Living systems carry out the reduction of N2 to ammonia (NH3) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber–Bosch process hydrogenates N2 but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH3 on addition of H2 to N2 have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp′Fe)3(μ3-N)2] (in which Cp′ = η5-1,2,4-(Me3C)3C5H2), which is prepared by reduction of [Cp′Fe(μ-I)]2 under an N2 atmosphere and comprises three iron centres bridged by two μ3-nitrido ligands. In solution, this complex reacts with H2 at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH3. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp′Fe)3(μ3-NH)2], by addition of H2 (10 bar) through an unusual solid–gas, single-crystal-to-single-crystal transformation. In solution, [(Cp′Fe)3(μ3-NH)2] further reacts with H2 or H+ to form NH3. The reduction of N2 to NH3 is mediated in living systems by the enzyme nitrogenase and in the chemical industry by the Haber–Bosch process; both systems rely on iron-based catalysis. Now, a molecular tri(iron)bis(nitrido) complex, prepared by reduction of a bis(iron)bis(iodo) precursor under an N2 atmosphere, has been isolated and shown to promote the formation of NH3 from H2.

In the marine bacterium, Dinoroseobacter shibae the transcription factor rhizobial iron regulator A (RirA) is involved in the adaptation to iron-limited growth conditions. In vitro iron and sulfide content determinations in combination with UV/Vis and electron paramagnetic resonance (EPR) spectroscopic analyses using anaerobically purified, recombinant RirA protein suggested a [3Fe–4S]1+ cluster as a cofactor. In vivo Mössbauer spectroscopy also corroborated the presence of a [3Fe–4S]1+ cluster in RirA. Moreover, the cluster was found to be redox stable. Three out of four highly conserved cysteine residues of RirA (Cys 91, Cys 99, Cys 105) were found essential for the [3Fe–4S]1+ cluster coordination. The dimeric structure of the RirA protein was independent of the presence of the [3Fe–4S]1+ cluster. Electro mobility shift assays demonstrated the essential role of an intact [3Fe–4S]1+ cluster for promoter binding by RirA. The DNA binding site was identified by DNase I footprinting. Mutagenesis studies in combination with DNA binding assays confirmed the promoter binding site as 3′-TTAAN10AATT-5′. This work describes a novel mechanism for the direct sensing of cellular iron levels in bacteria by an iron-responsive transcriptional regulator using the integrity of a redox-inactive [3Fe–4S]1+ cluster, and further contributes to the general understanding of iron regulation in marine bacteria.

In this work, two iron(II) coordination compounds with a N2O2 coordinating Schiff base-like ligand bearing a redox active tetrathiafulvalene (TTF) unit and pyridine or trans-1,2-bis(4-pyridylethylene) as an axial ligand are synthesized. Crystals suitable for single X-ray structure analysis were obtained for the new ligand. The complexes were characterized by magnetic susceptibility measurements, T-dependent UV-vis spectroscopy, and cyclic voltammetry. Both complexes display spin transition behavior below room temperature with T-1/2 values of 146 and 156 K. The mononuclear iron(II) complex [(FeL)-L-TTF(py)(2)] is relatively stable up to 400 K compared to similar complexes, showing no loss of axial ligands upon heating. Temperature dependent Mossbauer spectroscopy was conducted for the coordination polymer {[(FeL)-L-TTF(bpee)]}(n) to get more information regarding the origin of the stepwise spin crossover (SCO) behavior observed in the magnetic measurements. The change of the spin state is accompanied by a change of the optical properties, which can be monitored by VT-UV-vis spectroscopy for the mononuclear complex and has been analyzed in theoretical studies. The redox behavior of the iron(II) complexes reveals three reversible redox steps which are located at the iron center and at the TTF unit of the ligand. Oxidation of the TTF unit induces characteristic changes in the UV-vis spectrum that can be followed by spectroelectrochemical UV-vis spectroscopy. Addressing the potential of the iron-centered redox process results in similar changes in the UV-vis spectrum, which indicates an electronic coupling of the redox active unit with the metal center under certain circumstances.

