Ortho-metalation through C-H bond activation has been widely used as a means to generate bidentate ligands that support metal complexes with catalytic activities in hydroamination and alkyne hydroalkoxylation reactions. For this reason, many ligand systems based on alpha,beta-unsaturated ketones, esters and acetophenones have been the topic of intense research in recent years. With late transition metals, chelation of the acetophenone type ligands can be easily achieved through oxidative addition of the ortho C-H bond. However, due to the high oxidation states of early transition metals, this route cannot be utilized to successfully coordinate and synthesize analogous early transition metal complexes. As discussed in Chapter Two, to achieve site-specific activation, acetophenone imines were treated with nBuLi.;The major goal of the research presented in this dissertation involved the synthesis and characterization of a series of niobium and tantalum imido complexes with these mono-anionic ortho-metalated acetophenone imine ligands. The products were characterized using NMR spectroscopy, mass spectrometry, and elemental analysis. These low symmetry complexes are produced with only one or two isomers in all cases and display interesting correlations between the steric bulk of the ligands employed and the isomers isolated. Crystal structures of several new niobium and tantalum complexes are presented as confirmation of the structural isomers produced. The reactions of the activated acetophenone imine ligands with several group V imido precursors demonstrate a few trends. For example, depending on the steric bulk of the imido group, more than one structural isomer was frequently generated and the ratio of these structural isomers varied depending upon the reaction temperature employed. In order to fully understand the steric and electronic effects of the acetophenone imine ligands on the structure and properties of the resulting metal complexes, more detailed studies were undertaken through the synthesis of a series of niobium and tantalum complexes by varying the size of the substituents on both imine and imido groups. Full details on this study are presented in Chapter Three.;To delineate the reactivity of these new metal complexes and to obtain potential catalyst precursors, we also undertook an investigation of the substitution chemistry accessible by derivatization of the metal halogen bond. Each derivatized complex was produced as a single isomer and fully characterized using NMR spectroscopy, elemental analysis, and X-ray crystallography. The successful isolation and characterization of derivatization reaction products of the niobium imido complex L2NbCl(NAr) (L = ortho-metalated acetophenone imine, Ar = 2,6-iPr2C6H 3) with carbon and oxygen donor ligands such as Me, Me3SiCH 2, PhC≡kC, Me3SiC≡C, CF3SO3, and MeO are described. These more reactive sigma-donor ligands were readily installed in place of the chloride ligand through salt metathesis reactions. The trimethylsilylmethyl complex showed significant beta-agostic interactions between the methylene group and the niobium center. Similar strategies to derivatize the chloride complex utilizing lithiated amides (LiNMe2, LiNEt2, LiNiPr2, and LiNC5H 10) resulted in the production of a niobium-hydride species due to beta-hydrogen elimination processes. All of the resulting C1 symmetric complexes were formed as predominantly a single isomer and fully characterized using a combination of 1H and 13C NMR spectroscopy, elemental analyses, and X-ray crystallography, when possible.
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