Linear free energy relationships (LFERs) are quantitative structure-activity relationships that are linear when the reaction mechanism is invariant. While LFERs are invaluable tools guiding catalyst design, their applications have been largely limited to organic aromatic chemistry and heterogeneous catalysis. In transition metal chemistry, one long-standing challenge of establishing LFERs is isolation and quantification of ligand effects because ligands play a role in both electronic tuning of catalytic activity as well as spatial configuration. Our goal is to construct LFERs for transition metal chemistry by quantifying barrier trends obtained upon systematic catalyst design. The selected probe reaction is a one-step oxo-insertion mechanism of CH4 hydroxylation with bio-inspired N-donor-bound [Cu2O2]2+ complexes. We adopt the activation strain model to decompose barriers to interaction and strain energies. An LFER is constructed by isolating and quantifying interaction and strain energies using scalar descriptors and its barrier prediction accuracy is examined for imidazole-bound dicopper catalysts for CH activation. Our analysis identifies the Hammett parameter (σp) describing N-donor electrophilicity as a reasonable descriptor for interaction energy and that Sterimol parameters (B1) and bite angles (Θ) are essential for describing strain energy. These two LFERs are capable of describing interaction and strain energies, respectively. Lastly, we demonstrate that the combined (interaction + strain) LFER performs reliably for CH activation barriers and possesses transferability across different levels of theory. This study bridges a gap between the need of high throughput catalyst screening for transition-metal complexes and the challenge of accurate characterization of reaction barriers. Going forward, this work will focus on expanding this approach across metal-oxo active centers for CH activation.

Framework for developing linear free energy relationships for transition metal complex catalysts

ACS Spring 2022

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