Intervalence Charge Transfer in Cationic Heterotrinuclear Fe(III)−Rh(I)−Cr(0) Triads of the Polyaromatic Cyclopentadienyl−Indenyl Ligand

The challenge to realize polymetallic assemblies of unambiguous structure and stereochemistry, in which the nature of the intervalence transition (IT) is rationalized, has been faced by investigating the syn and anti isomers of η6-Cr(CO)3{η5-[(2-ferrocenyl)indenyl]Rh(CO)2} and their mixed-valence cations. Crystallographic studies and DFT calculations provide a detailed description of the structural and electronic features of these complexes, evidencing a significant difference in geometrical distortions and frontier MO composition between syn and anti isomers. Mixed-valence cations are generated and monitored by low-temperature spectroelectrochemistry in the visible, IR, and near-IR regions. The IT bands in the near-IR spectra are rationalized in the framework of Marcus−Hush theory and at quantum chemistry level by density functional theory. Noteworthy, the results reported provide rare experimental evidence that the presence of a third metal center (Rh) increases the metal−metal (Fe−Cr) interaction with respect to the structurally correlated binuclear system

Designing Molecules for Metal−Metal Electronic Communication: Synthesis and Molecular Structure of the Couple of Heterobimetallic Isomers [η⁶-(2-Ferrocenyl)indene]-Cr(CO)₃ and [η⁶-(3-Ferrocenyl)indene]-Cr(CO)₃

The heterobinuclear isomers [η6-(2-ferrocenyl)indene]-Cr(CO)3 (1) and [η6-(3-ferrocenyl)indene-Cr(CO)3 (2) have been prepared and the crystal structure determination showed that the Fe(C5H5) and Cr(CO)3 groups in the two molecules are disposed in different conformations with respect to the Cp-indene bridging ligand, cisoid in 1 and transoid in 2. Preliminary electrochemical (CV) and spectroscopic (IR and near-IR) results obtained for the corresponding monooxidized 1+ and 2+ demonstrate the existence of stronger electronic coupling in 1+ than in 2+

Mixed Valence Properties in Ferrocenyl-Based Bimetallic FeCp−Indenyl−ML_<i>n</i> Complexes: Effect of the ML_<i>n</i> Group

A series of ferrocenyl-based complexes of general structure [η5-(2-ferrocenyl)indenyl]MLn [MLn = RuCp*, FeCp, IrCOD, Mn(CO)3, and Cr(CO)2NO] were synthesized with the aim of tuning the effect of the nature of the second metal group MLn on the magnitude of the metal−metal electronic coupling in their mixed valence ions generated by electrochemical oxidation. The electronic interaction was probed by determining different and independent physical properties, the potential splitting in the cyclic voltammograms, and the IT bands in the near-IR spectra, which were rationalized in the framework of Marcus−Hush theory and at the quantum chemistry level by the density functional theory and TD density functional theory methods. On the basis of the obtained results, we were able to establish a trend based on the magnitude of the Fe−M electron transfer parameters Hab and α ranging from weakly to moderately coupled mixed valence ions

Intervalence Charge Transfer in Cationic Heterotrinuclear Fe(III)−Rh(I)−Cr(0) Triads of the Polyaromatic Cyclopentadienyl−Indenyl Ligand

The challenge to realize polymetallic assemblies of unambiguous structure and stereochemistry, in which the nature of the intervalence transition (IT) is rationalized, has been faced by investigating the syn and anti isomers of η6-Cr(CO)3{η5-[(2-ferrocenyl)indenyl]Rh(CO)2} and their mixed-valence cations. Crystallographic studies and DFT calculations provide a detailed description of the structural and electronic features of these complexes, evidencing a significant difference in geometrical distortions and frontier MO composition between syn and anti isomers. Mixed-valence cations are generated and monitored by low-temperature spectroelectrochemistry in the visible, IR, and near-IR regions. The IT bands in the near-IR spectra are rationalized in the framework of Marcus−Hush theory and at quantum chemistry level by density functional theory. Noteworthy, the results reported provide rare experimental evidence that the presence of a third metal center (Rh) increases the metal−metal (Fe−Cr) interaction with respect to the structurally correlated binuclear system

Designing Molecules for Metal−Metal Electronic Communication: Synthesis and Molecular Structure of the Couple of Heterobimetallic Isomers [η⁶-(2-Ferrocenyl)indene]-Cr(CO)₃ and [η⁶-(3-Ferrocenyl)indene]-Cr(CO)₃

The heterobinuclear isomers [η6-(2-ferrocenyl)indene]-Cr(CO)3 (1) and [η6-(3-ferrocenyl)indene-Cr(CO)3 (2) have been prepared and the crystal structure determination showed that the Fe(C5H5) and Cr(CO)3 groups in the two molecules are disposed in different conformations with respect to the Cp-indene bridging ligand, cisoid in 1 and transoid in 2. Preliminary electrochemical (CV) and spectroscopic (IR and near-IR) results obtained for the corresponding monooxidized 1+ and 2+ demonstrate the existence of stronger electronic coupling in 1+ than in 2+

