The Coordination Chemistry and Reactivity of Pd Phosphine 1-Azaallyl Complexes

Kyle M. Jackman, The University of Western Ontario

Abstract

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 [Pd^II(CH₃)(P^AzA)]₂ dimer when coordinated to Pd. Thermolysis of the dimer results in the formation of two new Pd complexes, a [Pd^I(P^AzA)]₂ dimer and a mononuclear [Pd^II(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 [Pd^I(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 [Pd^I(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.