Doctor of Philosophy
Blacquiere, Johanna, M.
Structurally responsive ligands (SRLs) are a class of ligands that change their coordination mode with a metal centre, which permits reactions that are difficult with non-SRLs, allow access to unique mechanisms, and allow isolation of unusual coordination complexes. The 1-azaallyl (1-AzA) family of SRLs are capable of isomerizing between η3-NCC and κ1-N modes that occupy two and one coordination sites, respectively. This thesis investigates a new class of phosphine 1^AzA (P^AzA) ligands and the reactivity of their coordination complexes with palladium.
The first-generation P^AzA ligand contains a phenyl linkage between the two heteroatoms and affords a [PdII(CH3)(P^AzA)]2 dimer when coordinated to Pd. Thermolysis of the dimer results in the formation of two new Pd complexes, a [PdI(P^AzA)]2 dimer and a mononuclear [PdII(P^AzA)2]complex via a unique mechanism. Distinct coordination modes of the 1-AzA are observed in all three complexes, and a previously unreported coordination mode is observed in the [PdI(P^AzA)]2 dimer. The formation of both Pd complexes is accompanied by difficult Csp3–Csp3 cross coupling, directly caused by the structurally responsive 1-AzA. The 1-AzA reduces barriers required for the reductive elimination step, and maintains the unusual dinuclear motif throughout the reaction. A direct synthesis for the [PdI(P^AzA)]2 dimer is also reported, and this complex is catalytically active for Kumada cross-coupling. Experimental evidence suggests the dinuclear Pd structure is maintained throughout the reaction mechanism. Thus, the phenyl-linked P^AzA ligand has been established as beneficial for fundamental reactivity proceeding via unusual mechanisms.
The second-generation P^AzA ligands contain a 1,8-naphthalene or 2,2'-biphenyl backbone. A P^AzA complex with the naphthalene backbone can only be isolated with the 1-AzA coordinated κ1-N. When the biaryl ligand is coordinated to Pd the first example of a Pd(P^AzA) complex with the 1-AzA coordinated η3-NCC coordination is observed. The bite angle of the biaryl ligand is ca. 15˚ larger than the naphthalene ligand, likely due to the increased distance between P and N, as well as rotation between the two aryls. The increased bite angle for the biaryl ligand allows for the isolation of the 1-Aza in the η3-NCC coordination mode, and a new ligand design strategy method for isolating Pd complexes in this coordination mode has therefore been reported.
Summary for Lay Audience
Catalysis is the process of increasing the rate of a chemical reaction by using an additive that is regenerated after each individual reaction takes place. Catalysts allow you to make molecules that are difficult or are inefficiently produced via other methods. It is one of the most important chemical discoveries in modern history, and it is so impactful that it is estimated that 90% of all commercial products use a catalyst at some point in the production chain. Transition metal catalysts are comprised of two equally important parts, the metal centre and the ligands that are bound to the metal centre, together these species from a transition metal complex. Structurally responsive ligands can change the way that they are bound to the metal centre, allowing for multiple different types of bonding to occur. These changes in binding can have a profound impact on the reactions that can be permitted by the complex and these transition metal complexes can consequently perform reactions difficult with other methods. Furthermore, the changes in binding allow for the isolation of a variety of unique and novel transition metal complexes. Together, these advantages could allow the synthesis of new and impactful fine chemicals and reduce the number of steps required to synthesize a plethora of natural products.
This thesis describes the synthesis and reactivity of a new series of ligands that contain phosphorus and a structurally responsive functional group. How the ligands are bound to the transition metal palladium is thoroughly explored, and new ligand design strategies influencing metal-ligand binding are discussed. Additionally, direct evidence of the beneficial effects of the structural responsiveness of the ligand is reported as it makes fundamental reaction steps easier, and it is shown to allow unusual reaction pathways where it is key in maintaining the catalyst structure.
Jackman, Kyle M., "The Coordination Chemistry and Reactivity of Pd Phosphine 1-Azaallyl Complexes" (2022). Electronic Thesis and Dissertation Repository. 8926.