Mixed Valence Properties in Ferrocenyl-Based Bimetallic FeCp−Indenyl−ML_<i>n</i> Complexes: Effect of the ML_<i>n</i> Group

A series of ferrocenyl-based complexes of general structure [η5-(2-ferrocenyl)indenyl]MLn [MLn = RuCp*, FeCp, IrCOD, Mn(CO)3, and Cr(CO)2NO] were synthesized with the aim of tuning the effect of the nature of the second metal group MLn on the magnitude of the metal−metal electronic coupling in their mixed valence ions generated by electrochemical oxidation. The electronic interaction was probed by determining different and independent physical properties, the potential splitting in the cyclic voltammograms, and the IT bands in the near-IR spectra, which were rationalized in the framework of Marcus−Hush theory and at the quantum chemistry level by the density functional theory and TD density functional theory methods. On the basis of the obtained results, we were able to establish a trend based on the magnitude of the Fe−M electron transfer parameters Hab and α ranging from weakly to moderately coupled mixed valence ions

Synergistic Effect of Sn and Fe in Fe–N_<i>x</i> Site Formation and Activity in Fe–N–C Catalyst for ORR

Iron–nitrogen–carbon (Fe–N–C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O2 in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe–N–C materials were co-doped with Sn as a secondary metal center. Sn–N–C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H2SO4 electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn–N–C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe–N4 site density, whereas the possible synergistic interaction of Fe–N4 and Sn–Nx found no confirmation. The presence of Fe–N4 and Sn–Nx was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe–N–C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe–N–C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (jk = 2.11 vs 1.83 A g–1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests

Charge Transfer through Isomeric Unsaturated Hydrocarbons. Redox Switchable Optical Properties and Electronic Structure of Substituted Indenes with a Pendant Ferrocenyl

A family of (ferrocenyl)indenes, (2-ferrocenyl)indene, (2-ferrocenyl)tetramethylindene, (2-ferrocenyl)hexamethylindene, (3-ferrocenyl)indene, and (3-ferrocenyl)hexamethylindene, and the corresponding monooxidized cations have been prepared. The results of a structural and spectroelectrochemical study are discussed. The availability of pairs of isomers with known geometries and differently methylated indenes allowed the detailed investigation of how slight geometric and electronic modifications affect their physical properties. The molecular structures have been determined by X-ray diffraction and compared with the fully optimized structures calculated with state-of-the-art DFT methods. Calculated and crystallographic structures agree in establishing the dependence of the orientation of the indene moiety and the ferrocenyl cyclopentadienyl rings on the degree of methylation. The UV−vis spectra and in particular the appearance upon oxidation of a new near-IR absorption, whose energy and intensity increase with the degree of methylation and cyclopentadienyl-indene planarity, are rationalized in the framework of the Hush theory and at quantum chemistry level by DFT and TD-DFT calculations

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Manuscript title: Operando visualization of the hydrogen evolution reaction with atomic-scale precision at different metal-graphene interfacesPublished in Nature Catalysis</div

Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe–N–C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis

Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe–N–C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe–N–C catalysts or because of its capability to increase the Fe–Nx site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe–Nx site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe–N–C catalysts. The Fe–N–C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0–16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors. The carbon with the highest sulfur content was also activated through steam treatment at 800 °C for different durations, which allowed us to modulate the carbon pore volume and surface area (1296–1726 m2 g–1). The resulting catalysts were tested in O2-saturated 0.5 M H2SO4 electrolyte, and the site density (SD) was determined using the NO-stripping technique. Here, we demonstrate that sulfur doping has a porogenic effect increasing the microporosity of the carbon support, and it also facilitates the nitrogen fixation on the carbon support as well as the formation of Fe–Nx sites. It was found that the Fe–N–C catalytic activity [E1/2 ranges between 0.609 and 0.731 V vs reversible hydrogen electrode (RHE)] does not directly depend on sulfur content, but rather on the microporous surface and therefore any electronic effect appears not to be determinant as confirmed by X-ray photoemission spectroscopy (XPS). The graph reporting Fe–Nx SD versus sulfur content assumes a volcano-like shape, where the maximum value is obtained for a sulfur/iron ratio close to 18, i.e., a too high or too low sulfur doping has a detrimental effect on Fe–Nx formation. However, it was highlighted that the increase of Fe–Nx SD is a necessary but not sufficient condition for increasing the catalytic activity of the material, unless the textural properties are also optimized, i.e., there must be an optimized hierarchical porosity that facilitates the mass transport to the active sites