Ferrocenyl-terminated redox stars: synthesis and electrostatic effects in mixed-valence stabilization

Electrostatic vs. electronic interactions within oxidized multinuclear Pt(bipyridine)(dithiolene) complexes

Bi- and trimetallic platinum(bipyridine)(dithiolene) complexes involving different organic linkers between the redox active platinum(dithiolene) moieties have been synthesized and studied by electrochemistry and spectroelectrochemistry. Cyclic voltammetry experiments carried out on these multinuclear complexes in dichloromethane using [NBu4][PF6] as the supporting salt show only one oxidation process involving the metallacycles. Nevertheless, spectro-electrochemical studies, carried out under the same conditions, show a different evolution of the spectra during oxidation depending on the position of the metallacycles on the phenyl ring. For instance, only the 1,3-disubtituted (Pt21,3-P) and 1,3,5-trisubstituted (Pt31,3,5-P) complexes show the growth of an absorption band in the NIR region upon oxidation while it was not observed for the other complexes. In contrast, electrochemical studies carried out using the poorly coordinating supporting electrolyte, [Na][B(C6H4(CF3)2)4], indicate sequential oxidation of the redox centers with ΔE values varying according to the nature of the bridge and the distance between metal centers. For all the investigated multinuclear complexes, spectro-electrochemical experiments performed in the presence of [Na][B(C6H4(CF3)2)4] show an absorption band in the NIR region consistent with appreciable electrostatic effects and charge delocalization in the mixed valent intermediates.

A convenient synthesis of ferrocene-(ethynylphenyl)thioacetates

Ferrocene is popular within the field of molecular electronics due to its well-defined electronic properties. However, where the conductance of highly-conjugated oligophenylethylenes has been widely studied, work on analogous ferrocenyl systems has been relatively rare, possibly due to difficulties associated with the synthesis of molecules containing terminal thioacetates, which are often used to bind molecules to metallic electrodes. Herein, a widely applicable synthetic methodology is demonstrated which can be used to synthesize a variety of conjugated ferrocene-alkyne systems terminated with thioacetates, including symmetric, asymmetric and multi-ferrocene systems. Conjugation of the ferrocene units to their terminal atoms is then shown through the use of both UV/Vis spectroscopy and cyclic voltammetry. This work paves the way for future studies and applications of conjugated ferrocene systems in the field of nanoelectronics.

From sandwich complexes to dendrimers: journey toward applications to sensing, molecular electronics, materials science, and biomedicine

This review links various areas of inorganic chemistry around the themes developed by our research group during the last four decades. It is firstly based on the electronic structure of iron sandwich complexes, showing how the metal electron count dictates their reactivities, with various applications (via C-H activation, C-C bond formation) as reducing and oxidizing agents, redox and electrocatalysts and precursors of dendrimers and catalyst templates through bursting reactions. Various electron-transfer processes and consequences are explored, including the influence of the redox state on the acidity of robust ligands and the possibility to iterate in situ C-H activation and C-C bond formation to build arene-cored dendrimers. Examples of how these dendrimers are functionalized are illustrated using the cross olefin metathesis reactions, with application to the synthesis of soft nanomaterials and biomaterials. Mixed and average valence complexes give rise to remarkable subsequent organometallic reactions, including the salt influence on these reactions. The stereo-electronic aspect of these mixed valencies is pointed out in star-shaped multi-ferrocenes with a frustration effect and other multi-organoiron systems, with the perspective of understanding electron-transfer processes among dendrimer redox sites involving electrostatic effects and application to redox sensing and polymer metallocene batteries. Dendritic redox sensing is summarized for biologically relevant anions such as ATP^2- with supramolecular exoreceptor interactions at the dendrimer periphery in parallel with the seminal work on metallocene-derived endoreceptors by Beer's group. This aspect includes the design of the first metallodendrimers that have applications in both redox sensing and micellar catalysis with nanoparticles. These properties provide the opportunity to summarize the biomedical (mostly anticancer) applications of ferrocenes, dendrimers and dendritic ferrocenes in biomedicine (in particular the contribution from our group, but not only). Finally, the use of dendrimers as templates for catalysis is illustrated with numerous reactions including C-C bond formation, click reactions and H₂ production reactions.

