Synthesis and Reactivity of Iron(VI) Nitrido Complexes

This project is concerned with the design, synthesis and reactivity of iron(VI) nitrido complexes. The highest oxidation state known for iron is +6, and there are only two examples of iron(VI) complexes. The iron (VI) oxo complex, FeO42-, is a powerful oxidant that is potentially useful as a green reagent for organic synthesis and wastewater treatment. On the other hand, the other iron(VI) complex, [FeVI(Me3cyclam-acetato)(N)]2+ (Me3cyclam-acetato is an anionic pentadentate N4O ligand), is a highly unstable nitrido complex that decomposes above 77 K. Hence, the search of more stable iron(VI) nitrido species that can exhibit oxidizing properties similar to those of FeO42- is of fundamental interest. Our group has previously found that ruthenium(VI) nitrido complexes bearing salentype ligands are highly electrophilic/oxidizing and they are potentially useful nitrogenation reagents. Selective nitrogentation of organic substrates is an important step in the synthesis of various pharmaceutical and other useful products. However, ruthenium is an expensive and toxic metal. Moreover, in the reaction of RuVI≡N with organic substrates (Ru≡N + Org → Ru-NOrg), the useful nitrogenated product remains tightly bound to the ruthenium centre. On the other hand, iron is cheap and non-toxic, and iron complexes are intrinsically much more labile than ruthenium. Hence, it is anticipated that the nitrogenated product would be readily released from the metal centre if FeVI≡N is employed. More importantly, it is possible to design catalytic nitrogenation processes based on FeVI≡N by using an external nitrogen source. In our preliminary investigation we have successfully synthesized a highly reactive iron(VI) nitrido complex [FeVI(N)(TAML)]- (TAML is a tetraanionic macrocyclic tetraamido ligand) that readily undergoes nitrogenation reactions at ambient conditions with substrates such as styrene. Moreover, the nitrogenated product (2-phenylaziridine) is readily released from the iron centre, suggesting that it is possible to render this process catalytic by using an external nitrogen source, such as N3- or NH3/NaClO. In this project we propose to design, synthesize and fully characterize a series of iron(VI) nitrido complexes bearing various tetra-anionic and tri-anionic anxilliary ligands. We will investigate their reactivity towards a variety of organic substrates and develop catalytic nitrogenation processes based on these iron(VI) nitrido complexes. Our proposed studies should contribute to a significant advance in high-valent iron chemistry. It should also lead to the development of green and efficient catalytic systems for the selective nitrogenation of organic substrates based on cheap and non-toxic iron(VI) nitrido complexes under mild conditions.