Structural Insights into the Nature of Fe⁰ and Fe^I Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents

Mechanistic studies of the reduction of Fe^III and Fe^II salts by aryl Grignard reagents in toluene/tetrahydrofuran mixtures in the absence of a supporting ligand, as well as structural insights regarding the nature of the low-valent iron species obtained at the end of this reduction process, are reported. It is shown that several reduction pathways can be followed, depending on the starting iron precursor. We demonstrate, moreover, that these pathways lead to a mixture of Fe⁰ and Fe^I complexes regardless of the nature of the precursor. Mössbauer and ¹H NMR spectroscopies suggest that diamagnetic 16-electron bisarene complexes such as (η⁴-C₆H₅Me)₂Fe⁰ can be formed as major species (85% of the overall iron quantity). The formation of a η⁶-arene-ligated low-spin Fe^I complex as a minor species (accounting for ca. 15% of the overall iron quantity) is attested by Mössbauer spectroscopy, as well as by continuous-wave electron paramagnetic resonance (EPR) and pulsed-EPR (HYSCORE) spectroscopies. The nature of the Fe^I coordination sphere is discussed by means of isotopic labeling experiments and density functional theory calculations. It is shown that the most likely low-spin Fe^I candidate obtained in these systems is a diphenylarene-stabilized species [(η⁶-C₆H₅Me)­Fe^IPh₂]⁻ exhibiting an idealized <i>C</i>_2<i>v</i> topology. This enlightens the nature of the lowest valence states accommodated by iron during the reduction of Fe^III and Fe^II salts by aryl Grignard reagents in the absence of any additional coligand, which so far remained rather unknown. The reactivity of these low-valent Fe^I and Fe⁰ complexes in aryl–heteroaryl Kumada cross-coupling conditions has also been investigated, and it is shown that the zerovalent Fe⁰ species can be used efficiently as a precursor in this reaction, whereas the Fe^I oxidation state does not exhibit any reactivity

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe–Fe]-Hydrogenases

A novel reaction pathway to synthesize the linear trinuclear clusters [Fe₃(CO)₅(κ²-diphosphine)­(μ-dithiolate)₂] via the direct reaction of the dinuclear hexacarbonyl precursor [Fe₂(CO)₆(μ-dithiolate)] with the mononuclear species [Fe­(CO)₂(κ²-diphosphine)­(κ²-dithiolate)] has been developed with diphosphine (dppe) and dithiolate (pdt = propanedithiolate) (<b>1</b>) or adt^Bn ({SCH₂}₂NBn = azadithiolate) (<b>2</b>). A crystallographic study was carried out on <b>2</b> and Mössbauer spectroscopy, and DFT calculations have been used to describe the electronic and structural properties of <b>1</b>. The electrochemical properties of <b>1</b> in the absence and in the presence of a weak acid have been the subject of a preliminary investigation

Single Asparagine to Arginine Mutation Allows PerR to Switch from PerR Box to Fur Box

Fur family proteins, ubiquitous in prokaryotes, play a pivotal role in microbial survival and virulence in most pathogens. Metalloregulators, such as Fur and PerR, regulate the transcription of genes connected to iron homeostasis and response to oxidative stress, respectively. In <i>Bacillus subtilis</i>, Fur and PerR bind with high affinity to DNA sequences differing at only two nucleotides. In addition to these differences in the PerR and Fur boxes, we identify in this study a residue located on the DNA binding motif of the Fur protein that is critical to discrimination between the two close DNA sequences. Interestingly, when this residue is introduced into PerR, it lowers the affinity of PerR for its own DNA target but confers to the protein the ability to interact strongly with the Fur DNA binding sequence. The present data show how two closely related proteins have distinct biological properties just by changing a single residue

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe–Fe]-Hydrogenases

A novel reaction pathway to synthesize the linear trinuclear clusters [Fe₃(CO)₅(κ²-diphosphine)­(μ-dithiolate)₂] via the direct reaction of the dinuclear hexacarbonyl precursor [Fe₂(CO)₆(μ-dithiolate)] with the mononuclear species [Fe­(CO)₂(κ²-diphosphine)­(κ²-dithiolate)] has been developed with diphosphine (dppe) and dithiolate (pdt = propanedithiolate) (<b>1</b>) or adt^Bn ({SCH₂}₂NBn = azadithiolate) (<b>2</b>). A crystallographic study was carried out on <b>2</b> and Mössbauer spectroscopy, and DFT calculations have been used to describe the electronic and structural properties of <b>1</b>. The electrochemical properties of <b>1</b> in the absence and in the presence of a weak acid have been the subject of a preliminary investigation

Proton-Coupled Intervalence Charge Transfer: Concerted Processes

The kinetics of proton-induced intervalence charge transfer (IVCT) may be measured electrochemically by generating one of the members of the IVCT couple in situ and following its conversion by means of the electrochemical signature of the other member of the couple. In the case of the diiron complex taken as an example, the reaction kinetics analysis, including the H/D isotope effect, clearly points to the prevalence of the concerted proton–intervalence charge transfer pathway over the stepwise pathways. A route is thus open toward systematic kinetic studies of proton-induced IVCT aiming at uncovering the main reactivity parameters and the factors that control the occurrence of concerted versus stepwise pathways