A decacationic ferrocene-based metallostar

Decacationic metallostars have been prepared by the reaction of permercurated ferrocene FeC₁₀(HgO₂CCF₃)₁₀ with superacidic (C₅F₅NH)(SbF₆) (pK _a = -11 estimated in H₂O) in multigram scale. In the resulting compound, [FeC₁₀Hg₁₀(NC₅F₅) _n ][SbF₆]₁₀, the labile pentafluoropyridine ligands are readily displaced by acetonitrile (MeCN) or tetrahydrothiophene (THT). In the X-ray structure of [FeC₁₀Hg₁₀(THT)₁₀][SbF₆]₁₀·24 MeCN no cation-anion contacts between mercury and fluorine were observed. Moreover, cyclic voltammetry measurements of [FeC₁₀(Hg(MeCN))₁₀]¹⁰⁺ and [FeC₁₀(Hg(THT))₁₀]¹⁰⁺ revealed a (quasi)reversible one-electron oxidation of Fe(ii) to Fe(iii). From the reaction of [FeC₁₀(Hg(MeCN))₁₀]¹⁰⁺ with MoF₆ as oxidant the ferrocenium cation [FeC₁₀(Hg(MeCN))₁₀]¹¹⁺ was obtained and characterized via single crystal XRD. These electrophilic metallostars are promising potential building blocks for the synthesis of dendritic architectures containing a robust, tenfold functionalized ferrocene core.

Comparison of crystal structure and DFT calculations of triferrocenyl trithiophosphite’s conformance

A triferrocenyl trithiophosphite was studied by X-ray single-crystal diffraction. Triferrocenyl trithiophosphite has nine axes of internal rotation: three P–S bonds, three C–S bonds and three Fe–cyclopentadienyl axes. Rotation around the P–S bonds results in a totally asymmetric structure with three ferrocenylthio groups exhibiting different orientations towards the phosphorus lone electron pair (LEP). A comparison of DFT calculations and X-ray diffraction data is presented, herein we show which conformations are preferred for a given ligand.

Electronic Coupling in 1,2,3-Triazole Bridged Ferrocenes and Its Impact on Reactive Oxygen Species Generation and Deleterious Activity in Cancer Cells

Mixed-valence (MV) binuclear ferrocenyl compounds have long been studied as models for testing theories of electron transfer and in attempts to design molecular-scale electronic devices (e.g., molecular wires). In contrary to that, far less attention has been paid to MV binuclear ferrocenes as anticancer agents. Herein, we discuss the synthesis of six 1,2,3-triazole ferrocenyl compounds for combined (spectro)electrochemical, electron paramagnetic resonance (EPR), computational, and anticancer activity studies. Our synthetic approach was based on the copper-catalyzed 1,3-dipolar azide–alkyne cycloaddition reaction and enabled us to obtain in one step compounds bearing either one, two, or three ferrocenyl entities linked to the common 1,2,3-triazole core. Thus, two series of complexes were obtained, which pertain to derivatives of 3′-azido-3′-deoxythymidine (AZT) and 3-azidopropionylferrocene, respectively. Based on the experimental and theoretical data, the two mono-oxidized species corresponding to binuclear AZT and trinuclear 3-azidopropionylferrocene complexes have been categorized as class II mixed-valence according to the classification proposed by Robin and Day. Of importance is the observation that these two compounds are more active against human A549 and H1975 non-small-cell lung cancer cells than their congeners, which do not show MV characteristics. Moreover, the anticancer activity of MV species competes or surpasses, dependent on the cell line, the activity of reference anticancer drugs such as cisplatin, tamoxifen, and 5-fluorouracil. The most active from the entire series of compounds was the binuclear thymidine derivative with the lowest IC₅₀ value of 5 ± 2 μM against lung H1975 cancer cells. The major mechanism of antiproliferative activity for the investigated MV compounds is based on reactive oxygen species generation in cancer cells. This hypothesis was substantiated by EPR spin-trapping experiments and the observation of decreased anticancer activity in the presence of N-acetyl cysteine (NAC) free-radical scavenger.

