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Photoredox catalysis enables distinctive and broadly applicable chemical reactions, but controlling their selectivity has proven to be difficult. The pursuit of enantioselectivity is a particularly daunting challenge, arguably because of the high energy of the activated radical (ion) intermediates, and previous approaches have invariably required pairing of the photoredox catalytic cycle with an additional activation mode for asymmetric induction. A potential solution for photoredox reactions proceeding via radical ions would be catalytic pairing with enantiopure counterions. However, although attempts toward this approach have been described, high selectivity has not yet been accomplished. Here we report a potentially general solution to radical cation–based asymmetric photoredox catalysis. We describe organic salts, featuring confined imidodiphosphorimidate counteranions that catalyze highly enantioselective [2+2]-cross cycloadditions of styrenes.

Crystallographic, spectroscopic, and computational studies on diiron(ii) hydride complexes reveal rapid hydride motions that cause major changes in the electronic structures of the iron sites.

For a large scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal and nitrogen doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the iron ion is coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando 57Fe Mössbauer spectroscopy and quantum chemistry. A Mössbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mössbauer signatures, implying a direct structural correspondence. Pyrrolic N-coordination, i.e. FeN4C12, is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coor-dination. These findings thus overcome the previously conflicting structural assignments for the active site, and moreover identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.

Large Spin-Orbit Coupling (SOC) is an intrinsic property of the heavy-elements that directly affects the electronic structures of the compounds. Herein we report the synthesis and characterization of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand. All magnetic measurements (SQUID, NMR) point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin-triplet. The apparent diamagnetism is explained by an extremely large SOC induced positive zero-field-splitting of more than 4500 cm −1 that leaves the M S = 0 magnetic sublevel thermally isolated in the electronic ground state.

A flexible NHC/pyridine macrocyclic ligand scaffold that provides two tridentate {CNC} compartments is shown to host two Cu ions in reversibly interconvertible redox states, CuICuI (1) and mixed-valent Cu1.5Cu1.5 (2). They were comprehensively characterized by X-ray diffraction (XRD) and multiple spectroscopic methods, including electron paramagnetic resonance (EPR), UV-Vis absorption and magnetic circular dichroism (MCD), in combination with TD-DFT and CASSCF calculations. 2 features an extremely short Cu⋯Cu distance (~2.5 Å; compared to ~4.0 Å in 1) and is a Robin Day class III mixed-valent system with a very high delocalization energy of 13000 cm-1, which is comparable to the mixed-valent Cu1.5Cu1.5 state of the biological CuA site. Electron self-exchange between 1 and 2 is found to be rapid despite large structural reorganizations, which is proposed to proceed via a sequential mechanism involving an active conformational isomer of 1, viz. 1’; the latter has been isolated and characterized by XRD. This electron transfer (ET) process is reminiscent of the conformationally gated ET pathway proposed for biological systems. This redox couple represents a unique pair of dicopper complexes with flexible ligand system, achieving fast electron self-exchange closely related to the function of the CuA site.

Magneto-structural correlations for air-stable four-coordinate Co(ii) complexes reveal the dependence of magnetic properties on the dihedral twist angle and establish valuable design guidelines for achieving high magnetic anisotropy.

Addition of dioxygen at low temperature to the nonheme ferrous complex, Fe(II)(Me(3)TACN)(OSi(Ph2))(2)O) (1), in 2-MeTHF produces a peroxo-bridged diferric complex, Fe(2)(III)(μ-O(2))(Me(3)TACN)(2)((OSi(Ph2))(2)O)(2) (2), which was characterized by UV-vis, resonance Raman, and variable field Mössbauer spectroscopies. Illumination of a frozen solution of 2 in THF with white light leads to homolytic O–O bond cleavage and generation of a Fe(IV)(O) complex 4 (ν(FeO) = 818 cm(−1); δ = 0.22 mm s(−1), ΔE(Q) = 0.23 mm s(−1)). Variable field Mössbauer spectroscopy measurements show that 4 is a rare example of a high-spin, S = 2 Fe(IV)(O) complex, and the first synthetic example to be generated directly from O(2). Complex 4 is highly reactive, as expected for a high-spin ferryl, and decays rapidly in fluid solution at cryogenic temperatures. This decay process in 2-MeTHF involves C–H cleavage of the solvent. However, the controlled photolysis of 2 in situ with visible light and excess phenol substrate leads to competitive phenol oxidation, via the proposed transient generation of 4 as the active oxidant.

