Coordination-Induced Spin Modulation: Overcoming Spin Blocking in C–H Methylation with High-Spin Ferrous Complexes

Understanding spin state changes and their influence on the reactivity of earth-abundant transition metal complexes is essential for unlocking the full potential of these elements. While precedented in biological contexts and related model complexes, control of metal spin state is underexplored as a mode for reactivity control in organometallic chemistry. Here we describe coordination-induced spin modulation as a strategy to overcome spin blocking to enable C-H functionalization with four-coordinate high-spin iron(II) dimethyl complexes. Addition of monodentate phosphines induced room-temperature C-H methylation of arenes, while evaluation of sterics and σ-donacity demonstrated PhPMe₂ as the optimal spin modulator (L). Mechanistic experiments, kinetics, a stereochemical probe, and computations corroborated spin-state lowering upon coordination of L, enabling previously spin-blocked substrate association to form a five-coordinate (bisphosphine)(L)Fe(CH₃)₂ intermediate directly observed by NMR spectroscopy. Lability of L allowed ligand dissociation and formation of the σ-agostic complex en route to C-H functionalization.