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. 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.