Isolation and stabilization of main group diradical species have posed a synthetic challenge over the years due to their intrinsic high reactivity. Herein we report on a large-scale synthesis and isolation of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand, which protects the Bi(I) center. The compound was characterized by its unique spectroscopic features (UV-vis and NMR), but more prominently, by its magnetic properties. Multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated by a spin-triplet. Further support for this electronic structure description was obtained through correlation of theory to experimental XRD, XAS, and UV-Vis data. However, all magnetic measurements (EPR, NMR and SQUID) point to a diamagnetic compound. This apparent discrepancy can be explained by an extremely large spin-orbit coupling (SOC) that leads to an unprecedented zero-field splitting of more than 8000 cm‒1, thus leaving a MS = 0 magnetic sublevel thermally isolated in the electronic ground state. The extremely large SOC effect is a result of the low-coordination number of the bismuth center in interplay with its heavy element nature.

The first confacial pentaoctahedron comprised of transition metal ions namely ZnII FeIIIA FeIIIB FeIIIA ZnII has been synthesized by using a dinucleating nonadentate ligand. The face-sharing bridging mode enforces short ZnII ⋅⋅⋅FeIIIA and FeIIIA ⋅⋅⋅FeIIIB distances of 2.83 and 2.72 A, respectively. Ab-initio CASSCF/NEVPT2 calculations provide significant negative zero-field splittings for FeIIIA and FeIIIB with |DA |>|DB | with the main component along the C3 axis. Hence, a spin-Hamiltonian comprised of anisotropic exchange, zero-field, and Zeeman term was employed. This allowed by following the boundary conditions from the theoretical results the simulation in a theory-guided parameter determination with Jxy =+0.37, Jz =-0.32, DA =-1.21, EA =-0.24, DB =-0.35, and EB =-0.01 cm-1 supported by simulations of high-field magnetic Mossbauer spectra recorded at 2 K. The weak but ferromagnetic FeIIIA FeIIIB interaction arises from the small bridging angle of 84.8° being at the switch from anti- to ferromagnetic for the face-sharing bridging mode.

The development of pyrimidine-based analogues of the well-known pyridinediimine (PDI) iron complexes enables access to a functional-group-tolerant methodology for the catalytic trimerization of ter...

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV=O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV=O complex 2 supported by the sterically demanding 1,4,7‐tri‐tert‐butyl‐1,4,7‐triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non‐classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton‐coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD‐J)., A highly reactive S=2 pseudotetrahedral oxoiron(IV) complex 2 supported by a sterically demanding 1,4,7‐tri‐tert‐butyl‐1,4,7‐triazacyclononane ligand has been synthesized and spectroscopically characterized as one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD‐J).

Copper-containing nitrous oxide reductase (N2OR) is the only known enzyme to catalyze the conversion of the environmentally critical greenhouse gas nitrous oxide (N2O) to dinitrogen (N2) as the final step of bacterial denitrification. Other than its unique tetranuclear active site CuZ, the binuclear electron entry point CuA is also utilized in other enzymes, including cytochrome c oxidase. In the CuA site of Pseudomonas stutzeri N2OR, a histidine ligand was found to undergo a conformational flip upon binding of the substrate N2O between the two copper centers. Here we report on the systematic mutagenesis and spectroscopic and structural characterization of this histidine and surrounding H-bonding residues, based on an established functional expression system for PsN2OR in E. coli. A single hydrogen bond from Ser550 is sufficient to stabilize an unbound conformation of His583, as shown in a Asp576Ala variant, while the additional removal of the hydrogen bond in a Asp576Ala/Ser550Ala double variant compelled His583 to stay in a bound conformation as a ligand to CuA. Systematic mutagenesis of His583 to Ala, Asp, Asn, Glu, Gln, Lys, Phe, Tyr, and Trp showed that although both the CuZ and CuA sites were present in all the variants, only the ones with a protonable side chain, i.e., His, Asp, and Glu, were able to mediate electron transfer at physiological pH. This observation is in line with a proton-coupled electron transfer mechanism at the CuA site of N2OR.

