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Design of Niobium and Tantalum Systems for Small Molecule Activation

Abstract

Chapter 1. This chapter comprises a detailed overview of the design and reactivity of pi-loaded group 5 transition metal complexes. The pi-loading strategy—coordination of two or more strongly pi-donating ligands to a single metal center—is discussed as it applies to promoting reactivity at group 5 transition metal-imido groups. Electronic structure studies of group 5 bis(imido) complexes are presented, and examples of catalytically and stoichiometrically active group 5 bis(imido) and chalcogenido-imido complexes are reviewed. These examples are intended to provide needed background information for chapters 2 and 3 of this dissertation and encourage future work exploring pi-loaded bis(imido) systems of the group 5 triad.

Chapter 2. Synthetic access to two pi-loaded Nb bis(imido) complexes is achieved. The 1,2- addition and [2+2] cycloaddition reactivity of these complexes is investigated, leading to activation of diverse substrates across the Nb-imido pi-bond. Studies of stoichiometric 1,2-addition revealed a significant substituent effect in bis(imido) complexes: more electron-donating substituents lead to more reactive Nb-N p-bonds. In addition, stoichiometric [2+2] cycloaddition reactivity of bis(imido) complexes with sulfur- and oxygen-containing heteroallenes is evaluated. One product generated through these studies, a carbamate imido complex, is induced to undergo retro-cycloaddition to eject isocyanate, generating the first terminal oxo imido Nb complex. Subsequently, this oxo imido complex is shown to engage in 1,2-addition across the Nb-oxo pi-bond; this represents a new reaction pathway in group 5 chemistry.

Chapter 3. Azide metathesis reactivity of Nb imido tetrazene complexes is evaluated. It is shown that addition of excess benzyl azide to a cyclohexyltetrazene Nb complex generates the corresponding benzyltetrazene Nb complex. Spectroscopic and computational investigations revealed that this conversion occurs via a series of concerted [3+2] cycloaddition and retrocycloaddtion reactions in which pi-loaded bis(imido) intermediates are formed. Two computational methods—the prototypical density functional theory (DFT) and the nascent density functional tight binding (xTB)—are employed to calculate the lowest energy pathway across the potential energy surface. Results using both methods are consistent with a concerted asynchronous mechanism, in which isolable intermediates are not formed, but bond-breaking and -making events do not occur simultaneously. This work highlights the utility of xTB methods for computational studies involving large transition metal complexes.

Chapter 4. Using dihydrogen as a reducing agent, reduction of tri- and tetramethyl tantalum complexes to yield low-valent, multinuclear tantalum polyhydrides is demonstrated. Control over the extent of metal center reduction is achieved by varying the number of methyl groups on the starting material. Reaction of a Ta(V)Me3 complex with dihydrogen afforded a bimetallic, hydride-bridged Ta(IV) complex. The analogous reaction of a Ta(V)Me4 complex in benzene or toluene yielded bimetallic Ta(III) complexes in which both metals are bound to the same face of a direduced μ-h4:h4-arene ligand.

Chapter 5. Access to a diverse array of organic and organometallic products containing newly formed C−C bonds is demonstrated. These products are constructed via successive methyl transfers from di-, tri-, and tetramethyl Ta(V) precursors to unsaturated small molecule substrates, including carbon monoxide and xylyl isocyanide. Two Ta(V) oxo enolate complexes are generated via reaction with carbon monoxide; notably, these complexes are shown to engage in 1,2-addition of silanes across the Ta-oxo pi-bond, the first example of this reactivity pattern for Ta. Insertion of xylyl isocyanide into the backbone of the supporting ligand with concomitant methyl transfer from the metal center generated a new, dianionic scorpionate ligand that supported a Ta(V) dimethyl chloro complex. This complex is shown to react with carbon monoxide to yield a pinacol- containing product—a new reaction pathway in early transition metal chemistry. 13CO-labeling experiments were employed to elucidate the mechanism of this transformation.

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