The mono-benzaldimine (HNCHPh) complex [( tBupyrpyrr2)­Fe­(HNCHPh)] (1-HNCHPh) has been prepared by reaction of [( tBupyrpyrr2)­Fe­(OEt2)] (1-OEt2) ( tBupyrpyrr2 = 2,6-bis­(3,5-di-tert-butyl-pyrrolyl)­pyridine) with one equivalent of benzyl azide. Compound 1-HNCHPh retains the cis-divacant octahedral coordination geometry akin to 1, as established by single crystal X-ray diffraction study. A bis-HNCHPh complex [( tBupyrpyrr2)­Fe­(HNCHPh)2] (2) was also prepared by the addition of two equivalents of benzyl azide to 1, and its molecular structure exhibits the two HNCHPh ligands coordinated trans to each other, thereby forming a square pyramidal coordination geometry at the FeII center. Reaction of 1 with excess benzyl azide yields [( tBupyrpyrr2)­Fe­(HNCHPh)2·PhCHNCH­(NH2)­Ph] (2-PhCHNCH­(NH2)­Ph), which contains an unstable benzylideneamino phenyl methanamine fragment, effectively hydrogen bonded to 2. Thermolysis of 2 or 2-PhCHNCH­(NH2)­Ph releases the HNCHPh self-coupling products hydrobenzamide (A), N-benzylidine benzylamine (B), and benzonitrile (C). Under catalytic conditions, free HNCHPh (cis/trans-HNCHPh mixture) is produced using 2.5 mol % of 1 in 90% spectroscopic yield. These studies provide a clearer understanding for the conversion of the HNCHPh in 2 or 2-PhCHNCH­(NH2)­Ph to the C–C and C–N coupled products. Reduction of 1-HNCHPh with KC8 yields the reductively coupled benzylamide complex [K­(OEt2)]2[( tBupyrpyrr2)2Fe2(μ2-NHCHPhCHPhNH)] (3) as the result of a new C–C bond formed between two radical benzylamide fragments.

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.

Reaction of the 6π-electron aromatic fourmembered heterocycle (IPr) 2 C 2 P 2 (1) (IPr = 1,3-bis(2,6diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) with [Fe 2 CO 9 ] gives the neutral iron tricarbonyl complex [Fe(CO) 3 -η 3 -{(IPr) 2 C 2 P 2 }] (2). Oxidation with two equivalents of the ferrocenium salt, [Fe(Cp) 2 ](BArF 24 ), affords the dicationic tricarbonyl complex [Fe(CO) 3 -η 4 -{(IPr) 2 C 2 P 2 }](BArF 24 ) 2 (4). The one-electron oxidation proceeds under concomitant loss of one CO ligand to give the paramagnetic dicarbonyl radical cation complex [Fe(CO) 2 -η 4 -{(IPr) 2 C 2 P 2 }](BArF 24 ) (5). Reduction of 5 allows the preparation of the neutral dicarbonyl complex [Fe(CO) 2 -η 4 -{(IPr) 2 C 2 P 2 }] ( 6). An analysis by various spectroscopic techniques ( 57 Fe Mössbauer, EPR) combined with DFT calculations gives insight into differences of the electronic structure within the members of this unique series of iron carbonyl complexes, which can be either described as electron precise or Wade-Mingos clusters.

We report high-valent iron complexes supported by N-heterocyclic carbene (NHC)-anchored, bis-phenolate pincer ligands that undergo ligand-to-metal charge transfer (LMCT) upon photoexcitation. The resulting excited states -with a lifetime in the picosecond range -feature a ligand-based, mixed-valence system and intense intervalence charge transfer bands in the near-infrared region. Upon oxidation of the complex, corresponding intervalence charge transfer absorp-tions are also observed in the ground state. We suggest that the spectroscopic hallmarks of such LMCT states provide useful tools to decipher excited-state decay mechanisms in high-valent NHC complexes. Our observations further indicate that NHC-anchored, bis-phenolate pincer ligands are not sufficiently strong donors to prevent the population of excited metal-centered states in high-valent iron complexes.

Iron compounds containing a bridging oxo or sulfido moiety are ubiquitous in biological systems, but substitution with the heavier chalcogenides selenium and tellurium, however, is much rarer, with only a...