Hanging on by a thread. Formally zerovalent copper complexes with an Al(iii) or Ga(iii) support were investigated. The combined experimental and theoretical data corroborate the presence of an odd-electron σ-bond between Cu and the Group 13 center.

In vivo expression of metalloproteins requires specific metal trafficking and incorporation machinery inside the cell. Synthetic designed metalloproteins are typically purified without the target metal, which is subsequently introduced through in vitro reconstitution. The extra step complicates protein optimization by high-throughput library screening or laboratory evolution. We demonstrate that a designed coiled-coil iron-sulfur protein (CCIS) assembles robustly with [4Fe-4S] clusters in vivo. While in vitro reconstitution produces a mixture of oligomers that depends on solution conditions, in vivo production generates a stable homotrimer coordinating a single, diamagnetic [4Fe-4S]2+ cluster. The multinuclear cluster of in vivo assembled CCIS is more resistant to degradation by molecular oxygen. Only one of the two metal coordinating half-sites is required in vivo, indicating specificity of molecular recognition in recruitment of the metal cluster. CCIS, unbiased by evolution, is a unique platform to examine iron-sulfur protein biogenesis and develop synthetic multinuclear oxidoreductases.

The essential process of iron-sulfur (Fe/S) cluster assembly (ISC) in mitochondria occurs in three major phases. First, [2Fe-2S] clusters are synthesized on the scaffold protein ISCU2; second, these clusters are transferred to the monothiol glutaredoxin GLRX5 by an Hsp70 system followed by insertion into [2Fe-2S] apoproteins; third, [4Fe-4S] clusters are formed involving the ISC proteins ISCA1–ISCA2–IBA57 followed by target-specific apoprotein insertion. The third phase is poorly characterized biochemically, because previous in vitro assembly reactions involved artificial reductants and lacked at least one of the in vivo-identified ISC components. Here, we reconstituted the maturation of mitochondrial [4Fe-4S] aconitase without artificial reductants and verified the [2Fe-2S]-containing GLRX5 as cluster donor. The process required all components known from in vivo studies (i.e., ISCA1–ISCA2–IBA57), yet surprisingly also depended on mitochondrial ferredoxin FDX2 and its NADPH-coupled reductase FDXR. Electrons from FDX2 catalyze the reductive [2Fe-2S] cluster fusion on ISCA1–ISCA2 in an IBA57-dependent fashion. This previously unidentified electron transfer was occluded during previous in vivo studies due to the earlier FDX2 requirement for [2Fe-2S] cluster synthesis on ISCU2. The FDX2 function is specific, because neither FDX1, a mitochondrial ferredoxin involved in steroid production, nor other cellular reducing systems, supported maturation. In contrast to ISC factor-assisted [4Fe-4S] protein assembly, [2Fe-2S] cluster transfer from GLRX5 to [2Fe-2S] apoproteins occurred spontaneously within seconds, clearly distinguishing the mechanisms of [2Fe-2S] and [4Fe-4S] protein maturation. Our study defines the physiologically relevant mechanistic action of late-acting ISC factors in mitochondrial [4Fe-4S] cluster synthesis, trafficking, and apoprotein insertion.

Treatment of [MoCl4(THF)2] with MOtBu (M = Na, Li) does not result in simple metathetic ligand exchange but entails disproportionation with formation of the well-known dinuclear complex [(tBuO)3Mo≡Mo(OtBu)3] and a new paramagnetic compound, [Mo(OtBu)5]. This particular five-coordinate species is the first monomeric, homoleptic, all-oxygen-ligated but non-oxo 4d1 Mo(V) complex known to date; as such, it proves that the dominance of the Mo=O group over (high-valent) molybdenum chemistry can be challenged. [Mo(OtBu)5] was characterized in detail by a combined experimental/computational approach using X-ray diffraction; UV/vis, MCD, IR, EPR, and NMR spectroscopy; and quantum chemistry. The recorded data confirm a Jahn–Teller distortion of the structure, as befitting a d1 species, and show that the complex undergoes Berry pseudorotation. The alkoxide ligands render the disproportionation reaction, leading the formation of [Mo(OtBu)5] to be particularly facile, even though the parent complex [MoCl4(THF)2] itself was also found to be intrinsically unstable; remarkably, this substrate converts into a crystalline material, in which the newly formed Mo(III) and Mo(V) products cohabitate the same unit cell.

