An iron(II) complex with a cyclic tetradentate ligand containing four N-heterocyclic carbenes was synthesized and characterized by means of NMR and IR spectroscopies, as well as by single-crystal X-ray structure analysis. The iron center exhibits an octahedral coordination geometry with two acetonitrile ligands in axial positions, showing structural analogies with porphyrine-ligated iron complexes. The acetonitrile ligands can readily be substituted by other ligands, for instance, dimethyl sulfoxide, carbon monoxide, and nitric oxide. Cyclic voltammetry was used to examine the electronic properties of the synthesized compounds.

N-heterocyclic carbenes (NHCs), and macrocyclic NHCs in particular, have been applied successfully to the stabilization of high oxidation states on transition metal complexes. This access to high oxidation states has enabled their application in oxidative catalysis including aziridination and epoxidation. However, the number of macrocyclic tetra-NHC ligands is still limited, especially those featuring anionic charge, which is beneficial in this regard. In this manuscript, we report a facile and high yielding synthesis for only the second example of such a dianionic, macrocyclic tetra-NHC ligand. This 16-atom macrocycle has the ring size and charge of a porphyrin but with the increased electron donation of NHCs. Its Fe(II) and Fe(III) complexes are reported, and their reactivities for ligand addition and oxidation were tested. Multiple oxidation catalysis reactions were tested with both the Fe(II) and Fe(III) complexes with reagents such as trimethylamine-Noxide, oxygen (from air), diazodiphenylmethane, and P-tert-butyl-dibenzo-7λ 3 phosphanorbornadiene ( t BuPA) to explore the possibilities for single site oxidation reactions. Article pubs.acs.org/Organometallics

Organometallic Fe complexes with exceptionally high activities in homogeneous epoxidation catalysis are reported. The compounds display Fe(II) and Fe(III) oxidation states and bear a tetracarbene ligand. The more active catalyst exhibits activities up to 183 000 turnovers per hour at room temperature and turnover numbers of up to 4300 at -30 °C. For the Fe(III) complex, a decreased Fenton-type reactivity is observed compared with Fe(II) catalysts reported previously as indicated by a substantially lower H2 O2 decomposition and higher (initial) turnover frequencies. The dependence of the catalyst performance on the catalyst loading, substrate, water addition, and the oxidant is investigated. Under all applied conditions, the advantageous nature of the use of the Fe(III) complex is evident.

Three five-coordinate iron(IV) imide complexes have been synthesized and characterized.T hese novel structures have disparate spin states on the iron as afunction of the R-group attached to the imide,w ith alkylg roups leading to low-spin diamagnetic (S = 0) complexes and an aryl group leading to an intermediate-spin (S = 1) complex. The different spin states lead to significant differences in the bonding about the iron center as well as the spectroscopic properties of these complexes.M çssbauer spectroscopyc onfirmed that all three imide complexes are in the iron(IV) oxidation state.T he combination of diamagnetism and 15 Nlabeling allowed for the first 15 NN MR resonance recorded on an iron imide.M ultireference calculations corroborate the experimental structural findings and suggest how the bonding is distinctly different on the imide ligand between the two spin states.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

The dioxygen reactivity of a cyclic iron(ii) tetra-NHC-complex (NHC: N-heterocyclic carbene) is investigated. Divergent oxidation behavior is observed depending on the choice of the solvent (acetonitrile or acetone). In the first case, exposure to molecular oxygen leads to an oxygen free Fe(iii) whereas in the latter case an oxide bridged Fe(iii) dimer is formed. In acetone, an Fe(iii)-superoxide can be trapped, isolated and characterized as intermediate at low temperatures. An Fe(iii)-O-Fe(iii) dimer is formed from the Fe(iii) superoxide in acetone upon warming and the molecular structure has been revealed by single crystal X-ray diffraction. It is shown that the oxidation of the Fe(ii) complex in both solvents is a reversible process. For the regeneration of the initial Fe(ii) complex both organic and inorganic reducing agents can be used.

The tetradentate ligand system L 1 combines two Nheterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL 1 (MeCN) 2 ](PF 6 ) 2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH ⧧ = 15.1 kcal mol −1 , and ΔS ⧧ = −4.7 cal mol −1 K −1 ). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL 1 (CO)(MeCN)](PF 6 ) 2 (2) or neutral [FeL 1 (N 3 ) 2 ] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1−3. Mossbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe−C NHC bonds in the basal plane constituted by the macrocyclic ligand L 1 . Oxidation of 1 has been monitored by UV−vis spectro-electro chemistry, and the resulting iron(III) complex [FeL 1 (MeCN) 2 ](PF 6 ) 3 (4) has been isolated after chemical oxidation. SQUID and Mossbauer data have shown an S = 1 / 2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL 1 } core are basically invariant with respect to changes in the metal ion's oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C 2 N 2 } hybrid ligand L 1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C 4 } tetracarbene ligand. N-Heterocyclic carbenes (NHCs) have attracted a great deal of attention over the past two decades. NHCs, most of them imidazol-2-ylidenes, are now established as a prime ligand class in organometallic chemistry, with numerous applications in homogeneous catalysis.

No abstract

The first stable formal Cu(iii) NHC and its unusual reactivity with acetate are reported. Several products of this reaction are identified and fully characterised. It reactivity is extensively investigated and additionally explored by means of theoretical, electrochemical and isotope labelling experiments.