Nickel Complexes of Diphosphine Ketone and Imine Ligands : Metal-Ligand Cooperation and Application in Hydrosilylation and Alkyne Cyclotrimerization Reactions

Abstract

Recent years have witnessed the application of homogenous catalysts in many chemical transformations, impacting chemistry in both industry and academia. The sustainability of the catalysis itself can however still be a point of improvement, as many of the efficient catalytic transformations rely on expensive, rare and generally relatively toxic metals. The transition to more sustainable alternatives such as nickel can benefit from the design of new types of molecular catalysts in which an organic part (a ligand) cooperates with the metal to facilitate chemical reactions. Such metal-ligand cooperation can for example arise when relatively weakly binding π-ligands such as imines (C=N) and ketones (C=O) are covalently tethered to strongly binding phosphorus ligands. In this thesis, the utility of this kind of ligands in Ni-catalyzed reactions is investigated. The role of the side-on π-coordinated C=O and C=N sites is extensively studied by both experimental and computational analysis. First, the metal-ligand hemilability at tethered ketone π-acceptor ligands can be used as a promising strategy in nickel catalysis, improving the activity and selectivity of a particular transformation. The ketone ligand shows versatile coordination at its π-acceptor site of which the binding mode responds to the electronic properties of the metal center and the specific requirements of elementary steps. The adaptive character of the ligand provided by the π-acceptor C=O moiety opens up mechanistic pathways that lead to an enhanced catalytic performance in alkyne cyclotrimerization, as demonstrated by comparison with various related Ni–complexes for which this specific π-hemilabile reactivity is not accessible. In addition, the ability of the ligand to adapt its coordination mode according the nature of the substrate/coligand used is an interesting property to further explore for the development of catalytic protocols involving other types of substrates, potentially opening up new venues in cooperative catalysis. Secondly, the general applicability of a nickel complex of a diphosphine–imine ligand in an industrially relevant process such as alkene hydrosilylation is highlighted, showing compatibility towards a broad range of olefins containing sensitive group functionalities. Furthermore, mechanistic investigations reveal that PPh3 positively affects the selectivity of the hydrosilylation. This finding may have general implications in hydrosilylation reactions: the choice of the coligand can potentially affect the outcome of the reaction, both in term of activity and selectivity. In addition to the contribution of PPh3 in hydrosilylation reactions, the synthesis and reactivity of the diphosphine–aminosilyl derivative of the Ni–catalyst offer a better mechanistic understanding of the reaction. Remarkably, the aminosilyl unit is the reactive site in transformations involving hydrosilane substrates; it suggests that transient Si–N bond formations are of importance in the catalytic hydrosilylation performance of nickel complexes of nitrogen-containing ligands. The findings that both the aminosilyl and the PPh3 fragments plays a role in the catalytic performance of hydrosilylation processes can open up opportunities for the design of new types of metal catalysts of nitrogen-containing ligands. A careful design of organometallic compounds that incorporate both of these concepts can become key features for the optimization of metal catalysts towards a certain reactivity