X-ray structures of the halo-substituted complexes [FeIII (5-X-salMeen)2 ]ClO4 (X=F, Cl, Br, I) [salMeen=N-methyl-N-(2-aminoethyl)salicylaldiminate]at RT have revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit with two perchlorate counter ions linking them by N-Hamine ⋅⋅⋅Operchlorate interactions. At 90 K, the two complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in the FeIII -salRen (R=alkyl) spin-crossover (SCO) system. At both temperatures, crystal packing shows dimerization through C-Himine ⋅⋅⋅Ophenolate interactions, a key feature for SCO cooperativity. Moreover, there are noncovalent contacts between the complex cations through type-II halogen-halogen bonds, which are novel in this system. The magnetic profiles and Mossbauer spectra concur with the structural analyses and reveal 50 % SCO of the type [HS-HS]↔[HS-LS] with a broad plateau. In contrast, [FeIII (5-Cl-salMeen)2 ]BPh4 ⋅2MeOH is LS and exhibits a temperature-dependent crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on SCO.

In soluble methane monooxygenase enzymes (sMMO), dioxygen (O2) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate Q that performs the challenging oxidation of methane to meth...

Iron complexes supported by novel π-acidic bis(imino)pyrazine (PPzDI) ligands can be functionalized at the nonligated nitrogen atom, and this has a marked effect on the redox properties of the resu...

One-electron oxidation of [(Py5Me2)RuII(N3)]+ to [(Py5Me2)RuIII(N3)]2+ switches on metallo-Staudinger reactivity towards PPh3, forming [(Py5Me2)RuII(N(H)PPh3)]2+ through a putative [Ru–NNN–PPh3] intermediate.

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the "oxidant"as well as "amino"group donor. It activates the high-spin Fe(II) (St = 2) catalyst [Fe(acac)2(H2O)2] (1) to generate a high-spin (St = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with St = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)2-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N-O bond homolysis. However, it does not generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (J = -524 cm-1) to an iminyl radical, [Fe(III)(acac)2-NH·], giving St = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N-O bond cleavage to generate the active iron-nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II features a weak elongated Fe-N bond which, together with the unpaired electron density along the Fe-N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis., Journal of the American Chemical Society, 144 (6), ISSN:0002-7863, ISSN:1520-5126

The ligand susan(6-Me) (susan(6-Me) 4,7-dimethyl-1,1,10,10-tetra(6 methyl-2-pyridylmethyl)-1,4,7,10-tetraazadecane) allows the synthesis of a peroxo complex that is only a transient species under catalytic conditions with the closely related ligand susan (susan - 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10- tetraazadecane). For the synthesis of a peroxo complex - analogous to that of susan(6-Me) we present here the three air- sensitive complexes [(susan){(FeCl2)-Cl-II}(2)], [(susan){Fe-II(OTf)(2)}(2)], and [(susan){Fe-II(mu-OH)(2)Fe-II}](ClO4)(2) as potential precursors. [(susan){Fe-II(mu-OH)(2)Fe-II}](ClO4)(2) shows weak antiferromagnetic coupling and a g -16 EPR signal. Dissolving [(susan){Fe-II(OTf)(2)}(2)] in CH3CN forms Rsusan){Fe-II(CH3CN)(2)}(2)](4+) that shows a spin transition upon cooling as evidenced by UV-Vis, H-1 NMR, and Mbssbauer spectroscopies. In the UV-Vis-NIR spectra the combination of (i) the energy of the MLCT, (ii) the energy, and (iii) the splitting of the d-d transitions provide insight into the overall electron donation and into the different sigma-donor/pi-donor/pi-acceptor capabilities of the exogenous ligands. These experimental signatures provide insight into the molecular structures in solution e.g. during reactions with O-2.

