Activation of Diazene and the Nitrogenase Problem:  An Investigation of Diazene-Bridged Fe(II) Centers with Sulfur Ligand Sphere. 1. Electronic Structure

Abstract

Mössbauer and UV−vis spectroscopic measurements of the two diazene-bridged systems [{Fe‘N H S 4 '} 2 (N 2 H 2 )] (‘N H S 4 ' 2 - = 2,2‘-bis(2-mercaptophenylthio)diethylamine(2−)) and [{Fe‘S 4 '(PPr 3 )} 2 (N 2 H 2 )] (‘S 4 ' 2 - = 1,2-bis(2-mercaptophenylthio)ethane(2−)) have been combined with self-consistent-field Xα scattered-wave (SCF-Xα -SW) calculations in order to describe the electronic properties of diazene coordinated to Fe(II)−sulfur centers. For both model systems, the experiments and calculations indicate a Fe(II)−N 2 H 2 −Fe(II) structure with Fe(II) low-spin centers. The stabilization of diazene is due to σ-donation from its highest occupied MO and π -backbonding into its lowest unoccupied MO (LUMO). The intense absorption band at approximately 600 nm is ascribed to a charge-transfer transition from metal-d orbitals into the diazene LUMO. Importantly, the diazene LUMO also is the LUMO of the complexes and is located between the fully occupied and empty orbitals of the iron centers. It is concluded that the Fe(II) high-spin configuration present in a sulfur environment of lower ligand-field strength leads to reduction of diazene. Therefore diazene is likely to be involved in a reduction pathway of dinitrogen bound symmetrically (i.e., with both nitrogen atoms) to the iron−molybdenum cofactor (FeMoCo) of the enzyme nitrogenase.