with monomeric Cp2Zr=NCMe3(THF) (Cp = eta5-C 5H5), the electron-donating ethylenebis(tetrahydro)indenyl (ebthi) ligand promotes clean and quantitative C-H bond addition reactions across the Zr=N bond with alkane C-H bonds. To investigate the factors responsible for this observed increase in reactivity, a series of complexes of the type CPxnZr=NCMe3(THF) (Cpx = eta5-C5RYH5-y) are evaluated for C-H bond activation. Complexes bearing ligands that present an electron-donating environment (e.g., Cp*CpZr=NCMe3(THF), where Cp* = eta 5-C5Me5) activate n-alkanes, while complexes bearing less electron-donating ligands react only with sp 2-hybridized C-H bonds. Adverse steric interactions, such as those generated by two bulky Cp* ligands in the Cp*2Zr=NCMe3(THF) system, also preclude the activation of n-alkanes. Thus, an optimal balance of stereoelectronic properties for the C-H bond activation of sp3 C-H bonds is promoted by partially alkylated ancillary Cp ligands.;Chapter 4. The Cp*CpZr=NCMe3(THF) system allows for the first direct measurement of kinetic selectivity in sp2 and sp3 C-H bond activation with Group 4 M=NR complexes. The feature of kinetic control provides an opportunity for the design of mechanistic experiments capable of probing the 1,2-RH-addition event. Kinetic isotope effect studies suggest that imido complexes activate terminal alkynes via rate-determining metallacycle formation followed by rearrangement, while a direct 1,2-RH-bond addition mechanism operates for other substrates. |