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO2). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO2, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO2 in their reduced states. The Ni(0) d10 species associates with CO2 to form a putative Aresta-type Ni-η2-CO2 complex, where electron transfer to CO2 through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d9 intermediate binding CO2 can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d7-to-d9 configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO2 activation for complexes based on redox-innocent ligands., A series of 3d transition-metal complexes (M = Mn, Fe, Co, Ni, Cu, and Zn) coordinated by a new redox-innocent PNP pincer ligand system were synthesized and structurally as well as electrochemically analyzed to illuminate the role of the metal center in molecular electrochemical carbon dioxide (CO2) activation.

Aqueous solutions of the iron(III) complex of N,N,N′-tris(2-pyridylmethyl)ethylenediamine-N′-acetate (tpena) react with hypochlorite (ClO-) to produce the reactive high-valent [FeIV(O)(tpena)]+. Under catalytic conditions, in bicarbonate-buffered media (pH 8) with a set ionic strength (10 mM NaCl), kinetic analysis shows that two equivalents of [FeIV(O)(tpena)]+ per one ClO- are produced, with benign chloride ions the only byproduct. An unprecedented supramolecular activation of ClO- by {(HCO3)⊂[(tpena)FeIII(μ-O)FeIII(Htpena)]}2+ is proposed. This mode of activation has great advantage for use in the catalytic oxidation of C-H bonds in water since: (i) the catalyst scaffold is protected from oxidative degradation and (ii) undesirable radical side reactions which produce toxic chlorinated compounds are circumvented by this novel coactivation of water and ClO-. The unique activation mechanism by the Fe-tpena system makes possible the destruction of organic contaminants as an add-on technology to water disinfection by chlorination, demonstrated here through (i) the catalytic oxidation of micropollutant metaldehyde, and (ii) mineralization of the model substrate formate. The resting-state speciation at pH 3, 5, 7, and 9, as well as the catalytically active iron speciation are characterized with Mössbauer and EPR spectroscopy and supported by DFT calculations. Our study provides fundamentally new insights into the design and activation mode of iron-based catalysts relevant to applications in water remediation.

Nitrous oxide reductase (N2OR) is the only known enzyme reducing environmentally critical nitrous oxide (N2O) to dinitrogen (N2) as the final step of bacterial denitrification. The assembly process of its unique catalytic [4Cu:2S] cluster CuZ remains scarcely understood. Here we report on a mutagenesis study of all seven histidine ligands coordinating this copper center, followed by spectroscopic and structural characterization and based on an established, functional expression system for Pseudomonas stutzeri N2OR in Escherichia coli. While no copper ion was found in the CuZ binding site of variants H129A, H130A, H178A, H326A, H433A and H494A, the H382A variant carried a catalytically inactive [3Cu:2S] center, in which one sulfur ligand, SZ2, had relocated to form a weak hydrogen bond to the sidechain of the nearby lysine residue K454. This link provides sufficient stability to avoid the loss of the sulfide anion. The UV-vis spectra of this cluster are strikingly similar to those of the active enzyme, implying that the flexibility of SZ2 may have been observed before, but not recognized. The sulfide shift changes the metal coordination in CuZ and is thus of high mechanistic interest., Variants of all seven histidine ligands of the [4Cu:2S] active site of nitrous oxide reductase mostly result in loss of the metal site. However, a H382A variant retains a [3Cu:2S] cluster that hints towards a structural flexibility also present in the intact site.

