Research master thesis: Ruthenium and Iridium Complexes Bearing N-Heterocyclic Carbene/Olefin Ligands for Olefin Metathesis

Abstract

Olefin metathesis has emerged as a versatile and widely applicable methodology for carbon-carbon bond formation, driving advancements in various branches of chemistry. In this research, a systematic investigation was conducted to explore the impact of ligand design and substrate properties on the catalytic activity of ruthenium and iridium complexes in ring-opening metathesis polymerization (ROMP). Ten catalyst precursors, incorporating ruthenium or iridium metals in conjunction with N-heterocyclic carbene (NHC) or N-heterocyclic olefin (NHO) ligands, were screened for the ROMP of strained (norbornene) and low-strain (cis-cyclooctene) cyclic olefins. Among the complexes tested, the known, in situ generated [RuCl2(p-cymene)IMes] complex demonstrated the highest activity for both norbornene and cyclooctene polymerization, while iridium complexes generally exhibited inferior performance. Computational investigations based on density functional theory (DFT) were employed to validate the geometries of the complexes, assess the ligand donor abilities, and elucidate the energetics and reaction kinetics of the ROMP process. The results highlighted the crucial role of ligand steric and electronic properties in governing the catalytic performance of these complexes. The substrate-dependent nature of the key reaction steps, particularly the formation and cleavage of metallacyclobutane intermediates, was emphasized, with the cleavage step identified as the rate-determining step for both norbornene and cyclooctene reactions. This comprehensive study provides valuable insights into the activity of ruthenium and iridium complexes for ROMP, emphasizing the significance of ligand design and substrate effects, and paving the way for future developments of enhanced olefin metathesis catalysts.