The 1,2,3-triazole bridged bi- and triferrocenyl compounds were prepared via a “click” reaction. Their corresponding mono-oxidized forms have been categorized as class II MV species. The biferrocenyl thymidine derivative showed remarkable anticancer activity against human A549 and H1975 cancer cells and negligible activity against nonmalignant human BEAS-2B cells. The anticancer activity mechanism is mainly due to ROS generation, and it originates from the combination of electronic coupling and the thymidine moiety, combined all together in one molecular scaffold.

Redox-Switchable Chalcogen Bonding for Anion Recognition and Sensing

Inspired by the success of its related sigma-hole congener halogen bonding (XB), chalcogen bonding (ChB) is emerging as a powerful noncovalent interaction with a plethora of applications in supramolecular chemistry and beyond. Despite its increasing importance, the judicious modulation of ChB donor strength remains a formidable challenge. Herein, we present, for the first time, the reversible and large-scale modulation of ChB potency by electrochemical redox control. This is exemplified by both the switching-ON of anion recognition via ChB oxidative activation of a novel bis(ferrocenyltellurotriazole) anion host and switching-OFF reductive ChB deactivation of anion binding potency with a telluroviologen receptor. The direct linking of the redox-active center and ChB receptor donor sites enables strong coupling, which is reflected by up to a remarkable 3 orders of magnitude modulation of anion binding strength. This is demonstrated through large voltammetric perturbations of the respective receptor ferrocene and viologen redox couples, enabling, for the first time, ChB-mediated electrochemical anion sensing. The sensors not only display significant anion-binding-induced electrochemical responses in competitive aqueous-organic solvent systems but can compete with, or even outperform similar, highly potent XB and HB sensors. These observations serve to highlight a unique (redox) tunability of ChB and pave the way for further exploration of the reversible (redox) modulation of ChB in a wide range of applications, including anion sensors as well as molecular switches and machines.

A Modular Strategy for Expanding Electron-Sink Capacity in Noncanonical Cluster Assemblies

A modular synthetic strategy is described whereby organometallic complexes exhibiting considerable electron-sink capacity may be assembled by using only a few simple molecular components. The Fe₂(PPh₂)₂(CO)₅ fragment was selected as a common electroactive component and was assembled around aromatic cores bearing one, two, or three isocyanide functional groups, with the resultant complexes possessing electron-sink capacities of two, four, and six electrons, respectively. The latter complex is noteworthy in that its electron-sink capacity was found to rival that of large multinuclear clusters (e.g., [Ni₃₂C₆(CO)₃₆]^6– and [Ni₃₈Pt₆(CO)₄₈]^6–), which are often considered as benchmarks of electron-sink behavior. Moreover, the modular assembly bearing three Fe₂(PPh₂)₂(CO)₅ fragments was observed to undergo reduction to a hexaanionic state over a potential window of about −1.4 to −2.1 V (vs Fc/Fc⁺), the relatively compressed range being attributed to potential inversions operative during the addition of the second, fourth, and sixth electrons. Such complexes may be designated noncanonical clusters because they exhibit redox properties similar to those of large multinuclear clusters yet lack the extensive network of metal–metal bonds and the condensed metallic cores that typify the latter.

By use of a modular synthetic strategy and relatively few molecular components, organometallic complexes exhibiting considerable electron-sink capacity have been characterized. Complexes bearing one, two, or three Fe₂(PPh₂)₂(CO)₅ fragments bound to aromatic isocyanide cores were found to possess electron-sink capacities of two, four, and six electrons, respectively, the latter rivaling the electron-sink capacity of large polynuclear cluster benchmarks.