Living systems carry out the reduction of N 2 to ammonia (NH 3 ) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber–Bosch process hydrogenates N 2 but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH 3 on addition of H 2 to N 2 have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp′Fe) 3 (μ 3 -N) 2 ] (in which Cp′ = η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ), which is prepared by reduction of [Cp′Fe(μ-I)] 2 under an N 2 atmosphere and comprises three iron centres bridged by two μ 3 -nitrido ligands. In solution, this complex reacts with H 2 at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH 3 . In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp′Fe) 3 (μ 3 -NH) 2 ], by addition of H 2 (10 bar) through an unusual solid–gas, single-crystal-to-single-crystal transformation. In solution, [(Cp′Fe) 3 (μ 3 -NH) 2 ] further reacts with H 2 or H + to form NH 3 .

Chromyl(V) complexes of eleven different meso-aryl- and [Formula: see text]-alkylcorrole ligands have been prepared and characterized by analytical, spectroscopical, electrochemical and structural means. All seven new complexes show the expected optical spectra of chromyl(V) corroles with the Soret band situated slightly above (meso-arylcorroles) or below ([Formula: see text]-alkylcorrole) 400 nm, and with an isotropic room temperature EPR signal for the [Formula: see text] ion Cr(V) at about [Formula: see text]. Cyclic voltammetry evidences two or three quasi-reversible one-electron redox steps, which are characterized as two ligand-centered processes and one metal-centered process based on spectroelectrochemical measurements and a Hammett analysis. Chemical reduction to a chromyl(IV) corrolate was successfully performed using NaHg and NEt3. Citric acid, however, produces a different reduction product, for which an isocorrole structure is proposed. Six chromyl(V) corroles could be analysed by seven single crystal diffraction analyses. The obtained molecular data prove the presence of remarkably similar CrN4O coordination units independent of the corrole substitution pattern, and of macrocyclic conformations which can be understood as comprised mainly of a doming mode, a more or less pronounced saddling mode, and one out of two different and dominating ruffling modes.

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)]2 (1-M, M = Fe, Co; Cp' = η5-1,2,4-tri-tert-butylcyclopentadienyl) with [ImDippNLi]2 (ImDippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NImDipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe3)2}] (3-Co) and ImDippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by 57Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Arsenic monophosphide (AsP) species supported by two different N-heterocyclic carbenes were prepared by reaction of (IDipp)PSiMe3 (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with (IMes)AsCl3 (2) (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) to afford the dichloride [(IMes)As(Cl)P(IDipp)]Cl (3), which upon reduction with KC8 furnished heteroleptic [(IMes)AsP(IDipp)] (4). The corresponding mono- and dications [(IMes)AsP(IDipp)][PF6], [5]PF6, and [(IMes)AsP(IDipp)][GaCl4](2,) [6][GaCl4](2), respectively, were prepared by one-electron oxidation of 4 with ferrocenium hexafluorophosphate, [Fc]PF6, or by chloride abstraction from 3 with two equivalents of GaCl3, respectively. Compounds 4-6 represent rare examples of heterodiatiomic interpnictogen compounds, and X-ray crystal structure determinations together with density functional theory (DFT) calculations reveal a consecutive shortening of the As-P bond lengths and increasing bond order, in agreement with the presence of an arsenic-phosphorus single bond in 4 and a double bond in 6(2+). The EPR signal of the cationic radical [5](+.) indicates a symmetric spin distribution on the AsP moiety through strong hyperfine coupling with the As-75 and P-31 nuclei.

The reaction of the phosphorus and arsenic dihalides (WCA-IDipp)ECl2 (E = P, As) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA = B(C6F5)(3), IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with (IDipp)PSiMe3 afforded the monochlorides (WCA-IDipp)E(Cl)P(IDipp), which can be transformed into the cationic dipnictenes [(WCA-IDipp)EP(IDipp)][GaCl4] or neutral radicals [(WCA-IDipp)EP(IDipp)] by treatment with GaCl3 or reduction with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine.

