The structural, spectroscopic, and electrochemical properties of a series of six-coordinate iron(II) complexes of the types [(MTC)Fe(NCMe)2]2+, [(MTC)Fe(CO)(NCMe)]2+, and [(MTC)Fe(CO)2]2+, where MTC represents four different macrocyclic tetra(NHC) ligand scaffolds (NHC = N-heterocyclic carbene), are compared to better understand the effects of ligand variations on electronic structures. The set of parameters evaluated includes Fe-C bond lengths, IR stretching frequencies of the axial ligands, zero-field 57Fe M & ouml;ssbauer data, 13C NMR chemical shifts and FeIII/FeII redox potentials. Several new complexes, as well as some known complexes for which structural information was so far lacking, have been characterized by X-ray crystallography. It is found that for most spectroscopic properties there is a large effect of changing the MTC ring size, but much less of an influence of the peripheral NHC ligand substituents. Additionally, a set of complexes [(MTC)Fe(CO)(L/X)]n+ are included in this study where the ligand trans to the axial CO is either a neutral (L = NCMe, CO) or anionic (X- = Cl-, OCP-) donor. Understanding the trends in molecular and electronic structures for the present series of complexes with ligand variations in both equatorial and axial positions will help to rationalize reactivity trends observed for this versatile MTC/iron(II) platform.

We report an investigation into the hydrogen exchange mechanism in a dinickel(II) dihydride complex with C 2v symmetry. To obtain atomic level information on the low-concentration intermediate species, we use parahydrogen in combination with nuclear magnetic resonance (NMR). Unexpectedly, despite the chemical equivalence of the two hydride sites imposed by the mirror symmetry of the model, we observe spontaneous conversion of the nuclear singlet order into in-phase longitudinal magnetization-something that normally requires sophisticated pulse sequences. The magnetization is retained upon reductive elimination of dihydrogen from the dihydride species and observed as an enhanced hydrogen NMR signal. This is explained by a chain of nuclear relaxation interference processes that do not require asymmetric intermediates; we confirm this in two ways: using analytical relaxation theory and with brute-force numerical simulations.

Correction for “Bis(amidophenolate)-supported pnictoranides: Lewis acid-induced electromerism in a bismuth complex” by Simon B. H. Karnbrock et al. , Chem. Sci. , 2025, 16 , 14178–14185, https://doi.org/10.1039/D5SC03374H.

The use of ammonia as a potential zero-carbon fuel has attracted high interest in the quest for novel sustainable energy carriers. This requires a deep understanding of the principles of the ammonia oxidation reaction (AOR) to guide the development of efficient catalysts. Here, we report the complex [RuII(TPA)(NH3)2](PF6)2 (1(PF 6 ) 2 ; TPA = tris(2-pyridylmethyl)amine), and we demonstrate that its 1e- oxidation in MeCN is partially ligand centered and finally produces [RuII(TPA)(MeCN)(NH3)](PF6)2 (2(PF 6 ) 2 ). Deprotonation of 1 3+ (pK A = 12.6) is derived as the rate-determining step, and bimolecular N-N coupling of the key intermediate [Ru(TPA)(NH3)(NH2)]2+ (4 2+ ) is proposed based on 15N labeling experiments and electrochemical studies. 1(PF 6 ) 2 is then investigated as a homogeneous AOR electrocatalyst, and the formation of N2 and H2 is confirmed during electrolysis at a moderate operating potential. DFT computations established a complete PCET map to evaluate potential reaction pathways and, in combination with spectroscopic and electrochemical studies as well as product analysis using 15N labeling, allowed us to propose a catalytic cycle where N-N bond formation occurs via intermolecular coupling (I2M) at an early amido stage (4 2+ ) of the PCET sequence. Circumventing higher metal oxidation states gives rise to a relatively low AOR overpotential of 0.96 V for 1 2+ . Complex 2 2+ is identified as an important intermediate that is catalytically competent and initiates a second cycle, while [RuII(TPA)(MeCN)2]2+ (XI 2+ ) represents an off-cycle product leading to gradual loss of catalytic activity.

