Dinitrogen Activation Mediated by the (P 2 P Ph )Fe Complex: Electronic Structure, Dimerization Mechanism, and Magnetic Coupling.

Herein, we report the estimation of the extent of dinitrogen activation by different charged and structural forms of (P 2 P Ph )Fe biomimetic catalysts, which, in the presence of light, exhibit significant yield in the N 2 -to-NH 3 conversion. Complete active space self-consistent field (CASSCF) calculations have been used to determine the electronic structure of different reduced forms of the mononuclear complexes: the neutral (P 2 P Ph )Fe(N 2 ) 2 adduct and the anionic [(P 2 P Ph )Fe(N 2 )] - and [(P 2 P Ph )Fe(N 2 )] 2- complexes. These calculations also revealed that the extent of reduction of a dinitrogen molecule reaches up to one electron (N 2 1- ) due to the back-bonding from the Fe center, in agreement with the changes observed in the vibration frequency of the N-N bond in these complexes. In addition, the energy profile of the dimerization of the mononuclear (P 2 P Ph )Fe(N 2 ) 2 complex to the dinuclear mono-N 2 -bridged [(P 2 P Ph )Fe] 2 (μ-N 2 ) complex has been determined by means of density functional theory (DFT) calculations. A three-step mechanism has been proposed for the dimerization, favored by both kinetics and thermodynamics criteria. Finally, the magnetic coupling constant in the diiron (μ-N 2 ) complex is estimated from CASSCF/NEVPT2 calculations. Such a dinuclear complex presents a strong antiferromagnetic coupling resulting from the interaction between two S = 1 d 6 Fe 2+ ions, bridged by a highly activated dinitrogen molecule (N 2 2- ) with two electrons on the π* orbitals.