Dinitrogen Activation Mediated by the (P₂P^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 (P2PPh)Fe biomimetic catalysts, which, in the presence of light, exhibit significant yield in the N-2-to-NH3 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 (P2PPh)Fe(N-2)(2) adduct and the anionic [(P2PPh)Fe(N-2)](-) and [(P2PPh)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 (P2PPh)Fe(N-2)(2) complex to the dinuclear mono-N-2-bridged [(P2PPh)Fe](2)(mu-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 (mu-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) Fe2+ ions, bridged by a highly activated dinitrogen molecule (N-2(2-)) with two electrons on the pi* orbitals.