The half-sandwich complex [Cp'Fe(μ-I)]2 (1; Cp' = η5-1,2,4-(Me3C)3C5H2) is cleaved when heated in toluene to form a cation-anion pair [{Cp'Fe(η6-toluene)}+{Cp'FeI2}-] (2), in which the two Fe(ii) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with C2H4I2, the thermally stable 15VE species [Cp'FeI2] (3) can be isolated, in which the Fe(iii) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CH2SiMe3)2 to form the unprecedented Fe(iii) (S = 3/2) bis(alkyl) complex [Cp'Fe(CH2SiMe3)2] (4). The respective spin states of complexes 2-4 are confirmed by single-crystal X-ray crystallography, zero-field 57Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(ii) alkyl species [Cp'FeCH(SiMe3)2], which resists the reaction with H2 as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H2 (8 bar) in cyclohexane to yield iron hydrides [{Cp'Fe}2(μ-H)3] (5) and [Cp'Fe(μ-H)2]2 (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp'Fe(η6-C6H6)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp'Fe)2(μ2-η5:η5-C12H12)] (7). Further evidence for the intermediacy of [Cp'Fe(η6-C6H6)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF6 proceeded via C-C bond cleavage instead of metal oxidation to form [Cp'Fe(C6H6)][SbF6] (8).

A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR2] is presented. In these complexes tmeda is readily replaced by an NHC or a bidentate phosphine ligand to form 3- and 4-coordinate compounds, respectively. [(ItBu)Co(CH2SiMe3)2] (ItBu = 1,3-di-tert-butylimidazolin-2-ylidene) serves as an efficient, homogeneous olefin hydrogenation pre-catalyst and allows the preparation of the novel cobalt bis(alkyne) complex [(ItBu)Co(η2-PhCCPh)2].

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Chemically inert oxidometal(V) corrols of molybdenum and rhenium undergo clean ligand-exchange reactions upon the action of SiCl4 . The resulting dichlorido complexes show trigonal prismatic coordination of the metal ion with the chlorine atoms residing in a cis configuration, and were studied by optical and resonance spectroscopy as well as DFT calculations. In situ reactivity studies with carbon nucleophiles indicate high reactivity for chlorine replacement. Treatment with sodium cyclopentadienide paves the way to robust molybdenum corrolocene half-sandwich complexes. These organometallic compounds are the first corrole species that stabilize an air-stable and diamagnetic low spin d2 -MoIV center. Structural, spectroelectrochemical, and chemical investigations prove a reversible MoIV /MoV redox couple close to the Fc/Fc+ potential for these systems. The high stability of the compounds in both redox states calls for future applications in catalysis and as redox switch.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Heteroleptic N-heterocyclic dicarbene-diphosphorus species were prepared by reaction of the carbene-phosphinidene adduct (IPr)PSiMe3 (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with the carbene-phosphorus trichloride adduct (IMes)PCl3 (2, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), which furnished the dichloride [(IPr)PPCl(IMes)]Cl (3). Reduction of 3 with potassium graphite (KC8) afforded [(IPr)PP(IMes)] (4). The corresponding radical cation [(IPr)PP(IMes)]˙+ (5˙+) is isolated as [5]PF6 by reaction of 4 with ferrocenium hexafluorophosphate, whereas complexes containing the corresponding dication [(IPr)PP(IMes)]2+ (62+) can be isolated as the gallate and borate salts [6](GaCl4)2 and [6](BArF)2 by chloride abstraction from 3 with GaCl3 or sodium tetrakis[bis(3,5-trifluoromethyl)phenyl]borate (NaBArF), respectively. The asymmetric set of N-heterocyclic carbene ligands allows to establish 1JPP coupling constants of 249 Hz for 4 and 543 Hz for [6](GaCl4)2. Based on X-ray diffraction analyses, the molecular structures of 4, 5˙+ and 62+ reveal a consecutive shortening of the P-P bond lengths, in agreement with the presence of a phosphorus-phosphorus single bond in 4 and a double bond in 62+, which is best described as a dicationic diphosphene according to density functional theory (DFT) calculations.

Reversible N2 coordination to [Cp′Fe(IiPr2Me2)] induces a spin-state change.