Terminal metal nitrides have been proposed as key intermediates in a series of pivotal chemical transformations. However, exploring the chemical activity of transient tetragonal iron(V) nitrides is largely impeded by their facile dimerization in fluid solutions. Herein, in situ EPR and Mössbauer investigations are presented of unprecedented oxygenation of a paramagnetic iron(V) nitrido intermediate, [FeVN(cyclam‐ac)]+ (2, cyclam‐ac−=1,4,8,11‐tetraazacyclotetradecane‐1‐acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam‐ac)]+ (3). Further theoretical studies suggest that during the reaction a closed‐shell singlet O atom is transferred to 2. Consequently, the N−O bond formation does not follow a radical coupling mechanism proposed for the N−N bond formation but is accomplished by three mutual electron‐transfer pathways between 2 and the O atom donor, thanks to the ambiphilic nature of 2., In situ EPR and Mössbauer investigations reveal unprecedented oxygenation of a transient paramagnetic iron(V) nitrido intermediate yielding an iron nitrosyl species. Computational studies suggest that the N−O bond formation involves transfer of a formally closed‐shell singlet O atom to the (FeN)2+ core concurrent with synergetic orbital interactions.

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster that features an iron centre with three bonds to sulfides, and characterize examples of the cluster in three oxidation levels using crystallography, spectroscopy, and ab initio calculations. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the three-coordinate iron centre in these clusters has a surprising low-spin electronic configuration due to the planar geometry and short Fe-S bonds. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH2 bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.

Iron–nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shel...

The vibrational properties of spin-crossover complexes [Fe(H2B(pz)2)2(L)] (pz = pyrazole) containing L = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands are investigated by temperature-dependent infrared and Raman spectroscopy. For comparison, the analogous cobalt(II) complexes [Co(H2B(pz)2)2(L)] (L = bipy and phen) and iron(II) compounds with L = 4,4'-dimethyl-2,2'-bipyridine and 4,7-dimethyl-1,10-phenanthroline coligands are studied. Highly intense, structured bands (giant Raman features, GRFs) are observed in the resonance Raman spectra of all Fe(II) complexes between 400 and 500 cm-1 at low temperatures in the HS state which, for the SCO complexes, is excited by the Raman laser. On the basis of magnetic field Mossbauer and saturation magnetization data electronic Raman effects are excluded to account for these features. Furthermore, detailed vibrational analysis also allows excluding a vibrational resonance Raman effect involving one of the modes of the individual complexes as a possible origin of the GRFs. Consequently, these features are attributed to coherent two-phonon excitation of metal-ligand stretching vibrations in molecular dimers coupled by π-π stacking interactions.

Mo nitrogenase (N2ase) utilizes a two-component protein system, the catalytic MoFe and its electron-transfer partner FeP, to reduce atmospheric dinitrogen (N2) to ammonia (NH3). The FeMo cofactor c...

Metal–superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe–, Co–, and Ni–superoxo complexes have been reported to date, well-defined Mn–superoxo complexes remain rather rare. Herein, we report two mononuclear MnIII–superoxo species, Mn(BDPP)(O2•–) (2, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDPBrP)(O2•–) (2′, H2BDPBrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O2 into solutions of their MnII precursors, Mn(BDPP) (1) and Mn(BDPBrP) (1′), at −80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin MnIII center (SMn = 2) antiferromagnetically coupled to a superoxo radical ligand (SOO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2′ were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form MnIII–hydroperoxo species, Mn(BDPP)(OOH) (5) and Mn(BDPBrP)(OOH) (5′). Complexes 5 and 5′ can be independently prepared by the reactions of the isolated MnIII-aqua complexes, [Mn(BDPP)(H2O)]OTf (6) and [Mn(BDPBrP)(H2O)]OTf (6′), with H2O2 in the presence of NEt3. The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5′., Two mononuclear MnIII−superoxo complexes, synthesized by bubbling O2 into solutions of their MnII precursors at −80 °C and well-characterized spectroscopically and computationally, were shown to be capable of abstracting a H atom from TEMPO-H to form MnIII−hydroperoxo species.

