The Effect of Hartree-Fock Exchange on Scaling Relations and Reaction Energetics for C-H Activation Catalysts

High-throughput computational catalyst studies are typically carried out using density functional theory (DFT) with a single, approximate exchange-correlation functional. In open shell transition metal complexes (TMCs) that are promising for challenging reactions (e.g., C-H activation), the predictive power of DFT has been challenged, and properties are known to be strongly dependent on the admixture of Hartree-Fock (HF) exchange. We carry out a large-scale study of the effect of HF exchange on the predicted catalytic properties of over 1200 mid-row (i.e., Cr, Mn, Fe, Co) 3d TMCs for direct methane-to-methanol conversion. Reaction energy sensitivities across this set depend both on the catalytic rearrangement and ligand chemistry of the catalyst. These differences in sensitivities change both the absolute energetics predicted for a catalyst and its relative performance. Previous observations of the poor performance of global linear free energy relationships (LFERs) hold with both semi-local DFT widely employed in heterogeneous catalysis and hybrid DFT. Narrower metal/oxidation/spin-state specific LFERs perform better and are less sensitive to HF exchange than absolute reaction energetics, except in the case of some intermediate/high-spin states. Importantly, the interplay between spin-state dependent reaction energetics and exchange effects on spin-state ordering means that the choice of DFT functional strongly influences whether the minimum energy pathway is spin-conserved. Despite these caveats, LFERs involving catalysts that can be expected to have closed shell intermediates and low-spin ground states retain significant predictive power.

Automated summary

High-throughput computational catalyst studies using density functional theory have shown that the predictive power of DFT in open shell transition metal complexes can be influenced by the admixture of Hartree-Fock exchange. The sensitivities of reaction energies in these complexes depend on catalytic rearrangement and ligand chemistry, with specific linear free energy relationships performing better than absolute reaction energetics and being less sensitive to HF exchange. The choice of DFT functional strongly impacts the pathway of minimum energy conservation and the predictive power of closed shell intermediates and low-spin ground states in catalysts.