The reaction of organoazides with a linear chromium(I) silylamide leads to the observation of rare monomeric and dimeric imido chromium complexes, as well as labile azide adducts. Further, the first trigonal chromium oxide complex is reported, obtained from the reaction of a linear chromium(II) silylamide with either acylazide or Me 3 NO.

Iron complexes of tetradentate macrocyclic ligands containing N-heterocyclic carbene (NHC) donors have been referred to as organometallic heme analogues, but they usually lack the redox noninnocence under oxidizing conditions that is characteristic of porphyrins. Here we report a novel NHC/N-donor hybrid macrocyclic ligand containing two trans NHC moieties, a pyridine and a redox active carbazolide fragment. Its FeII, FeIII and formal FeIV complexes have been isolated and comprehensively characterized, where UV/vis and 57Fe Mössbauer spectroscopies, SQUID magnetometry and density functional theory (DFT) calculations reveal that the latter are best described as FeIII systems antiferromagnetically coupled to a carbazolide-based organic π-radical. Two different redox series are obtained depending on the axial ligands: nitriles such as MeCN give low-spin (LS) configurations of the metal ion, while in case of weakly coordinating solvents and triflate anions the iron adopts an intermediate-spin (IS) configuration; MeCN binding constants have been determined. As in other heme analogues with NHC-based macrocycles, the strong equatorial σ-donor character raises the energy of the Fe(dx2-y2) orbital, making high-spin (HS) iron species inaccessible. The combined features of equatorial ligand redox noninnocence, restriction to LS/IS surfaces and tunability via the axial coligands makes this a promising platform for bioinspired reactivity such as the generation of reactive Fe/Ox intermediates.

Artificial heme enzymes offer unique opportunities to disentangle structure-function relationships and design novel biocatalysts. Mimochromes (MCs) are artificial, small-sized heme proteins able to reproduce the structural and functional features of natural heme enzymes. Here, we report the spectroscopic and structural investigation of a Mimochrome VI (MC6) analogue, Lys9DabMC6*a, for which we were previously able to isolate two distinct regioisomers. Mössbauer and EPR spectroscopy revealed distinct pH-dependent high-spin and quantum mixed-spin states in the Fe(III) complexes for both regioisomers. A detailed structural characterization was performed by NMR spectroscopy on the diamagnetic Co(III) analogues, providing high-resolution structures of the two isolated regioisomers. Both species show the intended helix-heme-helix sandwich fold but differ in interhelical orientation, axial histidine positioning, and second-sphere interactions, despite having the same peptide composition. Based on the reported electronic properties and structural features, we retrospectively attempt to elucidate the differences in substrate affinity and turnover frequency between the two regioisomers. Our results provide useful insights for the rational evolution of heme-based artificial minienzymes and highlight the minimal determinants required to achieve catalytic diversity.

Late transition metal hydroxides are important intermediates in many catalytic processes. In [Ni,Fe] carbon monoxide dehydrogenase enzymes (CODHs), which catalyze the reversible CO/CO2 interconversion, CO insertion in the Ni-(μ-OH) bond of a Ni/Fe-bridging hydroxide is the proposed C-O bond-forming step during CO oxidation. Here, we report that CO reacts with the dinickel(II) complex LNi2(μ-OH) (L3- is a pyrazolato-centered ligand with two {N3} compartments), finally leading to HLNiII(CO); this is coupled to the formation of CO2 and Ni(CO)4, which is the driving force of the overall reaction. Mechanistic details have been investigated by 13CO labeling experiments and DFT calculations, and the product derivative HLNiII(CNBn) has been crystallographically characterized. The findings demonstrate that a nickel(II)-bound μ-OH can serve as a nucleophile toward CO and promote its oxidation to CO2, which is suggested to proceed via a metal-bridging O-protonated carbonite intermediate, emulating a key step of [Ni,Fe]-CODH reactivity. For CO2 release, in the present system, one of the nickel ions serves as a terminal electron sink and the corresponding ligand {N3} site of L3- as a proton acceptor.

