Catalytic N₂ Reduction to Silylamines and Thermodynamics of N₂ Binding at Square Planar Fe

The geometric constraints imposed by a tetradentate P₄N₂ ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal Fe⁰(N₂)­(P₄N₂) complex catalyzes the conversion of N₂ to N­(SiR₃)₃ (R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N₂ silylation catalyst to date (up to 65 equiv N­(SiMe₃)₃ per Fe center). Elevated N₂ pressures (>1 atm) have a dramatic effect on catalysis, increasing N₂ solubility and the thermodynamic N₂ binding affinity at Fe⁰(N₂)­(P₄N₂). A combination of high-pressure electrochemistry and variable-temperature UV–vis spectroscopy were used to obtain thermodynamic measurements of N₂ binding. In addition, X-ray crystallography, ⁵⁷Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe⁰, Fe^I, and Fe^II complexes reveals that the free energy of N₂ binding across three oxidation states spans more than 37 kcal mol^–1.