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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.