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Interazioni elettroniche metallo-metallo in complessi bimetallici e trimetallici di leganti policiclici aromatici di struttura flessibile e rigida
Full text linkIn the last years many efforts have been directed toward the synthesis and study of ferrocenyl-based conjugated ligands since they can be used as building-blocks for the synthesis of poly-ferrocenyl conjugated systems or as starting material for the synthesis of new homo- and hetero-bimetallic complexes with innovative physical properties. Furthermore, this ferrocenyl-conjugated derivatives represent suitable models for the study of electronic communication between terminal units connected by an unsaturated hydrocarbon bridge in terms of tuning the electron properties and the reactivity of materials.
Systems where a metal is connected to a ferrocenyl unit by an unsaturated hydrocarbon bridge, associated with the redox switchable electron donor/acceptor capability of ferrocene/ferrocenium couple, are expected to display attractive properties that depend not only by the nature of the bridge but also by the redox state of the pendant ferrocenyl.
Hush analysis of InterValence Charge Transfer band (IVCT) of mixed-valence bimetallic complexes formed by oxidation of one metal unit, has revealed as the magnitude of the electronic coupling is largely dependent on the structure and stereochemistry of the bridging ligand and as it increases if the spacer is forced to adopt a planar geometry.
While the charge or energy transfer phenomena in homo- and hetero-bimetallic mixed-valence complexes have been widely investigated, studies of charge and energy transfer processes in systems formed by a ferrocenyl unit linked to an unsatured organic group have received minor attention. In these systems, the ferrocenyl unit, upon of her oxidation, acts as an electron-acceptor group generating new bands in the near-IR region which can be assigned to aryl→ferrocenium transition.
The classical electron transfer Hush model, developed for interpretation of IVCT bands of bimetallic and organic molecules, has been recently extended for interpretation of LMCT (Ligand to Metal Carghe Transfer) and MLCT (Metal to Ligand Carghe Transfer) bands to get insight into electron transfer processes between an organic and a metallic group in monometallic complexes. To this purpose, we have prepared the following complexes: (2-ferrocenyl)-indene, (2-ferrocenil)-tetramethylindene, (2-ferrocenyl)-hexamethylindene, (3-ferrocenyl)-indene e (3-ferrocenyl)-hexamethylindene. Furthermore, the indene unit has been replaced by a benzothiophene unit obtaining a couple of isomers: (3-ferrocenyl)- benzothiophene and (2-ferrocenyl)-benzothiophene). Subsequently, complexation with (CH3CN)3Cr(CO)3 of benzene ring of (ferrocenyl)-indenes has lead to a series of hetero-bimetallic Fe/Cr complexes. Availability of couples of monometallic and bimetallic isomers with known geometries with differently methylated indene unit has allowed the detailed investigation of how slight geometric and electronic modifications affect LMCT (in monometallic complexes) and IVCT ( in bimetallic complexes) charge transfer processes.
The big interest directed toward the study of bimetallic complexes characterized the last decades has not been accompanied by an equal attention toward the electronic transfer processes in mixed—valence trimetallic systems. This can be due to the difficult synthetic routes to obtain trimetallic species and to the complexity of the interactions that they can present. It is evident that the characterization of the redox properties and the study of the metal-metal interactions in complexes with more than two metallic centres offer the possibility of a better understanding of the electron transfer processes in multinuclear complexes.
