Metal-Ligand Cooperation (MLC) has been under study over the past two decades as a powerful tool for small molecule activation and functionalization. However, more mechanistic details are needed in order to understand the detailed steps that are enabled by the bifunctional cooperation between ligand and metal. In this regard, the hydroboration reaction offers a useful platform through which to assess the details of bifunctional reaction pathways and catalyst speciation. This dissertation focuses on the synthesis, characterization, and catalytic activity of base-metal complexes with cooperative N-, S-, and O-donor ligands to explore reaction pathways that are a consequence of diverging from traditional phosphine-based ligands. In Chapter 1 concepts and examples of MLC, especially as applied to hydroboration catalysis, are presented. In Chapter 2, three new Zn(II)-(κ²-SNS)₂ complexes were synthesized to directly compare the bifunctional catalytic activity rendered by amido and thiolate SNS ligands. Although all three complexes catalyzed carbonyl hydroboration, a detailed catalyst speciation study showed that the Zn amido complex reacts with pinacolborane (HBpin) to generate Zn-H and an unbound borylamido ligand. Subsequent substrate-derived zinc alkoxide formation followed by a second equiv of HBpin generates the product, regenerating the Zn hydride catalyst. In contrast, the Zn thiolate complex adds HBpin to the ligand imine unit, followed by aldehyde deoxygenation to give a benzothiazoline heterocycle and [Zn](OBpin). Reaction of the latter with HBpin then gives pinBOBpin and Zn-H, leading to the same active catalyst as that derived from the Zn amido precatalyst! For these systems, then, the bifunctional N- and S-donors serve to activate the catalyst rather than participating in a bifunctional catalytic cycle. Dissociation of the borylamido SNS ligand in Chapter 2 led us to reinvestigate a previously reported Cu(I) amido complex Cu[(κ²-SNS)(IPr) that was proposed to hydroborate carbonyls via an outer sphere process [IPr = bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Indeed, we showed that this complex also undergoes ligand borylation-dissociation to form the active catalyst [CuH(IPr)]₂ which had been reported previously as a carbonyl hydrosilylation catalyst. To compare these complexes with their heavier Group 10 analogue, we prepared and structurally characterized the silver amido SNS complex. Interestingly, this complex was not able to serve as a carbonyl hydroboration catalyst. Then we sought to use the MLC catalyst activation strategy to prepare an especially active Zn hydride hydroboration catalyst. Using a bidentate amine-pyrollide ligand with an aryl ether side-group, the 5-coordinate Zn complex, Zn(κ²-ONN)₂(DDI) (2.11Zn) was prepared and structurally characterized (DDI = 4,5-dichloro-1,3-dimethylimidazol-2-ylidene). On treatment with excess HBpin, formation of ONN(Bpin)₂ [(Bpin)₂-L3] gave rise to the reactive NHC-stabilized ZnH₂ catalyst that effected the rapid hydroboration of nitriles and quinoline derivatives under ambient conditions with only 0.01 and 0.05 mol% catalyst loading, respectively. In Chapter 3, in an attempt to prepare a cobalt complex containing both amido and thiolate SNS ligands, we obtained instead the Co(II) dithiolate complex, Co(κ³-SNS)(DDI) (3.2Co). This complex showed a unique selectivity for aldehyde hydroboration, over other functional groups such as ketones, cyanides, nitriles and olefins. A DFT study, in collaboration with Prof. Erin Johnson from Dalhousie University, showed that 3.2Co bifunctionally assembles the HBpin and aldehyde substrates, with Co binding the aldehyde oxygen and sulfur binding the boron of HBpin. With aromatic aldehyde substrates, interesting aromatic-aromatic dispersion effects led to catalyst inhibition which could be reversed by simply rinsing off the product with hexane. These effects were not observed for catalytic hydroboration of aliphatic aldehydes. In Chapter 4 we focused on expanding our MLC investigation to include additional donors beyond N and S. First, a dimeric Zn(II)-(κ⁴-NSNO) complex (4.1Zn) was synthesized and evaluated as a catalyst for nitrile dihydroboration to compare aryloxide and amido donors for B-H bond activation. In fact, 4.1Zn successfully catalyzed dihydroboration of a range of different aromatic and aliphatic nitriles under neat condition. Mechanistic studies determined that the aryloxide donor activates the B-H bond in the first step and the mechanism then likely proceeds through an inner-sphere insertion. As detected by our kinetic study, at high turnovers the catalyst decomposes when Bpin also binds to the amido donor. To compare the potential of other donors for B-H bond activation, a series of divalent NiᴵᴵX(κ³-NNN) complexes were synthesized, with X = bromide (4.3Ni), phenoxide (4.4Ni), thiophenoxide (4.5Ni), 2,5-dimethylpyrrolide (4.6Ni), diphenylphosphide (4.7Ni), and phenyl (4.8Ni), and employed as precatalysts for nitrile dihydroboration. Superior activity of the phenoxy derivative (vs. thiophenoxy or phenyl) suggests that B-H bond activation occurs at the Ni-X (vs. ligand Ni-N_pyrrolide) bond. Furthermore, stoichiometric treatment of 4.3Ni-4.8Ni with a nitrile showed no reaction, whereas stoichiometric reactions of 4.3Ni-4.8Ni with pinacolborane (HBpin) afforded the same Ni-H complex for 4.3Ni, 4.4Ni and 4.6Ni. Considering that only 4.3Ni, 4.4Ni and 4.6Ni successfully catalyzed nitrile dihydroboration reaction, we suggest that the catalytic cycle involves a conventional inner sphere pathway initiated by substrate insertion into Ni-H. In summary, our investigations confirm the importance of mechanistic studies and catalyst speciation for studies involving potential bifunctional catalysis. In Chapter 5 we summarize the findings of this thesis, placing them in the context of the current state of the art and speculating on future investigations they may enable.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/44462 |
| Date | 04 January 2023 |
| Creators | Ataie, Saeed |
| 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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