Oxidoiron(IV) species are crucial intermediates for the functionalization of C–H bonds by biological oxidases and oxygenases, and understanding the relations between ligation, electronic structure and reactivity of the FeIV=O unit is key to the rational design of bioinspired catalysts. Here, a series of FeIV=O complexes based on a macrocyclic tetracarbene ligand and with different anionic trans-axial ligands (Lax = CF3COO−, tBuS−, AdaS−) has been comprehensively characterized, including X-ray crystallography in two cases as well as zero-field and applied-field 57Fe Mössbauer spectroscopy, magnetic susceptibility measurements, magnetic circular dichroism (MCD), and helium tagging infrared photodissociation (IRPD). This establishes a detailed picture of the electronic structures of these ferryl complexes featuring a well separated S = 1 ground state, and the influence of the trans-ligand. The confinement to triplet-only pathways in C−H bond activations and comparison with the parent FeIV=O complex (Lax = MeCN) allows to uncover trends in hydrogen atom abstraction (HAA) ability without perturbations from different extents of two-state reactivity (TSR). Kinetic studies on HAA for C‒H substrates with BDEs up to 78 kcal mol−1 show relatively minor effects of the axial ligands, with rates following an electrophilc trend in the order Lax= MeCN > CF3COO–  RS–. This contrasts the anti-electrophilic trend found for a related series of tetramethylcyclam (TMC) based FeIV=O complexes for which TSR is dominant. For the present systems, temperature-dependent KIEs in combination with computations at the DFT and ab initio level of theories suggest a semiclassical model with modest tunneling contributions that are most pronounced in case of a strongly donating trans-thiolate. This study thus provides a combined experimental and computational basis for understanding intrinsic axial ligand effects and the effect of a trans-axial thiolate in S = 1 ferryl intermediates.

Nitrogenase accumulates reducing equivalents in hydrides and couples H2 elimination to the reductive binding of N2 at a diiron edge of its FeMo cofactor (FeMoco). Here, we describe that oxidation of a pyrazolato-based dinickel(II) dihydride complex K[L(Ni-H)2] (1K), either electrochemically or chemically using H+ or ferrocenium, triggers H2 elimination and binding of N2 in a constrained and extremely bent bridging mode in [LNi2(1,2-N2)] (3N2). Spectroscopic and computational evidence indicate that the electronic structure of 3N2 is best described as NiII–(N2•–)–NiII, with a rare 1e‒ reduced and significantly activated N2 substrate (n(NN) = 1894 cm–1). 3N2 is also formed upon 1e‒ oxidation of K[LNiI2] (2K) under N2. This is an unusual and counterintuitive scenario where the oxidation of a dinickel(II) dihydride, or of a dinickel(I) complex, induces the reductive activation of N2. Detailed (spectro)electrochemical studies confirm that N2 binding by the {LNi2} platform only occurs in the regime of the mixed valent NiIINiI species, while both oxidation and reduction induce the release of N2 from 3N2; the latter represents a redox-induced electron transfer (RIET) process where metal re-duction leads to N2•– oxidation due to intramolecular back electron transfer. These findings offer new perspectives for understanding the multi-e‒/H+ scenarios of N2 fixation via hydride intermediates akin to the FeMoco function, and for the development of synthetic platforms that avoid strongly reducing conditions for N2 activation.