Fusing pyrene and ferrocene into a chiral, redox-active triangle

A macrocycle that integrates three ferrocene-pyrene dyads in a triangular C2-symmetric arrangement is synthesised as a racemate in a simple one-pot approach. Crystal structural analysis reveals two enantiomeric conformers that pack alternatingly via π-π stacking and interconvert dynamically in solution. Electrochemical investigations indicate weak electrostatic interactions between Fc groups upon oxidation to a mixed valence triangle.

Formation of Metallic Polyferrocene Chains under Pressure

The effect of pressure on the structural reorganization of ferrocene, Fc = (C₅H₅)₂Fe, is studied using density functional theory (DFT) calculations assisted by evolutionary crystal structure prediction algorithms based on USPEX code. Pressure brings the individual molecules in close contact, and above 220 GPa, the 18-electron closed-shell molecular unit undergoes polymerization through the formation of quasi-one-dimensional (1D) chains, [(C₅H₅)₂Fe]_∞, termed as polyferrocene (p-Fc). Pressure induced polymerization (PIP) of Fc causes significant deviations from the 5-fold symmetry of the cyclopentadiene (Cp, C₅H₅ rings) and loss of planarity due to the onset of envelope-like distortions. This triggers distortions within the multidecker sandwich structures and σ(C-C) bond formation between the otherwise weak noncovalently interacting Cp rings in Fc crystals. Pressure gradually reduces the band gap of Fc, and for p-Fc, metallic states are found due to increased electronic coupling between the covalently linked Cp rings. Polyferrocene is significantly more rigid than ferrocene as evident from the 5-fold increase in its bulk modulus. Pressure dependent Raman spectra show a clear onset of polymerization in Fc at P = 220 GPa. Higher mechanical strength coupled with its metallicity makes p-Fc an interesting candidate for high pressure synthesis.

The core of the matter - arene substitution determines the coordination and catalytic behaviour of tris(1-phosphanyl-1'-ferrocenylene)arene gold(I) complexes

Changing the aromatic core of C3-symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound effects on their coordination behaviour towards gold(i). Depending on the arene (s-triazine, benzene, or trifluorobenzene), four different coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed fluctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

The modular syntheses of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes and their homotrinuclear gold(i) complexes are reported. Choosing the arene core allows fine-tuning of the exact oxidation potentials and thus tailoring of the electrochemical response. The tris[chloridogold(i)] complexes were investigated in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, showing cooperative behaviour vs. a mononuclear chloridogold(i) complex. Adding one, two, or three equivalents of 1,1′-diacetylferrocenium[tetrakis(perfluoro-tert-butoxy)aluminate] as an oxidant during the catalytic reaction (in situ) resulted in a distinct, stepwise influence on the resulting catalytic rates. Isolation of the oxidised species is possible, and using them as (pre-)catalysts (ex situ oxidation) confirmed the activity trend. Proving the intactness of the P–Au–Cl motif during oxidation, the tri-oxidised benzene-based complex has been structurally characterised.

Trinuclear gold(i) complexes of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes with four accessible oxidation states catalyse the ring-closing isomerisation of N-(2-propyn-1-yl)benzamide with different rates depending on their redox state.

The supramolecular redox functions of metallomacromolecules

Abstract

Metallomacromolecules are frequently encountered in redox proteins including metal-tanned hide collagen and play crucial roles involving supramolecular properties in biological electron-transfer processes. They are also currently found in non-natural families, such as: metallopolymers, metallodendrimers and metallodendronic polymers. This mini-review discusses the supramolecular redox functions of such nanomaterials developed in our research group. Electron-transfer processes are first examined in mono-, bis- and hexa-nuclear ferrocenes and other electron-reservoir organoiron systems showing the influence of supramolecular and reorganization aspects on their mechanism. Then applications of electron-transfer processes using these same organoiron redox systems in metallomacromolecules and their supramolecular functions are discussed including redox recognition/sensing, catalysis templates, electrocatalysis, redox catalysis, molecular machines, electrochromes, drug delivery device and nanobatteries.

