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

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