An Iron Pincer Complex in Four Oxidation States

The synthesis and characterization of a series of homoleptic iron complexes [Fe ((OCO)-O-benNHC)(2)](2-/1-/0/1+) supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene pincer ligand (OCO2-)-O-benzNHC (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((OCOK)-O-benzNHC)(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((OCO)-O-benzNHC)(2)] (2) and its crown-ether derivative [K-(18c6)(THF)(2)][Fe((OCO)-O-benzNHC)2] (3). Electrochemical studies of 2 suggested stable products upon further one- and two-electron oxidation. Hence, treatment of 2 with 1 equiv of AgPF 6 yields the charge-neutral species [Fe((OCO)-O-benzNHC)2] (4). Similarly, the cationic complex [Fe((OCO)-O-benzNHC)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 (OCO)-O-benzNHC 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 Fe-57 Mossbauer 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.