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.

Iron complexes with nitrido ligands are of interest as molecular analogues of key intermediates during N2-to-NH3 conversion in industrial or enzymatic processes. Dinuclear iron complexes with a bridging nitrido unit are mostly known in relatively high oxidation states (III/IV or IV/IV), originating from the decomposition of azidoiron precursors via high-valent FeN intermediates. The use of a tetra-NHC macrocyclic scaffold ligand (NHC = N-heterocyclic carbene) has now allowed for the isolation of a series of organometallic μ-nitridodiiron complexes ranging from the mid-valent FeIII–N–FeIII (1) via mixed-valent FeIII–N–FeIV (type 4) to the high-valent FeIV–N–FeIV (type 5) species that are interconverted at moderate potentials, accompanied by axial ligand binding at the FeIV sites. Magnetic measurements and electron paramagnetic resonance spectroscopy showed the homovalent complexes to be diamagnetic and the mixed-valent system to feature an S = 1/2 ground state due to very strong antiferromagnetic coupling. The bonding in the Fe–N–Fe moiety has been further probed by crystallographic structure determination, 57Fe Mössbauer and UV–vis spectroscopies, as well as density functional theory computations, which revealed high covalency and nearly identical Fe–N distances across this redox series. The latter has been rationalized in terms of the nonbonding nature of the combination of Fe d z 2 atomic orbitals from which electrons are successively removed upon oxidation, and these redox processes are best described as being metal-centered. The tetra-NHC-ligated μ-nitridodiiron series complements a set of related complexes with single-atom μ-oxido and μ-phosphido bridges, but the Fe–N–Fe core exhibits a comparatively high stability over several oxidation states. This promises interesting applications in view of the manifold catalytic uses of μ-nitridodiiron complexes based on macrocyclic N-donor porphinato­(2−) or phthalocyaninato(2−) ligands.

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B­(C6F5)3 resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two FeII cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the FeII-based t2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B­(C6F5)3 attachment at the cyanido N lone pair.

This article reports the efficient preparation of a series of unsymmetrically substituted thioethers through a two‐step procedure consisting of an initial metal‐free C−H sulfenylation of electron‐rich (hetero)arenes with newly prepared succinylthioimidazolium salts. Subsequent reaction of the arylthioimidazolium intermediates with Grignard reagents afford the desired thioethers. The synthetic protocol described is modular, scalable, and high yielding, and provides access to sulfides that are not easy to obtain through the existing methodologies. Importantly, no prefunctionalization of the initial (hetero)arene is required.

The combination of pyridonate ligands with transition metal ions enables the synthesis of an especially rich set of diverse coordination compounds involving various κ- and μ-bonding modes and higher nuclearities. With iron­(II) ions, this chemical space is rather poorly explored beyond some biomimetic models of the pyridone iron-containing hydrogenase. Here, the topologically new Fe5 and Fe4 clusters, Fe5(LH)6[N­(SiMe3)2]4 (1) and Fe4(LMe)6[N­(SiMe3)2]2 (2), were synthesized (LH = 2-pyridonate; LMe = 6-methyl-2-pyridonate). Complex 1 contained an unprecedented diamondoid Fe@Fe4 tetrahedron with a central-to-peripheral Fe–Fe distance of ∼3.1 Å. The crystal structure of complex 2 displayed an Fe4O6 butterfly motif containing a planar Fe4 arrangement. Mössbauer spectroscopy confirmed the high-spin ferrous character of all iron ions. SQUID magnetometry reveals that the Fe­(II) ions are involved in weak magnetic exchange coupling across the pyridonate bridges that results in antiferromagnetic interactions. The Fe4 cluster exhibits slow relaxation of magnetization under an applied magnetic field with an effective energy barrier of 38.5 K, rarely observed among the very rare examples of Fe­(II) cluster-based single-molecule magnets. Studies of protolytic substitution of the amido ligands demonstrated the lability of the diamondoid Fe5 core in 1 and the stability of the Fe4 rhomboid in 2.