A series of mononuclear Fe(II) triflate complexes based on the 3,3-bis(1-alkylimidazole-2-yl)propionate ester (BAIP) ligand scaffold are reported. In these complexes, the tripodal N,N,O-BAIP ester ligand is varied by (i) changing the ester moiety (i.e., n-Pr, tert-Bu esters, n-Pr amide), (ii) changing the methylimidazole moieties to methylbenzimidazole moieties, and (iii) changing the methylimidazole moieties to 1-ethyl-4-isopropylimidazole moieties. The general structure of the resulting complexes comprises two facially capping BAIP ligands around a coordinatively saturated octahedral Fe(II) center, with either a transoid or cisoid orientation of the N,N,O-donor manifold that depends on the combined steric and electronic demand of the ligands. In the case of the sterically most encumbered ligand, a four-coordinate all N-coordinate complex is formed as well, which cocrystallizes with the six-coordinate complex. In combination with the catalytic properties of the new complexes in the epoxidation/cis-dihydroxylation of cyclooctene with H2O2, in terms of turnover number and cis-diol formation, these studies provide a number of insights for further ligand design and catalyst development aimed at Fe-mediated cis-dihydroxylation.

Mononuclear iron(II) complexes with facially coordinating N,N,O-ligands were synthesized as accurate structural mimics of the 2-His-1-carboxylate facial triad found in mononuclear non-heme iron enzymes. Mimicking of the facial triad is achieved by designing sterically demanding ligands providing two histidine-like benzimidazole moieties and a coordinating carboxylate or ester moiety.

A series of [MoO2(acac′)2] [acac′ = acetylacetonato‐type ligand: dibenzoylmethane (3), 1‐benzoylacetone (4), bis(p‐methoxybenzoyl)methane (5), 2‐acetylcyclopentanone (6), 2‐acetylcyclohexanone (7), and 2‐acetyl‐1‐tetralone (8)] complexes have been synthesized in yields of 44–83 % by a simple synthetic method by using sodium molybdate and the desired acac‐type ligand as starting materials. All the complexes were characterized by IR, UV/Vis, NMR, and high‐resolution ESI‐MS, and for compounds 3, 4, and 8, solid‐state structures were obtained by X‐ray diffraction. All the complexes contain a cis‐dioxomolybdenum moiety, as proven by the characteristic Mo=O vibrations in the IR spectra and the occurrence of four sets of signals in the NMR spectra of the complexes bearing asymmetrical ligands (4 and 6–8), and confirmed by the solid‐state structures. The complexes were found to be active as catalysts in the dehydration of 1‐phenylethanol to styrene using technical‐grade toluene as the solvent in air at 100 °C. The highest catalytic activity was found for [MoO2{(tBuCO)2CH}2] (2), followed by [MoO2{(C6H5CO)2CH}2] (3). Both complexes were also found to be active in the dehydration of other alcohols, including allylic, aliphatic, and homoallylic alcohols, as well as secondary and tertiary alcohols, with 2 generally showing better activity and selectivity than 3. These catalytic results were compared with those previously obtained with the metal‐based catalyst Re2O7 and the benchmark acid catalyst H2SO4. The results were dependent on the substrate: By using 2, good selectivities but lower activities were generally obtained with tertiary alcohols, whereas good activities but lower selectivities were obtained with secondary alcohols. The industrially important dehydration of 2‐octanol to octenes was very efficiently catalyzed by 2. Overall, the [MoO2(acac′)2] complexes reported herein could offer a cheaper and more abundant metal‐based catalyst alternative to the previously reported rhenium‐based catalytic system for the dehydration reaction.

Iron-containing metalloenzymes that contain the 2-His-1-Carboxylate facial triad at their active site are well known for their ability to activate molecular oxygen and catalyse a broad range of oxidative transformations. Many of these reactions are synthetically challenging, and developing small molecular iron-based catalysts that can achieve similar reactivity and selectivity remains a long-standing goal in homogeneous catalysis. This review focuses on the development of bioinspired facial N,N,O ligands that model the 2-His-1-Carboxylate facial triad to a greater degree of structural accuracy than many of the polydentate N-donor ligands commonly used in this field. By developing robust, well-defined N,N,O facial ligands, an increased understanding could be gained of the factors governing enzymatic reactivity and selectivity.

The ability of the bis(imidazolyl)ketone ligand BMIK (bis(1-methyl-4,5-diphenylimidazolyl)ketone) to function as a redox active ligand has been investigated. The reduction of [M(BMIK)Cl] (M = Fe and Zn) complexes resulted in a pinacol-type coupling to form dinuclear complexes featuring very weak and abnormally elongated C-C bonds (1.729(5) and 1.708(3) Å for Fe and Zn, respectively). Oxidation of these complexes using ferrocenium in the presence of Cl ions regenerated the original [M(BMIK)Cl] complexes, showing the reversibility of the coupling process. This makes it a potentially interesting approach for the storage of electrons and application of the BMIK ligand as a redox active ligand.