(676e) Ru or Fe? Understanding Trends in C–H Activation Catalysis with High-Throughput Screening

Functionalization of polyolefin backbones can greatly enhance their value and versatility but remains a challenging prospect due to their chemical resistance arising from inert C–H bonds. Recent studies have demonstrated that a homogeneous Ru catalyst can selectively oxidize polyethylene, while an analogous Fe catalyst exhibited no catalytic activity. Yet, selective Fe-based C–H activation catalysts are ubiquitous not only in the chemical space of metalloenzymes but also synthetic catalysts. In this work, we perform a high-throughput virtual screening of Ru and Fe complexes using density functional theory (DFT) calculations to unearth the differences in their C–H activation reactivity, focusing on the roles of spin and oxidation state. We perform calculations on C–H activation intermediates for a large and diverse catalyst design space of mononuclear Ru and Fe complexes comprising realistic tetradentate macrocyclic ligands as well as combinations of small monodentate ligands that span ligand field strengths and coordinating atom identities. Our DFT calculations reveal that closed shell Fe and Ru complexes show tighter metal-oxo formation vs. hydrogen atom transfer (HAT) scaling relation with the oxo formation and HAT energies for both metals lying close to parity. For open shell complexes, we see more opportunity to break this scaling relation. Furthermore, we observe greater oxo formation favorability and HAT unfavorability in Ru complexes compared to Fe complexes. Nonetheless, Fe systems exhibit oxo formation energies within favorable ranges with a much lower upper bound on HAT. We find that these differences in reaction energetics at higher spin states can be attributed to C–H activation proceeding through spin forbidden pathways on Ru complexes compared to spin allowed pathways on Fe complexes. Thus, our broad catalyst screening efforts uncover the potential of the more earth-abundant Fe-based catalysts for polyolefin functionalization.