Unanticipated ability of the bis-iminopyridine ligand to participate in the reactivity of transition metal complexes: Synthetic and structural investigations of bis-iminopyridine-iron chloride and related complexes

Abstract

The discovery by Brookhart and Gibson that late transition metals supported by diimine ligands could sustain high activity for ethylene polymerization deposed the longstanding practice of employing only early metal d0 catalytic systems. This thesis focuses on our studies towards elucidation of the mechanism and determination of the nature of the active species responsible for the exceptional catalytic behaviour of the bisiminopyridine-Fe derivative. Alkylation of the catalyst precursor in Chapter 2 with a bulky, stable alkyl has shown a pronounced ability of the ligand system to be involved in the reactivity of the metal center, resulting in numerous transformations to the ligand backbone, including alkylation of the pyridine ring and the imine-C atom, deprotonation of the ketimine methyl groups, and dimerization and reduction. In Chapter 3, alkylation of the catalyst precursor with a more reactive alkyl led to reduction of the system by two electrons, with the ligand system being the recipient of the additional electrons at the expense of the metal center. Unexpectedly, the reduced system is also active for ethylene polymerization, producing polymer with the same activity and polymer quality as the divalent precursor, suggesting that activation of the catalyst involves an initial two electron reduction of the system. Further studies in Chapter 4, exploring activation of the complex with aluminum alkyls, have observed reduction of the metal center and transmetallation of the reduced ligand from Fe to Al as a possible deactivation pathway. The ability of the ligand to accept negative charge and assemble multi-nuclear structures leads to the proposed active species as a zwitterionic, cationic Fe alkyl. Expanding the reactivity studies to complexes of Cr in Chapter 5 highlights increased catalytic activity upon reduction towards a pseudo-monovalent Cr complex. Once again, the ligand accepts the added electron density and this ability to host electrons in the pi* orbital appears to be the rationale behind the catalytic activity experienced by the later-metal systems. Chapters 6 and 7 develop the reduction chemistry of bis-iminopyridine complexes of Fe and Cr respectively, revealing several dinitrogen complexes of Fe, and ultimately reduction and partial hydrogenation of a bridging Cr-N 2 complex towards complete cleavage of the N-N triple bond. Chapter 8 introduces two new Co-N2 complexes and describes a spontaneous reductive coupling of the dianionic version of the ligand upon complexation to Co, forming a dimenc, double dinitrogen complex. Incorporating the knowledge acquired in preceding chapters, two potential ligand scaffolds (dipyrroles and Schiff base pyrroles) were explored in Chapter 9 as supporting ligands for late metal polymerization catalysts. Although the results of catalytic testing remain preliminary, the pyrrole ligands continue to be of interest due to their ability to assemble heteropolymetallic clusters with potential for small molecule activation.