Graphical Abstract

Electrochemical Properties and Structure of Multi-Ferrocenyl Phosphorus Thioesters

The reaction of triferrocenylthiophosphite with elemental sulfur leads to triferrocenyltetrathiophosphate. The molecule of tetrathiophosphate adopts propeller-like all synclinal-conformation of the ferrocenyl fragments respective to the P=S bond. All ferrocenyl groups have nearly ideal eclipsed conformation of the cyclopentadienyl fragments. The Fc3S3P (1), Fc3S3P=O, (2) and Fc3S3P=S (3) demonstrate three reversible and well-separated ferrocenyl-based redox events. The electronic structures of 1-3 have been studied quantum-chemically; the energies and composition of frontier orbitals have been calculated.

Controlling Through-Space and Through-Bond Exchange Pathways in Bis-Cobaltocenes for Molecular Spintronics

Pinching molecules via chemical strain suggests intuitive consequences, such as compression at the pinched site and clothespin‐like opening of other parts of the structure. If this opening affects two spin centers, it should result in reduced communication between them. We show that for naphthalene‐bridged biscobaltocenes with competing through‐space and through‐bond pathways, the consequences of pinching are far less intuitive: despite the known dominance of through‐space interactions, the bridge plays a much larger role for exchange spin coupling than previously assumed. Based on a combination of chemical synthesis, structural, magnetic, and redox characterization, and a newly developed theoretical pathway analysis, we can suggest a comprehensive explanation for this non‐intuitive behavior. These results are of interest for molecular spintronics, as naphthalene‐linked cobaltocenes can form wires on surfaces for potential spin‐only information transfer.

Small anion-assisted electrochemical potential splitting in a new series of bistriarylamine derivatives: organic mixed valency across a urea bridge and zwitterionization

We report the synthesis of a new bistriarylamine series having a urea bridge and investigate its mixed-valence (MV) states by electrochemical and spectroelectrochemical methods. We found that the supporting electrolytes had unusual effects on potential splitting during electrochemical behavior, in which a smaller counteranion thermodynamically stabilized a MV cation more substantially than did a bulky one. The effects contrary to those reported in conventional MV systems were explained by zwitterionization through hydrogen bonding between the urea bridge and the counteranions, increasing the electronic interactions between two triarylamino units. Furthermore, we clarified the intervalence charge transfer characteristics of the zwitterionic MV state.

Diaqua-β-octaferrocenyltetraphenylporphyrin: a multiredox-active and air-stable 16π non-aromatic species

Herein the synthesis and properties of the first β-octaferrocenyltetraphenylporphyrin, {TPPFc₈(H₂O)₂}, in its extraordinary stable and non-aromatic 16π form are reported, showing seven separate reversible redox events.

Synthesis and Characterisation of Linear and Towards Cyclic Diferrocenes with Alkynyl Spacers

Ferrocenediyl systems offer a motif that incorporates multiple functionality and redox-active centers, enabling these units to be used as molecular scaffolds in linear and cyclic compounds. Herein, we discuss a new modular methodology for the synthesis and incorporation of ferrocenediyl motifs within extended conjugated systems. We have synthesized a family of compounds featuring ferrocenediyl-ethynyl units with various para-substituted aromatic linkages. Extended linear, open-chain species have been isolated and understanding towards the analogous cyclic compounds gained. The new compounds have been probed using NMR, mass spectrometry, cyclic voltammetry and X-ray crystallography to gain further understanding of their structural and electronic properties.

Oxidatively stable ferrocenyl-π-bridge-titanocene D–π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states

Coordination of Cu^I into ^RCp₂Ti(C₂Fc)₂ compounds results in well-separated Fe^III/II couples and mixed-valent states with distinct spectroscopic signatures.

