High‐Spin Iron(I) and Iron(0) Dinitrogen Complexes Supported by N‐Heterocyclic Carbene Ligands

The use of 1,3‐dicyclohexylimidazol‐2‐ylidene (ICy) as ligand has enabled the preparation of the high‐spin tetrahedral iron(I)‐ and iron(0)–N 2 complexes, namely [(ICy) 3 Fe(N 2 )][BPh 4 ] ( 1 ) and [(ICy) 3 Fe(N 2 )] ( 2 ), the electronic structures of which have been established by various spectroscopic characterization and DFT calculations. The frequency of the N–N stretching resonance of the iron(0)–N 2 complex is the lowest among the reported terminal N 2 complexes of iron, signifying the beneficial roles of strongly σ‐donating ligands in combination with the high‐spin low‐valent iron center in promoting N 2 ‐activation. The iron(0)–N 2 complex can convert reversibly to the low‐spin iron(II)‐N 2 hydride complex [(ICy) 2 (ICy′)Fe(N 2 )(H)] ( 4 ).