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. The addition of monodentate phosphines induced room-temperature C–H methylation of arenes, while the evaluation of sterics and σ-donacity demonstrated PhPMe2 to be an optimal spin modulator (L). Mechanistic experiments, kinetics, a stereochemical probe, and computations corroborated the spin-state lowering upon the coordination of L, enabling the previously spin-blocked substrate association to form a five-coordinate (bisphosphine)(L)Fe(CH3)2 intermediate directly observed by NMR spectroscopy. The lability of L allowed ligand dissociation and the formation of the σ-agostic complex en route to C–H functionalization.
Zhang et al. (2026) studied this question.