Starting from their six‐coordinate iron(II) precursor complexes [L8RFe(MeCN)]2+, a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L8H and its new octamethylated derivative L8Me, both providing four imidazol‐2‐yliden donors, were synthesized. Several five‐ and six‐coordinate iron(III) complexes with different axial ligands (Cl−, OTf−, MeCN) were structurally characterized by X‐ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and 57Fe Mößbauer). Depending on the axial ligands, either low‐spin (S=1/2) or intermediate‐spin (S=3/2) states were observed, whereas high‐spin (S=5/2) states were inaccessible because of the extremely strong in‐plane σ‐donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ‐oxo diiron(III) complexes were also studied.

Molecular first-row transition metal complexes for electrocatalytic CO2 reduction mostly feature N-donor supporting ligands, iron porphyrins being among the most prominent catalysts. Introducing N-heterocyclic carbene (NHC) ligation has previously shown promising effects for some systems, yet the application of NHC iron complexes for electrochemical CO2 reduction has so far remained unreported. Herein we show that the macrocyclic tetracarbene iron complex [LFe(NCMe)2](OTf)2 (1), which can be described as an organometallic heme analogue, mediates selective electrocatalytic CO2to-CO conversion with a faradaic efficiency of over 90% and a very high initial observed catalytic rate constant (kobs) of 7,800 s −1 . Replacement of an axial MeCN ligand by CO significantly increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (kobs = 3,100 s −1 ). Ferrous complexes with one or two axial CO ligands, [LFe(NCMe)(CO)](OTf)2 (1-CO) and [LFe(CO)2](OTf)2 (1-(CO)2), have been isolated and fully characterized. Based on linear sweep voltammogram (LSV) spectroelectro-IR (SEC-IR) studies for 1 and 1-CO, both under N2 and CO2 atmosphere, a mechanistic scenario in anhydrous acetonitrile is proposed. It involves two molecules of CO2 and results in CO and CO3 2− formation, whereby the first CO2 binds to the doubly reduced, pentacoordinated [LFe 0 (CO)] species. This work commences the exploration of the reductive chemistry by the widely tunable macrocyclic tetracarbene iron motif, which is topologically similar to hemes but electronically distinct as the strongly -donating and redox inactive NHC scaffold leads to metal-centered reduction and population of the exposed dz² orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.

μ‐Oxodiiron(III) species are air‐stable and unreactive products of autoxidation processes of monomeric heme and non‐heme iron(II) complexes. Now, the organometallic [(LNHC)FeIII‐(μ‐O)‐FeIII(LNHC)]4+ complex 1 (LNHC is a macrocyclic tetracarbene) is shown to be reactive in C−H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) (2) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic analysis gave an equilibrium constant for the disproportionation of 1, which is very small (Keq=7.5±2.5×10−8 m). Increasing Keq might by a useful strategy for circumventing the formation of dead‐end μ‐oxodiiron(III) products during Fe‐based homogeneous oxidation catalysis.

μ‐Oxodiiron(III) species are air‐stable and unreactive products of autoxidation processes of monomeric heme and non‐heme iron(II) complexes. Now, the organometallic [(LNHC)FeIII‐(μ‐O)‐FeIII(LNHC)]4+ complex 1 (LNHC is a macrocyclic tetracarbene) is shown to be reactive in C−H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) (2) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic analysis gave an equilibrium constant for the disproportionation of 1, which is very small (Keq=7.5±2.5×10−8 m). Increasing Keq might by a useful strategy for circumventing the formation of dead‐end μ‐oxodiiron(III) products during Fe‐based homogeneous oxidation catalysis.

Molecular first-row transition metal complexes for electrocatalytic CO₂ reduction mostly feature N-donor supporting ligands, iron porphyrins being among the most prominent catalysts. Introducing N-heterocyclic carbene (NHC) ligation has previously shown promising effects for some systems, yet the application of NHC iron complexes for electrochemical CO₂ reduction has so far remained unreported. Herein we show that the macrocyclic tetracarbene iron complex [LFe(NCMe)₂](OTf)₂ (<b>1</b>), which can be described as an organometallic heme analogue, mediates selective electrocatalytic CO₂-to-CO conversion with a faradaic efficiency of over 90% and a very high initial observed catalytic rate constant (<i>k</i>_obs) of 7,800 s⁻¹. Replacement of an axial MeCN ligand by CO significantly increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (<i>k</i>_obs = 3,100 s⁻¹). Ferrous complexes with one or two axial CO ligands, [LFe(NCMe)(CO)](OTf)₂ (<b>1-CO</b>) and [LFe(CO)₂](OTf)₂ (<b>1-(CO)₂</b>), have been isolated and fully characterized. Based on linear sweep voltammogram (LSV) spectroelectro-IR (SEC-IR) studies for <b>1</b> and <b>1-CO</b>, both under N₂ and CO₂ atmosphere, a mechanistic scenario in anhydrous acetonitrile is proposed. It involves two molecules of CO₂ and results in CO and CO₃²⁻ formation, whereby the first CO₂ binds to the doubly reduced, pentacoordinated [LFe⁰(CO)] species. This work commences the exploration of the reductive chemistry by the widely tunable macrocyclic tetracarbene iron motif, which is topologically similar to hemes but electronically distinct as the strongly s-donating and redox inactive NHC scaffold leads to metal-centered reduction and population of the exposed d_z² orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.