To this purpose, the research has been extended to the synthesis, characterization and study of a triferrocenylic model system: the anti-[(FeCp)3Td] (Td = trindene) complex
Key multi(ferrocenyl) complexes in the interplay between electronic coupling and electrostatic interaction
Full text linkIn this review, the properties of the most significant examples of multi(ferrocenyl) cations containing a number of ferrocenyl units from two to six are discussed and the results are compared with the outcomes of some of our recent studies on conjugated ferrocenyl complexes, in order to give an overview of how the nature of the bridging ligand, the distance between the redox-active centres, and the medium affect the electronic and electrostatic properties of the molecule
Mixed Valence Properties in Ferrocenyl-Based Bimetallic FeCp−Indenyl-MLn Complexes: Effect of the MLn Group
No full textA series of ferrocenyl-based complexes of general structure [eta(5)-(2-ferrocenyl)indenyI]ML(n) [ML(n) = RuCp*, FeCp, IrCOD, Mn(CO)(3), and Cr(CO)(2)NO] were synthesized with the aim of tuning the effect of the nature of the second metal group ML(n) 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 H(ab) and alpha ranging from weakly to moderately coupled mixed valence ions
Charge Transfer Properties in Cyclopenta[l]phenanthrene Ferrocenyl Complexes
No full textThe new complexes (2-ferrocenyl)cyclopenta[l]-phenanthrene and (2-ferrocenyl)(eta(5)-cyclopenta[l]phenanthrenyl)FeCp have been prepared and the charge transfer properties of their monocationic derivatives investigated. The cations were generated by chemical oxidation using ferrocenium(BF4) or acetylferrocenium(BF4) as the oxidative agent and monitored in the visible, IR, and near-IR regions. The electrochemistry of the two complexes and, for comparison, of the previously reported (eta(5)-cyclopenta[l]phenanthrenyl)FeCp was analyzed. The charge transfer bands in the near-IR spectral region of the monocations are rationalized in the framework of Marcus-Hush theory. In particular, the monometallic (2-ferrocenyl)cyclopenta[l]phenanthrene displays a single oxidation wave at a potential very close to that of (eta(5)-cyclopenta[l]phenanthrenyl)FeCp and its monocations exhibits a ligand-to-metal charge transfer band in the visnear-IR region. The unsymmetrical diiron species (2-ferrocenyl)(eta(5)-cyclopenta[l]phenanthrenyl)FeCp undergoes two consecutive and well-resolved one-electron oxidations producing, at the first oxidation step, a mixed-valence monocation which displays an intervalence charge transfer band in the vis-near-IR region
Intervalence Charge Transfer in Cationic Heterotrinuclear Fe(III)-Rh(I)-Cr(0) Triads of the Polyaromatic Cyclopentadienyl-Indenyl Ligand
No full textThe 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 eta(6)-Cr(CO)(3){eta(5)-[(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-ER 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
Synthesis of the prototypical cyclic metallocene triad: mixed-valence properties of [(FeCP)3(trindenyl)] isomers
No full text"Fusion" of three ferrocenes into a rigid cyclic array produces the syn,syn,anti-and syn,syn,syn isomers of (FeCp)3trindenyl whose cations display different mixed valence properties: multiple Intervalence-Transfer (IT) absorptions appear in the near-IR/IR for syn,syn,anti with characteristics indicative of localized-to-delocalized Class II-III behavior, while a single band in the visible/near-IR for syn,syn,syn is in favor of localized Class II behavior
Mapping charge delocalization in a peptide chain triggered by oxidation of a terminal ferrocene moiety
No full textTwo series of peptides of different length and rigidity, based on the strongly helicogenic α-aminoisobutyric acid (Aib) residue and containing a terminal ferrocene (Fc) unit, Fc−CO-(Aib)n−OMe (An, n = 1−5, OMe, methoxy) and Z−(Aib)n−NH−Fc (Bn, n = 1−5, Z, benzyloxycarbonyl), were prepared and investigated. We utilized the oxidation-state sensitive, spectroscopic tags of peptides, the CO and NH groups, to map charge delocalization triggered by oxidation of the terminal ferrocene
Charge transfer through isomeric unsaturated hydrocarbons. Redox switchable optical properties and electronic structure of substituted indenes with a pendant ferrocenyl.
No full textA 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
Metal-Metal Electronic Coupling in syn and anti Stereoisomers of Mixed-Valent (FeCp)2-, (RhL2)2-, and (FeCp)(RhL2)-as-Indacenediide Ions
No full textThe extent of metal-metal electronic coupling was quantified for a series of syn and anti stereoisomers of (FeCP)(2)-, (RhL2)(2)- and (FeCp)(RhL2)-(L-2 = 1,5-cyclooctadiene (cod), L = CO) as-indacenediide mixed-valent ions by spectroelectrochemical and DFT studies. The effect of the synlanti orientation of the metal units with respect to the planar aromatic ligand indicates that electron transfer occurs through the bridge rather than through space. The nature of the metal was found to be crucial: while homobimetallic diiron species are localised valence-trapped ions (Class II), the dirhodium analogues are almost delocalised mixed-valent ions (borderline and Class III). Finally, despite their redox asymmetry, even in the heterobimetallic iron-rhodium as-indacenediide complexes, strong metal-metal coupling is present. In fact, oxidation of the iron centre is accompanied by electron transfer from rhodium to iron and formation of a reactive 17-electron rhodium site. syn and anti Fe-Rh as-indacenediide complexes are rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species
Single Two-Electron Transfers and Successive One-Electron Transfers in Biferrocenyl-Indacene Isomers
No full textNovel biferrocenyl complexes of s- and as-dihydroindacenes have been prepared and the charge transfer properties of their mono- and dicationic derivatives, selectively generated by one-electron and two-electron oxidation, have been investigated. Mixed-valence cations are generated by chemical oxidation using acetylferricinium as an oxidant agent and monitored 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. The rigid and planar indacene platform bonded to two terminal redox groups displays a redox chemistry that can be switched from single two-electron transfers to two successive one-electron transfers by changing the supporting electrolyte from nBu(4)NPF(6) to nBu(4)NB(C(6)F(5))(4)
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