Iron terminal imido species are typically implicated as reaction intermediates in iron-catalyzed transformations. While a large body of work has been devoted to mid- and high-valent iron imidos, to date the chemistry of iron(II) imidos has remained largely unexplored due to the difficulty in accessing them. Herein, we present a study on the two-coordinate iron(II) imido complex [(IPr)Fe(NArTrip)] (3; IPr = 1,3-bis(2′,6′-diisopropylphenyl)imidazol-2-ylidene; ArTrip = 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide ArTripN3. Spectroscopic investigations in combination with DFT calculations established a high-spin S = 2 ground spin state for 3, consistent with its long Fe–N multiple bond of 1.715(2) Å revealed by X-ray diffraction analysis. Complex 3 exhibits unusual activity of nitrene transfer and C–H bond activation in comparison to the reported iron imido complexes. Specifically, the reactions of 3 with CH2=CHArCF3, an electron-deficient alkene, and CO, a strong π acid, readily afford nitrene transfer products, ArCF3CH=CHNHArTrip and ArTripNCO, respectively, yet no similar reaction occurs when 3 is treated with electron-rich alkenes and PMe3. Moreover, 3 is inert toward the weak C(sp3)–H bonds in 1,4-cyclohexadiene, THF, and toluene, whereas it can cleave the stronger C(sp)–H bond in p-trifluoromethylphenylacetylene to form an iron(II) amido alkynyl complex. Interestingly, intramolecular C(sp3)–H bond functionalization was observed by adding (p-Tol)2CN2 to 3. The unique reactivity of 3 is attributed to its low-coordinate nature and the high negative charge population on the imido N atom, which render its iron–imido unit nucleophilic in nature., The two-coordinate iron(II) imido complex (IPr)Fe(NArTrip) (3) exhibits a high-spin ground state (S = 2) and was found to be reactive toward electron-deficient alkene, diazo compounds, terminal alkyne, et al., in which diversified reactivities of nitrene transfer, C−H bond activation, and C−N bond formation have been observed. The reactivity pattern reflects the nucleophilic nature of the imido moiety of the high-spin iron(II) complex.

Previously, we reported the synthesis of Ti[N(o-(NCH2P(iPr)2)C6H4)3] and the Fe–Ti complex, FeTi[N(o-(NCH2P(iPr)2)C6H4)3], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe–Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at −2.16 and −1.36 V (versus Fc/Fc+), respectively. Two isostructural redox members, [FeTiL]+ and [FeTiL]− (2ox and 2red, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL]+ complex (1ox). The solid-state structures of the [FeTiL]+/0/– series feature short metal–metal bonds, ranging from 1.94–2.38 Å, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 Å). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and 57Fe Mössbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL]+,0,– series are primarily Fe-based and that the polarized Fe–Ti π-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%)., An isostructural redox series of Fe≡Ti complexes was investigated using a combination of spectroscopic methods and density functional theory to elucidate their electronic structures and to understand their polarized metal−metal bonding. Overall, the results support that the redox changes occur primarily at the Fe site though some electron density is delocalized to Ti. Hence, the Ti plays an important role in enhancing the redox flexibility of the single Fe site.

A specific investigation was carried out to study the influence of the Ni/Fe ratio for oxygen evolution reaction (OER) by using the hard templating method as a toolbox. Various compositions of homogeneously blended Ni–Fe oxide nanoparticles with a primary particle size of around 8 nm were simply prepared by using pore confinement of the tea leaves template. Based on the similar physical properties, including particle size and surface area, for all samples, it was verified that the OER activity in alkali electrolyte was mainly governed by the metal stoichiometry, where a maximum current density was obtained with a Ni/Fe ratio of 32/1. The higher catalytic performance of Ni32Fe oxide was attributed to lower reaction resistance and higher intrinsic activity, which are confirmed by electrochemical impedance spectroscopy and surface area analysis, respectively. The lowest overpotential (0.291 VRHE at 10 mA/cm2) as well as the highest current density (over 600 mA/cm2 at 1.7 VRHE) was achieved with Ni/Fe = 32/1 ...

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C–O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(I) complex, LtBuCo (where LtBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C–O cleavage, the cobalt(I) monocarbonyl complex LtBuCo(CO) and the dicobalt(II) carbonate complex (LtBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of LtBuCo rearranges to the κ2N,N′ binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to LtBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C–O bond rupture. A plausible intermediate in the reaction of CO2 with LtBuCo is the oxodicobalt(II) complex LtBuCoOCoLtBu, which has been independently synthesized through the reaction of LtBuCo with N2O. The rapid reaction of LtBuCoOCoLtBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by LtBuCo. LtBuCoOCoLtBu is a novel example of a thoroughly characterized molecular cobalt–oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the μ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(II) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations.

