Activation and protonation of dinitrogen at the FeMo cofactor of nitrogenase

The protonation of N2 bound to the active center of nitrogenase has beeninvestigated using state-of-the-art DFT calculations. Dinitrogen in thebridging mode is activated by forming two bonds to Fe sites, which results in areduction of the energy for the first hydrogen transfer by 123 kJ/mol. Theaxial binding mode with open sulfur bridge is less reactive by 30 kJ/mol andthe energetic ordering of the axial and bridged binding mode is reversed infavor of the bridging dinitrogen during the first protonation. Protonation ofthe central ligand is thermodynamically favorable but kinetically hindered. Ifthe central ligand is protonated, the proton is transferred to dinitrogenfollowing the second protonation. Protonation of dinitrogen at the Mo site doesnot lead to low-energy intermediates.Comment: 9 Pages, 6 figures, accepted in J. Chem. Phys. Typos corrected according to the galley proofs of JC