Date of Award

9-20-2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

C-O bond activation, mechanistic study, nickel catalysis, transition metal clusters

Supervisor

Johnson, S.

Rights

info:eu-repo/semantics/openAccess

Abstract

Inert C−O bond activation has been an important research area because ethers have the potential to replace organohalides as relatively economic and less toxic building blocks. Phosphine supported Ni complexes have been found to be reactive towards C−O bonds. A number of cross-coupling reactions using Ni catalysts for C−O activation have been developed. However, mechanistic understanding lags behind current application. Multiple mechanisms have been proposed based on DFT studies, while few intermediates have been isolated or observed experimentally. This dissertation focuses on the mechanistic details of Ni mediated C(sp2)−O activation. The number of ligand coordinated to Ni in the critical bond cleavage step remains unclear, and previous DFT studies have suggested that both L2Ni and LNi species might be the active species, where L is a neutral phosphine donor. The lack of easily accessible LNi(0) sources has been an obstacle to examining the ability of the LNi moiety to facilitate C−O bond cleavage. This work provides a synthesis to a series of (Cy3P)Ni(η6-arene) complexes that provide a source of the (Cy3P)Ni(0) moiety. We compared the stability of (Cy3P)Ni(η6-arene) complexes with different substituents. Arenes with electron-withdrawing substituents form the most thermodynamically stable adducts. More fluorinated arenes form less stable adducts. This trend is opposite to the adducts of L2Ni(0) species where more fluorinated substituents form more stable adducts. In the preparation of (Cy3P)Ni(η6-arene) complexes we also correct a longstanding error in the nature of the starting material [(Cy3P)2Ni]2(μ-N2) in solution. In our mechanistic studies of C(sp2)−O acitvation, (Cy3P)Ni(η6-arene) and [(Cy3P)2Ni]2(μ-N2) were used as the LNi(0) and L2Ni(0) sources. The reactions of naphthyl substrates with LNi(0) or L2Ni(0) produce different products. However, the isolation of these adducts proved impossible, so other substrates were examined. The reactions of alkenyl ethers with LNi(0) or L2Ni(0) at room temperature provided (Cy3P)2Ni(η2-alkenylether). The reactions of vinyl ethers (RH2COCH=CH2, R is aliphatic groups) with [(Cy3P)2Ni]2(μ-N2) produces esters through nickel mediated homocoupling of ethers. [(Cy3P)2Ni]2(μ-N2) can also mediate the coupling of acetaldehyde and vinyl ethers. A mechanism of the ether-acetaldehyde coupling was proposed based on the experimental and computational studies. Apart from the traditional mononuclear complexes, electron-deficient transition metal clusters are emerging as powerful catalysts to inert bond activations. They are able to cooperatively activate bonds under mild conditions. Our previous work reported the synthesis of a pentanuclear nickel clusters, [(iPr3P)Ni]5H6, which shows high reactivities with inert C−O bonds. This thesis examines the role of the supporting phosphine donor in cluster formation and reactivity. A series of phosphines were investigated; PtBuMe2 provided [(tBu2MeP)Ni]5H6, which is even more reactive than the PiPr3 analogue. PCy2Me is able to form a dinuclear complex [(Cy2MeP)2Ni]2(μ-H)2. In the case of PCy3 and PCyp3, P−C bond cleavage occurred and led to the production of a dinuclear Ni complex [(R3P)HNi]2(μ-H)(μ-PR2). We also proposed mechanisms of the production of these nickel hydride complexes based on the experimental studies.

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