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

Prokopchuk, Demyan E., Wiedner, Eric S., Walter, Eric D., Popescu, Codrina V., Piro, Nicholas A., Kassel, W. Scott, Bullock, R. Morris, Mock, Michael T.
Journal of the American Chemical Society 2017 v.139 no.27 pp. 9291-9301
Gibbs free energy, X-ray diffraction, ambient temperature, binding capacity, catalysts, catalytic activity, electrochemistry, electron paramagnetic resonance spectroscopy, geometry, iron, ligands, nitrogen, oxidation, silylation, solubility, ultraviolet-visible spectroscopy
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, Feᴵ, and Feᴵᴵ complexes reveals that the free energy of N₂ binding across three oxidation states spans more than 37 kcal mol–¹.