Formation of unexpected silicon- and disiloxane-bridged multiferrocenyl derivatives bearing Si-O-CH[double bond, length as m-dash]CH2 and Si-(CH2)2C(CH3)3 substituents via cleavage of tetrahydrofuran and trapping of its ring fragments

The formation of a family of silicon- and siloxane-bridged multiferrocenyl derivatives carrying different functional groups attached to silicon, including Fc₂(CH₃)₃C(CH₂)₂SiCH[double bond, length as m-dash]CH₂ (5), Fc₂(CH₂[double bond, length as m-dash]CH-O)SiCH[double bond, length as m-dash]CH₂ (6), Fc₂(OH)SiCH[double bond, length as m-dash]CH₂ (7), Fc₂(CH₂[double bond, length as m-dash]CH-O)Si-O-Si(O-CH[double bond, length as m-dash]CH₂)Fc₂ (8) and Fc₂(CH₂[double bond, length as m-dash]CH-O)Si-O-SiFc₃ (9) is described. Silyl vinyl ether molecules 6, 8 and 9 and the heteroleptic vinylsilane 5 resulted from the competing metathesis reaction of lithioferrocene (FcLi), CH₂[double bond, length as m-dash]CH-OLi or (CH₃)₃C(CH₂)₂Li with the corresponding multifunctional chlorosilane, Cl₃SiCH[double bond, length as m-dash]CH₂ or Cl₃Si-O-SiCl₃. The last two organolithium species have been likely formed in situ by fragmentation of the tetrahydrofuran solvent. Diferrocenylvinyloxyvinylsilane 6 is noteworthy since it represents a rare example of a redox-active silyl mononomer in which two different C[double bond, length as m-dash]C polymerisable groups are directly connected to silicon. The molecular structures of the silicon-containing multiferrocenyl species 5, 6, 8 and 9 have been investigated by single-crystal X-ray diffraction studies, demonstrating the capture and storage processes of two ring fragments resulting from the cleavage of cyclic THF in redox-active and stable crystalline organometallic compounds. From electrochemical studies we found that by changing the anion of the supporting electrolyte from [PF₆]^- to [B(C₆F₅)₄]^-, the redox behaviour of tetrametallic disiloxane 8 can be switched from a poorly resolved multistep redox process to four consecutive well-separated one-electron oxidations, corresponding to the sequential oxidation of the four ferrocenyl moieties.

Six States Switching of Redox-Active Molecular Tweezers by Three Orthogonal Stimuli

A six level molecular switch based on terpyridine(Ni-salphen)₂ tweezers and addressable by three orthogonal stimuli (metal coordination, redox reaction, and guest binding) is reported. By a metal coordination stimulus, the tweezers can be mechanically switched from an open "W"-shaped conformation to a closed "U"-shaped form. Theses two states can each be reversibly oxidized by the redox stimulus and bind to a pyrazine guest resulting in four additional states. All six states are stable and accessible by the right combination of stimuli and were studied by NMR, XRD, EPR spectroscopy, and DFT calculations. The combination of the supramolecular concepts of mechanical motion and guest binding with the redox noninnocent and valence tautomerism properties of Ni-salphen complexes added two new dimensions to a mechanical switch.

Ferrocene- and Biferrocene-Containing Macrocycles towards Single-Molecule Electronics

Cyclic multiredox centered systems are currently of great interest, with new compounds being reported and developments made in understanding their behavior. Efficient, elegant, and high‐yielding (for macrocyclic species) synthetic routes to two novel alkynyl‐conjugated multiple ferrocene‐ and biferrocene‐containing cyclic compounds are presented. The electronic interactions between the individual ferrocene units have been investigated through electrochemistry, spectroelectrochemistry, density functional theory (DFT), and crystallography to understand the effect of cyclization on the electronic properties and structure.