Iron complexes of a tris(phosphino)borane ligand as nitrogenase models

Biol. dinitrogen fixation - i.e. The redn. of N_2 to two equiv. of ammonia by addn. of protons and electrons - is thought to proceed through initial coordination of N_2 to one or more iron centers of the nitrogenase MoFe cofactor. To shed light on the mechanism of this transformation, a no. of model compds. have previously been prepd. that mimic the trigonal environment of the iron center in the MoFe cofactor by enforcing either pseudotetrahedral (PT) or trigonal-bipyramidal (TBP) geometry with robust phosphine-based multidentate ligands. In this contribution, we show that both geometries can be accessed in ferraboratranes derived from a tris(phosphino)borane ligands. In particular, we describe terminal iron-N_2 complexes that exhibit a TBP geometry as well as PT compds. with a Fe≡NR triple bond. Efforts towards the functionalization of the ironbound dinitrogen mol. are also discussed