Analyzing the Stability and Aggregation of Copper Hydride Monomers

The Science

Molecular copper hydrides (CuHs) are prominent catalysts used for transforming unsaturated hydrocarbons because of the selective and low-barrier insertion of alkynes and olefins into the copper-hydrogen bond. However, catalysis is compromised by aggregation and ligand dissociation of the transient monomeric CuH catalysts to form inactive CuH clusters. By isolating CuH monomers, researchers identified mechanistic details of the varied stages of aggregation and cluster formation through kinetic analysis and molecular dynamic simulations. These details can help guide the synthesis of more stable and active CuH catalysts.

The Impact

Monomeric hydride species commonly convert to hydride clusters, leading to catalyst deactivation. Remediating this process is critical to developing effective and stable catalysts. Insights into the kinetics of CuH conversion and aggregation highlight the efficacy of incorporating distal steric bulk away from the active site of the CuH. This change controls the kinetic stability of monomeric CuH without drastically interfering with on-cycle insertion chemistry. This strategy generated solution and thermally stable monomeric CuH catalysts for use with less reactive, unactivated olefins.

Summary

Molecular CuH catalysts have significantly contributed to the currently available, versatile routes for upgrading hydrocarbon feedstocks to chemicals. However, these catalysts often undergo aggregation that depletes active CuH species. The mechanisms of CuH aggregation and how it influences catalyst performance remain unclear. Researchers used kinetic studies of unactivated alkene hydroboration catalyzed by the isolable CuH catalyst (IPr*CPh₃)CuH (LCuH). They were able to delineate the mechanism of an aggregation pathway that continuously depletes active LCuH to form inactive CuH clusters during catalysis. LCuH deactivation is primarily controlled by the competitive kinetics of LCuH dimerization and alkene insertion into LCuH. They further demonstrated that ostensibly minor structural modifications to the N-heterocyclic carbene (NHC) peripheries in a series of (NHC)CuH congeners drastically affect LCuH dimerization while maintaining reactivity towards on-cycle alkene insertion, enabling the design of new NHC ligands that eliminate CuH aggregation. They developed a computational approach based on molecular dynamics simulations to elucidate the specific ligand modifications that substantially increase the kinetic stability of monomeric CuH catalysts. The combined experimental and computational studies provide strategies for ligand design that can be broadly applied to molecular catalysts that are susceptible to deactivation via aggregation pathways.

The results of this study show the impact of the synergy and close collaboration in the Institute for Integrated Catalysis between synthetic chemists, kineticists, computational chemists, and X-ray crystallographers to address knowledge gaps in molecular CuH catalysis.

Contact

Ba Tran, Pacific Northwest National Laboratory, ba.tran@pnnl.gov 

Funding

This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division, Catalysis Science Program, FWP 47319. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center located at Lawrence Berkley National Laboratory.