PNNL

The Science

DTBM-SEGPHOS is a key ligand in copper hydride catalysis, producing a highly reactive catalyst that has been extremely challenging to characterize. Thus, researchers have speculated on the nature of the (DTBM-SEGPHOS)CuH monomer (LCuH) species for over two decades. Scientists used a combination of low-temperature nuclear magnetic resonance spectroscopy and molecular dynamics simulations to characterize and better understand the reactivity of this species. This work provides compelling evidence for characterization of this transient species. While high reactivity had always been assumed, these results show just how rapidly it inserts alkenes even at very low temperatures (−40 °C). This high reactivity is the key to this catalyst overcoming decomposition tendency, because only a tiny amount is needed to catalyze the reaction.

The Impact

Despite its importance, LCuH has remained elusive because of its instability. Through this work on LCuH, researchers refined and developed processes for trapping highly reactive intermediates and examining their reactivity. The computational work allowed the team to rationalize the origin of the kinetic stability of LCuH. The approach can be broadened to other base metal hydride systems and help enable the design of future ligands, using the underexplored effect of distal steric bulk, to address catalyst deactivation by aggregation.

Summary

DTBM-SEGPHOS, a bulky diphosphine, is widely used in CuH-catalyzed transformations. The transient LCuH is often invoked as the active species. However, its instability has prevented spectroscopic characterization and mechanistic study, which has hindered developing an overall mechanistic understanding of these reactions. Researchers performed low-temperature nuclear magnetic resonance spectroscopic characterization of LCuH, which enabled quantitative kinetic analysis of reactions with cyclopentene and the structural identification of two CuH clusters. LCuH inserted into cyclopentene at −43 °C, highlighting its high reactivity toward olefins. LCuH deactivated to form L₂Cu₃H₃ and L₂Cu₄H₄ clusters, where LCuH dimerization initiated aggregation. Kinetic analysis of reactions of unactivated alkenes indicated that competing on-cycle alkene hydrocupration and LCuH dimerization affected catalyst performance, as the deactivation and turnover occurred on comparable timescales. Structure-activity analysis using atomistic simulations showed that the steric profile of DTBM-SEGPHOS increased the CuH dimerization barrier by ∼7.7 kcal mol⁻¹ relative to that of SEGPHOS, providing insight into the unique ability of DTBM-SEGPHOS to stabilize reactive monomers for reaction with a broad range of alkene substrates. These findings illustrate the fundamental design principle that steric control of aggregation governs CuH catalyst performance, explaining both the exceptional activity of (DTBM-SEGPHOS)CuH and the limits imposed by competing deactivation.

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 program, through the Catalysis Science Program within the Chemical Sciences, Geosciences and Biosciences Division, FWP 47319. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center, located at Lawrence Berkeley National Laboratory.