N-Donor and N-Heterocyclic Ligands for the Study of Early Transition Metal Reactivity
- Author(s): Ziegler, Jessica Ann
- Advisor(s): Arnold, John
- et al.
Chapter 1. Reaction of the free base corrole (Mes2(p-OMePh)corrole)H3 with tantalum trialkyl precursors TaMe3Cl2 and TaBn3NtBu resulted in the formation of the tantalum dichloride and tantalum imido corrole complexes via alkane elimination. The X-ray crystal structures of these two compounds have been determined and the structural parameters are discussed. The Ta center of the dichloride species was found to sit out of the plane of the corrole ring by 0.903 Å and is cis-ligated, similarly to what has been reported for group 4 porphyrin complexes. The imido complex reacted with triphenylmethanol and 4-methylbenzyl alcohol, resulting in different extents of protonation of the imido group.
Chapter 2. Incorporation of a Cp ligand into the Nb-BDI-imido system is shown to result in unusual κ1-coordination of the BDI ligand in the complex (κ1-N,BDI)Nb(NtBu)CpCl. Via chloride abstraction from this species, we develop synthetic routes to structurally analogous cationic and NbIV pseudo-four coordinate complexes where the BDI ligand returns to the κ2-coordination mode. We compare these to our previously reported tetrahedral niobium bis(imido) complexes to investigate the effects of the Cp ligand on this system. Substitution of the chloride with a hydride also causes return to bidentate binding of the BDI ligand. The X-ray crystal structures of these complexes have been determined, and the structural parameters reflecting the consequences of the electronic differences are discussed.
Chapter 3. Syntheses of high-valent dichloride and dimethyl niobium imido complexes bearing a bis(NHC)borate supporting ligand are described. Reaction of a dimethyl species with excess CO generates an equivalent of acetone, which inserts into a B-H bond of the bis(NHC)borate ligand to form a boryl-isopropoxide Nb(III) dicarbonyl complex. This mode of hydroboration reactivity also occurs readily upon treatment of the dichloride and dimethyl compounds with ketones, aldehydes, and isocyanates. Modification of the bis(carbene) ligand via hydroboration of benzophenone produces a dimethylniobium complex that undergoes intramolecular η6-arene coordination upon hydrogenation.
Chapter 4. A niobacyclopropane complex is shown to undergo insertion of ethylene and linear alkynes at the Nb-C bond of the metallacycle. This species also undergoes 1,2-addition reactions of tert-butylacetylene and H2 at the Nb-C bond, generating complexes bearing a secondary amide. The addition of dihydrogen generates a niobium-hydride, which upon heating inserts 2-butyne, and subsequently releases cis-2-butene. This reaction also generates a cyclometallated species, indicating that the intermediate generated upon alkyne insertion undergoes an intramolecular σ-bond metathesis. In the presence of H2, the hydride complex catalyzes the hydrogenation of 2-butyne generating both cis-2-butene and 1-butene. The results of a mechanistic study are presented, elucidating the role of intermediates involved in alkene isomerization.
Chapter 5. Carbodiimide cleavage by a Nb(III) synthon yields a series of bis(imido) isocyanide complexes. These species are shown to undergo [2+2] cycloaddition reactions with internal alkynes at one of the Nb-Nimido π-bonds. The initial metallacyclebutene intermediate undergoes insertion of the isocyanide ligand into the Nb-C bond, generating products containing an unsaturated iminoamine fragment. The bis(imido) complexes also undergo 1,2-additions with H2 and silanes. The outcome of these reactions differs for two bis(imido) species bearing different isocyanide ligands. An alkyl-isocyanide containing complex experiences nucleophilic attack of the carbene upon a formimidoyl intermediate generated upon reaction with either H2 or a silane. While the reaction of the aryl-isocyanide containing species with silane also likely generates a formimidoyl intermediate, this fragment couples to the silicon atom to generate a metallacyclopentane complex.
Chapter 6. The synthesis of a series of titanium imido-amido complexes supported by the bis(NHC)borate ligand is reported. Deprotonation of the compounds bearing an aryl(amide) generates anionic bis(imido) complexes charge-balanced by a potassium counterion. These complexes contain significantly elongated metal-nitrogen bonds, likely due to the π-loading effect of incorporating multiple imido groups on the same titanium center. Sequestration of the potassium atom leads to the isolation of a bis(imido) complex that undergoes cyclometallation in solution. This indicates that upon chelation of the cation, the nucleophilicity of the imido nitrogens increases.