In the endeavor to close the anthropogenic carbon cycle and produce goods in a de-fossilized future, electrochemical carbon dioxide reduction constitutes a powerful tool.[1] Herein,3d transition metal complexes range among the most effective catalysts to overcome the kinetic barriers for CO2 activation.[2] In a recent literature review, we analyzed the main reaction pathways of homogeneously catalyzed CO2 electroreduction from an organometallic perspective and classified them into two mechanisms: (1) direct coordination of carbon dioxide to the metal center prior to activation and (2) formation of a metal hydride as the intermediate reactive to CO2.[3] Although properties of the metal center such as hydricity, or substrate (CO2) and product (CO, HCO2H...) binding affinity, are crucial regarding the mechanism and the ability of the complex to mediate CO2 reduction, systematic variations of the metal within an identical, redox-innocent ligand backbone remain scarcely investigated. In this study,[4] we report on the synthesis, structures, and electrochemistry of 3d transition metal complexes (M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I) and Zn(II)) coordinated by a new redox-innocent PNP pincer ligand system (Figure 1). The ligand combines a large steric demand, partially shielding the metal center, with a high degree of aromaticity. With these features, the ligand only marginally interferes in the redox processes but still takes sufficient π-backbonding to stabilize low-valent metals. The coordination compounds derived thereof were analyzed by spectroscopic methods (e.g., NMR, EPR, and Mössbauer) as well as crystal X-Ray diffraction to determine their spatial and electronic structures. These findings served as the starting point for a further exploration of the electrochemical reduction of the complexes via cyclic voltammetry. While the Mn, Cu, and Zn complexes solely exhibit ligand reduction or decomposition processes, the distinct electrochemical waves found for the Fe, Co and Ni coordination compounds could be assigned to metal-centered reduction events from M(II) down to M(0). For both cobalt and nickel, the reductions appear to be accompanied by their chloride ligands being lost or exchanged with the acetonitrile solvent, a fact that we currently investigate[5] as these phenomena remain misjudged across many (CO2) electroreduction catalysts. In contrast to Co and Ni, Fe undergoes a more complex reduction pattern likely yielding a dimeric species. Despite their unequal reduction pathways, the evolution of the voltammograms of the complexes under CO2 atmosphere indicates an interaction of each of the complexes with the substrate molecule in their reduced metal states. The d10 Ni(0) species putatively forms an Aresta-type Ni-η2-CO2 complex, in which the electron transfer to the substrate through backbonding is insufficient to enable electrocatalytic activity. At contrast, CO2 binding at the d9 Co(0) intermediate likely leads to additional electron uptake and formation of a formal Co(I) metallacarboxylate complex able to promote turnover (Figure 2). Eventually, we related our findings to the few literature precedents that incorporate redox-innocent ligands. This assessment shows that beneficial characteristics in the electrochemical activation of CO2 by complexes based on redox-innocent ligands are an unsaturated coordination sphere (coordination number = 4 or 5) as well as a d7 to d9 configuration in the reduced oxidation state (+I or 0). The on-purpose design of complexes that simultaneously meet these three characteristics hence provides a promising strategy for catalyst development. In particular, dynamic structural and electronic changes under electrochemical conditions, such as the exchange of chlorido ligands with acetonitrile as faced in this study, must be controlled to ensure the primary operation of the metal centers in the desired catalytic manifold. References [1] P. De Luna, C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo, E. H. Sargent, Science 2019, 364, eaav3506. [2] R. Francke, B. Schille, M. Roemelt, Chem. Rev. 2018, 118, 4631. [3] N. W. Kinzel, C. Werlé, W. Leitner, Angew. Chem. Int. Ed. 2021, 60, 11628. [4] N. W. Kinzel,D. Demirbas, E. Bill, T. Weyhermüller, C. Werlé,N. Kaeffer, W. Leitner, Inorg. Chem. 2021, doi: 10.1021/acs.inorgchem.1c02909. [5] N. W. Kinzel, N. Kaeffer, W. Leitner in preparation. Figure 1