Imido Complexes of the Heavier Group 5 Metals Supported by an N

Chapter 1. The preparations of Nb(V) halide, alkyl, and aryl complexes supported by both the N, N'-diaryl-beta-diketiminato (BDI) and imido ligands are described. Characterization of the complexes by X-ray diffraction reveals a dependence of the bonding within the BDI framework on the geometry of the complex with respect to the position of the imido group. The dichloride complex was found to be suitable for introduction of a second imido ligand, leading to the rare Group 5 bis(imido) structural moiety. The synthesis, structural characterization, and reaction chemistry of these complexes will be discussed. Chapter 2. The outcome of the reaction of (BDI)Me 2Nb(NtBu) with carbon monoxide (CO) was found to be highly dependent on the reaction conditions employed, leading to a range of reaction products. Among them, the products of metal-based reduction, C=C bond formation, and C--H activation were observed. A reaction scheme accounting for the observed products is proposed based on both the conditions under which the different products form and the synthesis of stable analogues to various intermediates proposed to lie along the carbonylation reaction sequence. Chapter 3. The synthesis and reaction chemistry of cationic niobium and tantalum monomethyl complexes is discussed. The cationic complexes failed to irreversibly bind CO but were found to form phosphine-trapped acyl complexes in the presence of a combination of trialkylphosphines and CO. The outcome of this reaction is compared to the reaction of the cationic niobium complex with an aryl isocyanide, an isoelectronic variant of CO. Further stoichiometric reactions with the Nb-Me and Ta-Me bonds will be discussed, including (i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; (ii) protonolysis reactions with Ph3SiOH to form thermally robust cationic siloxide complexes; and (iii) the capability of the complexes to perform ethylene polymerization. Chapter 4. The stability of (BDI)Nb(III) imido complexes is discussed, including the characterization of the product of reductive ligand degradation. The use of strong pi-acids to stabilize the d 2 metal center led to complexes that were indefinitely stable at room temperature, providing a means of systematically investigating the application of these low-valent early-metal complexes toward the formation of new bonds (Nb=NR, Nb=0, Nb=C, C-C, and C-H) based on the reducing capability of the metal complex. Also, the details of a catalytic alkyne hydrogenation reaction will be discussed, including initial mechanistic investigations which suggest that the reaction is proceeding through a Nb(III)/Nb(V) redox cycle.