A cooperative ligand is defined as one which actively participates in substrate activation to facilitate catalysis with a metal ion in a synergistic fashion. This dissertation focuses on the synthesis and catalytic activity of base-metal complexes with cooperative SNS pincer ligands to explore unconventional reaction pathways that are a consequence of diverging from traditional phosphine-based ligands.
Two new NHC–Cu(I)-[κ2-SNS] complexes were synthesized to directly compare the bifunctional catalytic activity between the two SNS ligands. The Cu thiolate complex catalyzed ketone hydroboration but not hydrosilylation, while the Cu amido complex is a high-performing carbonyl reduction catalyst boranes and silanes, through a conventional outer-sphere mechanism.
The bifunctional reactivity of three M[SMeNSMe]2 complexes was computationally assessed by comparing the nucleophilicity of the M–Namido donor (M = Mn, Fe, Co), and the Mn analogue was identified as the most promising catalyst candidate. A combined experimental and mechanistic study of the chemoselective hydroboration of carbonyls by Mn[SMeNSMe]2 follows. The catalyst allows for room temperature hydroboration of carbonyls at low catalyst loadings (0.1 mol%) and reaction times (<30 min). Mechanistic studies highlight the significance of bifunctional amido bis(thioether) ligand to the success of the reaction. DFT calculations showed that thioether hemilability is crucial during catalysis for providing the active coordinating site. A non-traditional inner-sphere reaction pathway with carbonyl coordination to the metal center and amido-promoted B–H reactivity is proposed to be operative, as opposed to the traditional metal-hydride pathway enabled by phosphine-based bifunctional ligands.
The manganese(I)-thiolate complex Mn(κ3-SMeNS)(CO)3 is an active precatalyst for the photo-catalyzed dihydroboration of nitriles. Reaction optimization studies revealed that catalysis requires the presence of UV light to enter and remain in the catalytic cycle. Stoichiometric mechanistic studies showed that HBpin borylates the imine N=C of the ligand backbone in the absence of nitrile, forming an inactive off-cycle by-product. Isotopic labeling studies with 13CO revealed that the catalyst resting state features a single CO ligand coordinated to the Mn center. DFT calculations showed that the bifunctional thiolate donor, coordinative flexibility of the SMeNS ligand, and access to an open-shell intermediate are all crucial to accessing low-energy intermediates during catalysis.
The electronic structure of a Fe[N2S3] complex was investigated in detail. Cyclic voltammetry and spectroelectrochemistry studies show a reversible oxidation and reduction to stable species. The anionic redox partner of Fe[N2S3] was synthesized and characterized by X-ray diffraction, Mössbauer spectroscopy, and X-ray absorption techniques along with Fe[N2S3]. Both Mössbauer and X-ray absorption near edge structure (XANES) data indicated a ligand-based reduction and DFT studies of the Zn analogs allowed us to propose a new bonding scheme for the reduced ligand. Redox interconversions in these complexes are dominated by changes in electron population in the N2S3 ligand, and intimate mixing with the Fe dxy orbital due to a high degree of covalency.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/43221 |
| Date | 26 January 2022 |
| Creators | Elsby, Matthew |
| Contributors | Baker, R. Tom |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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