Fluorination of Alkoxide Ligands: Neither too much, Nor too little for Optimal Ligand Field Strength

Fluorination is generally expected to act predictably in ligand design: each added fluorine atom withdraws electron density, weakens σ-donation, and steadily modulates reactivity. However, reported catalytic systems show activity instead peaking at intermediate fluorination—a non-monotonic trend that simple donor-strength arguments cannot explain. To disentangle this non-monotonicity, we prepared a series of homoleptic Mo(V) alkoxides bearing fluorinated tert-butoxide ligands via Mo(IV) → Mo(V/III) disproportionation. As oxo-free d¹ species with trigonalbipyramidal geometry, these complexes provide a direct EPR handle on ligand-field splitting, indicating strengthened axial fields at partial fluorination. Computations reveal enhanced donor–acceptor overlap and favorable electrostatics at intermediate F-content, rationalizing the fluorination sweet spot observed experimentally in catalysis. 17O NMR calculations pinpoint the physical origin of this effect towards C-CF3 σ* acceptor orbitals. Notably, computational exploration of Schrock-type catalysts, for which the influence of fluoroalkoxide ligand is well-known, shows that the specific alkoxide orientation can have a net influence of up to 6 kcal/mol on the ground and transition state energies for the metallacyclobutane formation during the olefin metathesis process. The findings provide understanding of fluorination and can be used to develop new alkoxide ligands and analogous promising synthons for alkoxide-containing catalysts.