Biologically Relevant Heterodinuclear Iron–Manganese Complexes

The heterodinuclear complexes [Fe^IIIMn^II(L-Bn)­(μ-OAc)₂]­(ClO₄)₂ (<b>1</b>) and [Fe^IIMn^II(L-Bn)­(μ-OAc)₂]­(ClO₄) (<b>2</b>) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis­[(2-pyridylmethyl)­aminomethyl]]-6-[benzyl-2-(pyridylmethyl)­aminomethyl]-4-methylphenol} were synthesized and characterized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound <b>1</b> and reveal an Fe^IIIMn^IIμ-phenoxobis­(μ-carboxylato) core. A single location of the Fe^III ion in <b>1</b> and of the Fe^II ion in <b>2</b> was demonstrated by Mössbauer and ¹H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of <b>1</b> revealed a moderate antiferromagnetic interaction (<i>J</i> = 20 cm^–1) between the high-spin Fe^III and Mn^II ions in <b>1</b>, which was confirmed by Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex <b>1</b> are described. A quasireversible electron transfer at −40 mV versus Ag/AgCl corresponding to the Fe^IIIMn^II/Fe^IIMn^II couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex <b>2</b> were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (<i>J</i> = 5.72 cm^–1) within the Fe^IIMn^II pair consistent with that deduced from magnetic susceptibility measurements (<i>J</i> = 6.8 cm^–1). Owing to the similar value of the Fe^II zero-field splitting (<i>D</i>_Fe = 3.55 cm^–1), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex <b>2</b> was developed. This situation is reminiscent of that found in many diiron and iron–manganese enzyme active sites

Biologically Relevant Heterodinuclear Iron–Manganese Complexes

The heterodinuclear complexes [Fe^IIIMn^II(L-Bn)­(μ-OAc)₂]­(ClO₄)₂ (<b>1</b>) and [Fe^IIMn^II(L-Bn)­(μ-OAc)₂]­(ClO₄) (<b>2</b>) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis­[(2-pyridylmethyl)­aminomethyl]]-6-[benzyl-2-(pyridylmethyl)­aminomethyl]-4-methylphenol} were synthesized and characterized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound <b>1</b> and reveal an Fe^IIIMn^IIμ-phenoxobis­(μ-carboxylato) core. A single location of the Fe^III ion in <b>1</b> and of the Fe^II ion in <b>2</b> was demonstrated by Mössbauer and ¹H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of <b>1</b> revealed a moderate antiferromagnetic interaction (<i>J</i> = 20 cm^–1) between the high-spin Fe^III and Mn^II ions in <b>1</b>, which was confirmed by Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex <b>1</b> are described. A quasireversible electron transfer at −40 mV versus Ag/AgCl corresponding to the Fe^IIIMn^II/Fe^IIMn^II couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex <b>2</b> were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (<i>J</i> = 5.72 cm^–1) within the Fe^IIMn^II pair consistent with that deduced from magnetic susceptibility measurements (<i>J</i> = 6.8 cm^–1). Owing to the similar value of the Fe^II zero-field splitting (<i>D</i>_Fe = 3.55 cm^–1), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex <b>2</b> was developed. This situation is reminiscent of that found in many diiron and iron–manganese enzyme active sites

Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline or Benzimidazole Ligands

We have previously investigated cis/trans isomerization processes in phenoxido-bridged mixed-valent Fe^IIFe^III complexes that contain either one aniline or one anilide ligand. In this work, we compare the properties of similar complexes bearing one terminal protic ligand, either aniline or 1<i>H</i>-benzimidazole. Whatever the ligand, ¹H NMR spectroscopy clearly evidences that the complexes are present in CH₃CN as a mixture of cis- and trans-isomers in a close to 1:1 ratio. We show here that addition of NEt₃ indeed allows the deprotonation of these ligands, the resulting complexes bearing either anilide or benzimidazolide that are coordinated to the ferric site. The latter are singular examples of a high-spin ferric ion coordinated to a benzimidazolide ligand. Whereas the trans-isomer of the anilide complex is the overwhelming species, benzimidazolide species are mixtures of cis- and trans-isomers in equal proportions. Moreover, cyclic voltammametry studies show that Fe^IIIFe^III complexes with 1<i>H</i>-benzimidazole are more stable than their aniline counterparts, whereas the reverse is observed for the deprotonated species

A New FeMo Complex as a Model of Heterobimetallic Assemblies in Natural Systems: Mössbauer and Density Functional Theory Investigations

The design of the new FeMo heterobimetallic species [FeMo­(CO)₅(κ²-dppe)­(μ-pdt)] is reported. Mössbauer spectroscopy and density functional theory calculations give deep insight into the electronic and structural properties of this compound

Development of a Rubredoxin-Type Center Embedded in a <i>de Dovo</i>-Designed Three-Helix Bundle

Protein design is a powerful tool for interrogating the basic requirements for the function of a metal site in a way that allows for the selective incorporation of elements that are important for function. Rubredoxins are small electron transfer proteins with a reduction potential centered near 0 mV (vs normal hydrogen electrode). All previous attempts to design a rubredoxin site have focused on incorporating the canonical CXXC motifs in addition to reproducing the peptide fold or using flexible loop regions to define the morphology of the site. We have produced a rubredoxin site in an utterly different fold, a three-helix bundle. The spectra of this construct mimic the ultraviolet–visible, Mössbauer, electron paramagnetic resonance, and magnetic circular dichroism spectra of native rubredoxin. Furthermore, the measured reduction potential suggests that this rubredoxin analogue could function similarly. Thus, we have shown that an α-helical scaffold sustains a rubredoxin site that can cycle with the desired potential between the Fe­(II) and Fe­(III) states and reproduces the spectroscopic characteristics of this electron transport protein without requiring the classic rubredoxin protein fold