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The 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.

The electrochemical and near-IR properties of a series of (bis)ferrocenyl complexes, in which the ferrocenyl moieties are spanned by enyne ( E )-(–CH CH–C C–), diyne (–C C–) 2 , or diene ( E )-(–CH CH–) 2 C 4 unsaturated spacers, were investigated. The experiments performed on 1,4-diferrocenyl-( E )-1-buten-3-yne revealed behaviors indicative of through-bridge/metal–metal electron transfer. The enyne bridged (bis)ferrocenyl cation displays charge transfer properties intermediate between the dienyl and diynyl systems and can be assigned to weakly coupled Class II mixed valence species. The analysis of the collected data according to the classical two-state Hush theory revealed that the efficiency of the electron transfer increases from wholly sp-C to wholly sp 2 -C bridged complexes. Therefore, the degree of communication is modulated according to the structure of the bridging 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 ...

A series of heterobimetallic η 6 -[(ferrocenyl)indene]-Cr(CO) 3 complexes differing for the position of the ferrocenyl group, 1-(ferrocenyl)indene and 2-(ferrocenyl)indene, and the degree of indene methylation (tetramethyl- and hexamethyl-) have been prepared and studied with the aim to stabilise the mono- and dications generated by chemical and electrochemical oxidation, and at same time to tune the metal–metal electronic coupling in the mixed-valence cations. The magnitude of electronic delocalisation and spin density in the cations have been monitored by means of optical techniques (UV–Vis, near-IR, mid-IR) and EPR spectroscopy. The results have been rationalised in the framework of Marcus-Hush theory and at quantum chemistry level by DFT and TD-DFT methods, establishing that a metal-to-metal electronic coupling occurs the magnitude of which depends on the degree of indene methylation.

Mixed-valence compounds, which have at least two redox sites in different oxidation states linked by a bridging ligand that promotes electron transfer from one site to the other, are excellent benchmarks for the study of intramolecular electron transfer (1). Transition metal complexes that display multielectron redox chemistry are therefore attracting much interest due to their potential applications in molecular materials for catalysis and electronics (2). In particular, compounds containing multiple ferrocene (Fc) units are suitable models since these units display versatile organic chemistry and a well-defined redox behavior (3-5). Although there have been many reports of biferrocenyl complexes,(1c, 3, 5) experimental (6) and theoretical (7) studies on oligoferrocene systems are relatively scarce despite their potential to highlight those factors that influence the mechanistic pathway of intercomponent electron transfer.

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.

The extent of metal-metal electronic coupling was quantified for a series of syn and anti stereoisomers of (FeCp)(2)-, (RhL(2))(2)- and (FeCp)(RhL(2))- (L(2)=1,5-cyclooctadiene (cod), L=CO) as-indacenediide mixed-valent ions by spectroelectrochemical and DFT studies. The effect of the syn/anti 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.

The synthesis of 2-ferrocenylbenzo[b]-thiophene, 3-ferrocenylbenzo[b]thiophene, 1,1-bis(2-indene)-ferrocene, and the two isomers of 1,1'-bis(2-benzo[b]-thiophene)ferrocene was efficiently achieved by using the palladium-catalyzed Negishi C,C cross-coupling reaction of the appropriate bromobenzo[b]thiophene derivative with ferrocenylzinc chloride. The accessibility of differently substituted benzo[b]thiophenes and a comparison with indene analogues allowed an in-depth investigation on how the geometric modifications and the presence of sulfur affect their physical properties. The molecular structure of 3-ferrocenylbenzo[b]-(t)hiophene has been determined by X-ray diffraction. Electrochemistry and UV-vis-NIR spectroscopy, in particular the appearance upon oxidation of a charge transfer absorption in the NIR region, are rationalized through quantum chemistry calculations and in the framework of the Hush theory.

In 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 molecules.

In this article, the successful preparation of a new series of 310-helical peptides of different length containing two terminal ferrocenyl (Fc) units and based on the strongly foldameric α-aminoisobutyric (Aib) acid is reported. The synthesis of the FcCO(Aib)nNHFc (n = 1−5) homo-peptides was performed by solution methods. Moderate to good yields (26−85%) were obtained in each of the difficult coupling steps of FcCOOH and the corresponding H(Aib)nNHFc compounds by C-activation with the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide/7-aza-1-hydroxy-1,2,3-benzotriazole method. Information on the CO···HN intramolecularly hydrogen-bonded networks was initially obtained from FT-IR absorption measurements. The NH stretching (amide A) region allowed us to distinguish which amide protons are involved in intramolecular hydrogen bonds and indicates the formation of an incipient 310-helix structure for peptides containing at least two Aib residues. This conclusion was confirmed by 1H NMR titrations of the NH groups of the peptides in CDCl3 with dimethylsulfoxide and by crystallographic analysis of the Nα-acylated FcCO(Aib)5NHFc pentapeptide amide. The two redox-active Fc groups covalently bound to the termini of the foldameric peptide architectures were used as electrochemical probes. The end-to-end effects of electron holes generated by single and double oxidations were analyzed by means of electrochemical and spectroelectrochemical techniques. The results of these studies indicate that charge transfer across the peptide main chain does occur in the five peptides. In particular, in the pentapeptide 5, charge is transferred through an intramolecular Fe···Fe separation of 14 A. © 2012 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 71–81, 2013.

Novel 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 nBu4NPF6 to nBU4NB(C6F6)4

Two series of 3(10)-helical peptides of different lengths and rigidity, based on the strongly foldameric α-aminoisobutyric acid and containing a terminal ferrocenyl unit, have been synthesized. Oxidation-state sensitive spectroscopic tags of helical peptides, the N-H groups, allowed mapping of the charge delocalization triggered by oxidation of the terminal ferrocenyl moiety and were monitored by IR spectroelectrochemistry.

A series of ferrocenyl-based complexes of general structure [eta(5)-(2-ferrocenyl)indenyI]MLn [MLn = 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 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 H-ab and alpha ranging from weakly to moderately coupled mixed valence ions.

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