Iron enzymes are ubiquitous in nature. In particular, enzymes with iron-oxygen cofactors as active sites perform a vast variety of reactions. Both iron(III)-hydroxido and iron(IV)-oxido species have been observed to play a catalytically active role. In order to complement biochemical investigations, a large variety of synthetic compounds using these motifs were synthesized in past decades to study and understand their inherent reactivity. One such synthetic model complex is [FeIV(O)(Py5Me2)]2+, (Py5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, henceforth labeled L1), which was used as a model complex for epigenetically relevant iron(II)/alpha-ketoglutarate-dependent ten-eleven translocation 5-methylcytosine dioxygenases (TET). Additionally, [FeIII(OH)(Py5(OH)2)]2+ (Py5(OH)2 = pyridine-2,6-diylbis [di(pyridin-2-yl)methanol, henceforth labeled L2) was tested as a lipoxygenase model. We have complemented the available complexes of these related pentapyridyl complexes to include all oxidation states II-IV and performed detailed spectroscopic and spectrometric investigations. We found that iron(II) and iron(IV)-oxido compounds (cross-)comproportionate readily to form iron(III)-hydroxido species, which represents a major side reaction for model complex investigations. We also investigated the oxidative reactivity of a new iron(IV)-oxido complex.

Heavier transition metal carbyne analogs hold significant potential for cooperative activation of small molecules. However, complexes containing more than one heavier tetrylidyne ligand RE (E = Si, Ge, Sn, Pb) are rare due to the high oligomerization tendency of RE ligands. In this study, we describe the complex [Fe(SnAr')2] (1; Ar' = 2,6-Dipp2-C6H3, Dipp = 2,6- i Pr2-C6H3), which features adjacent Fe-Sn single and double bonds. Complex 1 exhibits versatile reactivity with transition metal and main group compounds. Treatment of complex 1 with Ni(COD)2 (COD = 1,5-cyclooctadiene) yields the tetranuclear complex [Fe(mu-SnAr')2Ni] (2), characterized by an unusual "push-pull" interaction between nickel(0) and the two coordinating Sn atoms, as revealed by quantum chemical studies. The reaction of complex 1 with AlBr3 results in Al-Br bond cleavage and Ar' migration to aluminum. CH3I adds oxidatively to the Sn atom that is singly bonded to Fe, while PMe3 coordinates to Fe, inducing reversible cleavage of the Fe=Sn double bond. In addition, complex 1 activates inorganic molecules. CO2 undergoes disproportionation to produce a carbonate-bridged Ar'Sn(mu-OCO2)SnAr' ligand, whereas CS2 is reductively coupled to form an ethylene tetrathiolate ligand ([C2S4]4-). The reaction with white phosphorus (P4) generates an unusual Ar'P4Sn2Ar' ligand. This multifaceted reactivity illustrates the behavior of the Fe and Sn sites in complex 1, suggesting that complexes of this type are promising reagents for small molecule activation.

Photoelectrochemical (PEC) water splitting offers a direct path to generate hydrogen from water and sunlight using semiconductor (SC) materials. The hydrogen can be stored and utilized as sustainable fuel to satisfy the energy demand in an environmental friendly economy.[1] Molecular-oxide hybrid devices provide great opportunities to combine high catalytic activity regarding photocatalytic water splitting with long-term stability.[2,3] The investigation of the hybrids requires a well-known oxide absorber, that is very photostable, enable a fast charge transfer to the catalyst and has a well-suited electronic bandstructure, where photo-excited hole states can capture the electrons from electrocatalytic water oxidation. One promising candidate is the oxide bismuth vanadate (BiVO 4 ) that shows high photocatalytic activity for PEC water splitting reaction.[4] We show a systematic study of the time-resolved photocatalytic activity of BiVO 4 thin film absorbers with a highly functionalized surface deposited on conducting Nb-doped SrTiO 3 substrates. The BiVO 4 films were photo-electrochemically investigated at neutral pH with an aqueous phosphate buffered electrolyte via cyclovoltammetry. Rotating ring disk electrode (RRDE) experiments in the dark and under illumination (LED 1.5 AM spectrum) allowed distinguishing between the oxygen evolution reaction and other processes such as photo-induced capacitive changes. The experiments aim to understand the electron charge transfer from water molecules into the BiVO 4 surface under illumination and different applied bias. First promising steps to extend such studies to hybrid systems are pursued, where high-efficient Ru-based molecular catalysts well-designed for OER are anchored on different SC surfaces. [1] E. L. Miller, Energy Environ. Sci. 2015, 8, 2809-2810. [2] R. Matheu et al., Nature Rev. Chem. 2019, 3, 331; [3] J. Odrobina, J. Scholz et al, ACS Catal. 7, 2017, 6235. [4] F. Abdi et al, J. Phys. D: Appl. Phys. 2017, 50, 193002.

