Synthesis and Characterization of Pyridine Imine Base Metal Catalysts

Abstract

Precious metals are widely used to catalyze industrial processes, but have drawbacks associated with their use. Replacing them with base metals would be advantageous, but base metals tend to facilitate one-electron transfers, which detracts from their potential for catalysis. To circumvent this issue and facilitate two-electron chemistry, two primary catalytic design principles have emerged: attaching the metal to either strong-field ligands or redox-active ligands. Each strategy has led to the synthesis of effective base metal catalysts, prompting the question of whether these two strategies could be implemented simultaneously in ligand design for improved catalysis. Consequently, the pyridine imine ligand family was investigated, with particular emphasis placed on iPrPIPh, which can cyclometalate to (iPrPI-C6H4-), a tridentate, formally anionic chelate that is structural similar to previous ligands studied by our lab and has potential redox-activity and strong-field character. Synthesis of stable PI Co complexes proved challenging; for instance, treatment of o-bromo-iPrPIPh with Co precursors generated cyclometalated intermediates with a tendency to decompose, producing dehalogenated iPrPIPh ligand. However, several synthesized compounds, (iPrPI-C6H4-)Co(py) and (iPrPI)2CoNs, exhibited preliminary catalytic activity. Attempts to modify (iPrPI-C6H4-)Co(py) into more promising precatalysts were unsuccessful due to the low lability of the pyridine. PI Ni complexes were also briefly investigated, and proved to be easier to synthesize, with better stability, but exhibited no catalytic activity in preliminary assessments. Further assessments of how well iPrPIPh base metal complexes catalyze benchmark reactions should be conducted in order to more accurately ascertain their potential for catalysis.