An Iron Pincer Complex in Four Oxidation States

The synthesis and
characterization of a series of homoleptic iron
complexes [Fe­(benzNHCOCO)2]2–/1–/0/1+ supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene
pincer ligand benzNHCOCO2– (II) is presented. While the reaction of 2 equiv of free ligand II with a ferrous iron precursor leads to the isolation of
the coordination polymer [Fe­(benzNHCOCOK)2]n (1), treatment of II with ferric iron salts allows for the synthesis and isolation of
the mononuclear, octahedral bis-pincer compound K­[Fe­(benzNHCOCO)2] (2) and its crown-ether derivative
[K­(18c6)­(THF)2]­[Fe­(benzNHCOCO)2]
(3). Electrochemical studies of 2 suggested
stable products upon further one- and two-electron oxidation. Hence,
treatment of 2 with 1 equiv of AgPF6 yields
the charge-neutral species [Fe­(benzNHCOCO)2]
(4). Similarly, the cationic complex [Fe­(benzNHCOCO)2]­PF6 (5) is obtained by addition
of 2 equiv of AgPF6. The characterization of complexes 1, 3, and 4 reveals iron-centered
reduction and oxidation processes; thus, preserving the dianionic,
closed-shell structure of both coordinated benzNHCOCO pincer
chelates, II. This implies a stabilization of a highly
Lewis acidic iron­(IV) center by four phenolate anions rather than
charge distribution across the ligand framework with a lower formal
oxidation state at iron. Notably, the overall charge-neutral iron­(IV)
complex undergoes reductive elimination of the pincer ligand, providing
a metal-free compound that can be described as a spirocyclic imidazolone
ketal (6). In contrast, the ligand–metal bonds
in 5, formally an iron­(V) complex, are considerably covalent,
rendering the assignment of its oxidation state challenging, if not
impossible. All compounds are fully characterized, and the complexes’
electronic structures were studied with a variety of spectroscopic
and computational methods, including single-crystal X-ray diffraction
(SC-XRD), X-band electron paramagnetic resonance (EPR), and zero-field 57Fe Mössbauer spectroscopy, variable-field and variable-temperature
superconducting quantum interference device (SQUID) magnetization
measurements, and multi-reference ab initio (NEVPT2/CASSCF)
as well as density functional theory (DFT) studies. Taken altogether,
the electronic structure of 5 is best described as an
iron­(IV) center antiferromagnetically coupled to a ligand-centered
radical.