Iron(IV) Alkyl Hydrazido Complexes: Electronic Structure, Fe–C Bond Homolysis, and N–C Bond Forming Migration Reactions

High-valent iron-alkyl complexes are rare, as they are typically prone to Fe–C bond homolysis. We show here an unusual way to access formally iron(IV) alkyl complexes through double silylation of iron(I) alkyl dinitrogen complexes to form an NNSi2 group. When the alkyl group is trimethylsilylmethyl, the formally iron(IV) compound is stable at room temperature. Spectroscopically validated computations show that the disilylhydrazido(2–) ligand stabilizes the formal iron(IV) oxidation state through a strongly covalent Fe–N -interaction, in which one -bond fits an "inverted field" description. This means that the two bonding electrons are localized on the metal and not the ligand, and an iron(II) resonance structure is a significant contributor as with the phenyl analogue. However, in contrast to the phenyl analogue which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, and this places one electron in the * orbital and weakens the Fe–N bonding, leading to longer Fe–N bonds. The reactivity of these hydrazido(2–) complexes has an interesting dependence on the specific alkyl group. When the alkyl group is methyl, the formally iron(IV) species undergoes migration of the carbon-based ligand to the NNSi2 group to form a new N–C bond, followed by an intriguing isomerization of the hydrazido ligand. This reactivity is not observed with the bulkier trimethylsilylmethyl complex. When the alkyl group is benzyl, yet another reactivity pathway is evident: the Fe–C bond homolyzes to give a three-coordinate iron(III) complex with a hydrazido(2–) ligand. DFT calculations are used to explain the differences between the behavior with the different alkyl groups. Overall, these formally iron(IV) compounds display a diverse set of reaction pathways associated with the specific alkyl groups.