An Iron Pincer Complex in Four Oxidation States

Abstract

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.

Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

“…Although we have not been able to structurally characterize the product(s), we suspect that a mixture of oligomeric species is formed. Dimerization is common for iron complexes with thiolate-containing ligands and, in particular, has been observed for ligands derived from the o -aminothiophenol 3 . − The formation of polymeric species has also been reported for some iron complexes with a benzimidazole OCO ligand related to SCS 2– …”

“…60−62 The formation of polymeric species has also been reported for some iron complexes with a benzimidazole OCO ligand related to SCS 2− . 63 In contrast, addition of 7 to Fe(HMDS) 2 in THF followed by addition of 18-crown-6 formed a bright yellow species (10) that is stable in solution at room temperature in the absence of air. Crystals of 10 suitable for X-ray diffraction were obtained from a concentrated diethyl ether solution stored at −40 °C overnight.…”

Intense Photoinduced Intervalence Charge Transfer in High‐Valent Iron Mixed Phenolate/Carbene Complexes

“…[Fe( NHC OCO) 2 )]•1.5 THF. [38] H atoms, the counter ion, and co-crystallized solvent molecules are omitted for clarity; thermal ellipsoids are shown at the 50 % probability level.…”

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes

“…Interestingly, while NHCs were first considered as fully innocent, pure-spectator ligands, the seminal report by Tumanskii and Apeloig in 2007 shows that imidazole-derived NHC ligands are also able to accept and delocalize radicals, or some spin density, within their heterocyclic scaffold [13], and, hence, that they could also be considered as redox-active ligands [14][15][16][17][18]. In parallel, redoxactive NHC ligands were designed on purpose, by combining the carbenic heterocycle with some known redox-active moieties, such as ferrocene [19][20][21][22], cobaltocenium [23], quinone [24][25][26], o-aminophenols [27][28][29][30][31], or more recently, a naphthalene diimide backbone [32,33]. The redox-active NHC ligands allowed the design of redox-switchable catalysts by a strong modification of the electronic properties through a redox stimulus [7,34], or permitted the access to multiple oxidation states of the complexes.…”