Revisiting NHC–Metal Bonding: π-Donation in Mid- to High-Valent Iron Nitrido Complexes Stabilizes the Fe(VI) Oxidation State | doi.page

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Revisiting NHC–Metal Bonding: π-Donation in Mid- to High-Valent Iron Nitrido Complexes Stabilizes the Fe(VI) Oxidation State

Journal of the American Chemical Society2026

Zihan Zhang, Weiqing Mao, Y Chen, Frank W. Heinemann, Andreas Scheurer, Frank Neese, Klaus Meyer

Abstract

N-anchored tripodal N-heterocyclic carbene ligands, satTIMMNMes and Me2TIMMNMes, were synthesized and used to stabilize a series of mid-valent Fe(IV) and high-valent Fe(V) and Fe(VI) nitrides. The Fe(IV) and Fe(V) species adopt trigonal-pyramidal and trigonal-bipyramidal geometries, respectively, whereas the Fe(VI) nitrides exhibit octahedral coordination, representing rare examples of structurally characterized iron(VI) nitrido complexes. All complexes were characterized by single-crystal X-ray diffraction, multinuclear (1H, 13C, 15N, and 19F) NMR, zero- and applied-field 57Fe Mössbauer, electron paramagnetic resonance, as well as vibrational and electronic absorption spectroscopy. Combined spectroscopic, electrochemical, and computational studies examined how systematic variation in the NHC backbone modulates the electronic structures of the [Fe≡N]n+ and Fe-C_NHC moieties. Structural and spectroscopic parameters of Me2TIMMNMes-based Fe(IV-VI) nitrides closely resemble those of parent TIMMNMes analogues, while more pronounced deviations are observed for satTIMMNMes derivatives. 57Fe Mössbauer spectroscopy at 80 K revealed an unusually negative isomer shift for the tetravalent (d4, S = 0) [(satTIMMNMes)FeIV≡N]+ (δ = -0.45 mm s-1), distinct from Me2TIMMNMes and TIMMNMes analogues (δ = -0.36 and -0.35 mm s-1), attributable to enhanced Fe-C_NHC covalency in satTIMMNMes ligand. Cyclic voltammetry, supported by theoretical calculations, quantifies relative ligand π-donation in the Fe(IV) nitrido complexes, complementary to established NHC σ-donor descriptors, yielding the trend satTIMMNMes Mes Me2TIMMNMes. Consistent with this trend, structural and EPR data indicate differing degrees of Jahn-Teller distortion among the paramagnetic Fe(V) nitrides (d3, S = 1/2). In contrast, the structural and spectroscopic differences are relatively diminished in highly covalent Fe(VI) nitrides (d2, S = 0).