High valent iron complexes have been identified or proposed as key intermediates in many iron-catalyzed chemical and biochemical processes. Among the family members, high valence Iron-imido species, isoelectronic to Iron-oxo species, have generally attracted great synthetic and spectroscopic interests in recent decades. Although the mechanism underlying the chemical reactivities of these species remains unclear, the spin transition process is proposed to play an important role in some mechanisms, emphasizing the importance to understand the possible correlation between spin state and reactivity. Herein, we represent a comprehensive study on the electronic structures of two three-coordinate bis-imido Fe(IV) complexes with a bulky NHC ligand as spectator ligands. They exhibit distinct physical and chemical properties: one has a diamagnetic ground state showing poor reactivity, the other is paramagnetic which can conduct hydrogen-atom-transfer(HAT)-based dehydration at room temperature. Mössbauer, EPR and magnetism studies reveal the paramagnetic species has an <i>S</i> = 1 ground state with strong axial-type magnetic anisotropy as a consequence of pseudo-double-degenerate ground orbital states with strong spin-orbit coupling. Theoretical calculations (DFT and CASSCF/NEVPT2) reveal the orientation between the two R-N(imido) bonds are the key structural factor that is responsible for the spin divergence. Our results highlight the steric effects of substituents on imido ligands in the present system. A hypothesis is also proposed to understand the electronic structure and associated HAT-based reactivity of most reported high valent iron-imido species.

Electronic structures on two iron(IV) bis-imido complexes: Steric hindrance perturbed spin divergence

ACS Spring 2022

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