Cooperative intermolecular interactions, usually observed in solid state, can confer useful properties to stimuli-responsive spin transition materials. Here, we demonstrate for the first time intramolecular cooperativity between the two Fe-Co subunits of a molecular cyanido-bridged square Fe2Co2 Prussian blue analogue (PBA) in solution, which upon single photon excitation sequentially undergo electron transfer coupled spin transition (ETCST) from a diamagnetic low-spin (LS) to a paramagnetic high-spin (HS) state. Ultrafast UV-vis and IR pump-probe spectroscopies show that irradiation into the IVCT band of the LS state induces electron transfer within one Fe-Co subunit followed by fast (360 fs) SCO to an intermediate HS/LS species and a further ETCST event in the other Fe-Co subunit then occurs on a ns timescale. Kinetic analysis reveals that this cooperative switching of the two Fe-Co subunits is caused by two coupled equilibria favouring the second ETCST step, and the free energy landscape for the square Fe2Co2 system is determined experimentally.

Isostructural Dy(III) and Er(III) complexes [L1 2Ln(H2O)5][I]3 ⋅ L1 2 ⋅ (CH2Cl2) (Ln=Dy (1), Er (3)) and [L2 2Ln(H2O)5][I]3 ⋅ L2 2 ⋅ (CH2Cl2)2 (Ln=Dy (2), Er (4)), with distorted pentagonal bipyramidal geometry (D5h) around the central metal were synthesized by utilizing two bulky phosphonamide ligands, adamantyl phosphonamide, (Ad)P(O)(NHiPr)2 (L1) and carbazolyl phosphoramide (Cz)P(O)(NHiPr)2 (L2). The resultant complexes were investigated for their magnetic properties in order to elucidate the impact of modification of the coordinating P-O bond environment either by increasing steric bulk and/or introduction of a third P-N bond at the central phosphorus atom. Magnetic studies revealed substantial energy barriers (Ueff) of 640 K and 491 K for Dy compounds 1 and 2, respectively, rendering them as some of the best-performing air-stable SIMs amongst the class of SIMs with D5h symmetry. Compounds 1 and 2 exhibit magnetization blocking (TB) at 6.5 K and 6 K, respectively, at a sweep rate of 20 Oe/s. Compound 1 benefits from increased lattice intermetallic distances due to bulky adamantyl substituent, but exhibits a significant deviation from linear axial (P)O-Dy-O(P) geometry (173.7(1)°). In addition to the deviation from linearity, the incorporation of a bulky adamantane (or carbazole) ligand in complex 1 (or 2) was found to result in relatively strong Dy…H-C agostic interactions, with distances of 3.698 Å (3.376 Å). These interactions are expected to induce transverse anisotropy. Ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations offer valuable insights into the dynamics of magnetic relaxation and the impact of axial bulkiness on the anisotropy of D5h systems. Beyond highlighting the crucial role of crystal field and symmetry in achieving high-temperature SIMs, this study also explores how the secondary coordination sphere can be engineered to create novel SIMs.

