Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate.

Both biological and industrial nitrogen reduction catalysts activate N2 at multinuclear binding sites with constrained Fe-Fe distances. This contrasts with molecular diiron systems, which routinely form linear N2 bridges to minimize steric interactions. Model compounds that capture the salient geometric features of N2 binding by the nitrogenase enzymes and Mittasch catalysts would contribute to understanding their high N2-reduction activity. It is shown in the present study that use of a geometrically flexible, dinucleating macrocycle allows for the formation of a bridging N2 ligand with an unusual Fe-CtN2-Fe angle of 150° (CtN2 = centroid of N2), a geometry that approximates the α-N2 binding mode on Fe(111) surfaces that precedes N2 bond cleavage. The cavity size of the macrocycle prevents the formation of a linear Fe-N2-Fe unit and leads to orbital interactions that are distinct from those available to the linear configuration.