Terminal N Dissociation in [(PNN)Fe(N )] (μ-N ) Leads to Local Spin-State Changes and Augmented Bridging N Activation.

Nitrogen fixation at iron centres is a fundamental catalytic step for N utilisation, relevant to biological (nitrogenase) and industrial (Haber-Bosch) processes. This step is coupled with important electronic structure changes which are currently poorly understood. We show here for the first time that terminal dinitrogen dissociation from iron complexes that coordinate N in a terminal and bridging fashion leaves the Fe-N -Fe unit intact but significantly enhances the degree of N activation (Δν≈180 cm , Raman spectroscopy) through charge redistribution. The transformation proceeds with local spin state change at the iron centre (S= →S= / ). Further dissociation of the bridging N can be induced under thermolytic conditions, triggering a disproportionation reaction, from which the tetrahedral (PNN) Fe could be isolated. This work shows that dinitrogen activation can be induced in the absence of external chemical stimuli such as reducing agents or Lewis acids.