Ru complexes are widely studied in photodynamic therapy. The type I mechanism of action is based on a photoinduced electron transfer from the complex to O2 and needs an electron donor to be catalytic. Little is known about electron donors among physiologically relevant compounds. Hence, we investigated the oxidation of ascorbate, NADH, cysteine, and glutathione with the canonical [Ru(bpy)3](PF6)2 as well as a derivative with a peripheral disulphide unit, [Ru(S-Sbpy)(bpy)2](PF6)2. The established reactivity order is ascorbate > NADH ∼ cysteine > glutathione.

Coordination of 10-Pn-4 bis(amidophenolate)-supported pnictoranide anions to [CpFe(CO) 2 ] + delivers 10-Pn-5 complexes for P, As and Sb, while for Bi a 12-Bi-5 species is isolated.

A trigonal arylimido iron complex is reported, which is found in an intermediate spin state. The iron bound imido unit is electronically flexible and acts as a nucleophile, reductant, or H atom abstractor. The latter is used for catalytic intramolecular C-H bond amination.

Nitrogenase accumulates reducing equivalents in hydrides and couples H2 elimination to the reductive binding of N2 at a di-iron edge of its FeMo cofactor (FeMoco). Here, we describe that oxidation of a pyrazolato-based dinickel(II) dihydride complex K[L(Ni-H)2] (1K), either electrochemically or chemically using H+ or ferrocenium, triggers H2 elimination and binding of N2 in a constrained and extremely bent bridging mode in [LNi2(μ1,2-N2)] (3N2). Spectroscopic and computational evidence indicate that the electronic structure of 3N2 is best described as NiII-(N2•-)-NiII, with a rare 1e- reduced and significantly activated N2 substrate (ṽ̃NN = 1894 cm-1). 3N2 is also formed upon 1e- oxidation of K[LNi2I] (2K) under N2. This is an unusual and counterintuitive scenario where the oxidation of a dinickel(II) dihydride, or of a dinickel(I) complex, induces the reductive activation of N2. Detailed (spectro)electrochemical studies and DFT calculations confirm that N2 binding by the {LNi2} platform occurs only in the regime of the mixed-valent NiIINiI species, while both oxidation and reduction induce the release of N2 from 3N2; the latter represents a redox-induced electron transfer (RIET) process where metal reduction leads to N2•- oxidation due to intramolecular back electron transfer. These findings offer new perspectives for understanding the multi-e-/H+ scenarios of N2 fixation via hydride intermediates inspired by the FeMoco function, and for the development of synthetic platforms that avoid strongly reducing conditions for N2 activation.

The catalytic oxidation of ammonia is relevant in the context of zero-carbon energy scenarios, for instance, in fuels cells or for hydrogen storage. Here, we report the diruthenium complex [LRu2(py)4(mu 1,2-N2H4)](PF6)3 (1(PF6)3; HL = 3,5-bis(bipyridyl)pyrazole) and demonstrate its ability to catalyze the oxidation of ammonia at a low overpotential of 0.76 V. Notably, complex 1(PF6)3 is a molecular catalyst with a highly preorganized dinuclear substrate binding pocket to enforce the close proximity of two ammonia molecules and induce metal-metal cooperativity. The formation of N2 and H2 from ammonia during constant potential electrolysis was analyzed using gas chromatography as well as mass spectrometry in combination with 15N-labeling. Detailed electrochemical and spectroscopic studies, including voltammetry-coupled ESI-MS and gas phase photodissociation spectroscopy, provided mechanistic insights and identified [LRu2(py)4(N2H2)]3+ (3 3+ ) as a key intermediate, and DFT computations were used to evaluate possible reaction pathways as well as the electronic structures of relevant species. The combined experimental and computational findings allowed to propose a catalytic cycle where N-N coupling occurs at an early stage of the multi-PCET sequence in species [LRu2(py)4(NH2)2]3+, giving 1 3+ and then 3 3+ en route to N2. Furthermore, [LRu2(py)4(MeCN)2]3+ (2 3+ ) was identified as an off-cycle product leading to gradual catalyst deactivation.