Air- and Thermally Stable Organometallic Fe(VI) Nitrides Enabled by a Tripodal Mesoionic Carbene Ligand: Access to a Water-Coordinated Fe(VI) Nitrido Complex.

A tripodal mesoionic carbene (MIC) ligand, tris-[4-(1-mesityl-3-methyl-1,2,3-triazol-5-ylidene)methyl]amine (TAMNMes), enables the synthesis and isolation of a family of exceptionally robust organometallic Fe(VI) nitrido complexes. One-electron oxidation of the corresponding Fe(V) nitrido precursor furnishes Fe(VI) nitrides bearing distinct equatorial ligands, including fluoride, chloride, triflate, and acetonitrile. Single-crystal X-ray diffraction analyses reveal distorted octahedral iron centers supported by the tripodal tris-MIC ligand, while zero-field 57Fe Mössbauer spectroscopy and multinuclear NMR spectroscopy establish diamagnetic d2, S = 0 electronic ground states. The Fe(VI) nitrides display remarkable persistence under ambient conditions and during prolonged room-temperature Mössbauer measurements. Electrochemical studies show that the Fe(VI/V) reduction potentials are strongly modulated by the equatorial ligand, while the cathodically shifted Fe(V/IV) couples highlight the enhanced σ-donor character of the tris-MIC scaffold compared to its tris-NHC analog. The relative lability of the equatorial ligands enables controlled interconversion among the Fe(VI) nitrides and, most notably, the reversible formation of a unique aqua-coordinated Fe(VI) nitrido complex, [(TAMNMes)FeVI(N)(OH2)](OTf)3. These results establish TAMNMes as a MIC-based complement to the tris-NHC-based TIMMNMes ligand platform, providing a powerful framework for stabilizing and tuning molecular iron complexes in exceptionally high oxidation states.