Organometallic electron reservoirs. Part 36. Binuclear electron reservoir complexes. Syntheses, reactivity, and electronic structure of the 37- and 38-electron fulvalene complexes

Electron Delocalization and Transfer Across Polyoxometalates-Based Subnanomaterials in Catalytic Reactions

Due to their identical building blocks and high surface-to-volume ratio, subnanomaterials exhibit significant properties compared to their bulk nanomaterial counterparts. The interactions between these building blocks can result in either equal or unequal sharing of electrons, leading to electron transfer in heterojunctions or electron delocalization within symmetric structures. Clusters, possessing electronic properties akin to atoms, can serve as reservoirs of electrons to stabilize crucial intermediates in catalytic reactions. This perspective provides a novel understanding of well-defined subnanomaterials with distinct architectures, such as cluster-based constructions and co-assembled heterojunctions, emphasizing the relationship between electronic structures and catalytic properties. The objective is to provide novel perspectives on the realm of subnanomaterials and cluster-based architectures.

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.

On the Roles of Electron Transfer in Catalysis by Nanoclusters and Nanoparticles

Electron transfer plays a major role in chemical reactions and processes, and this is particularly true of catalysis by nanomaterials. The advent of metal nanoparticle (NP) catalysts, recently including atomically precise nanoclusters (NCs) as parts of nanocatalyst devices has brought increased control of the relationship between NP and NC structures and their catalytic functions. Consequently, the molecular definition of these new nanocatalysts has allowed a better understanding and management of various kinds of electron transfer involved in the catalytic processes. This Minireview brings a chemist's view of several major aspects of electron-transfer functions concerning NPs and NCs in catalytic processes. Particular focus concerns the role of NPs and NCs as electron reservoirs and light-induced antenna in catalytic processes from H₂ generation to more complex reactions and sustainable energy production.

Ferrocenyl-terminated polyphenylene-type "click" dendrimers as supports for efficient gold and palladium nanocatalysis

Although dendrimer supports have been known as key parts of nanocatalysts, the capability of rigid dendrimers for this function has not yet been reported. Here, the study is focused on ferrocenylmethylenetriazolyl-terminated dendrimers (FcMTPD) as supports of remarkably efficient nanogold and nanopalladium catalysts. A biphasic system is elaborated to evaluate the catalytic activity of FcMTPD-supported Au and Pd nanoparticles (NPs) for the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ at 20 °C, and FcMTPD-supported PdNPs are found to be the best nanocatalysts with a rate constant k_app = 7.8 × 10^-2 s^-1. Excellent catalytic results are also obtained in this reaction for FcMTPD-supported AuNPs with a rate constant k_app = 5.6 × 10^-2 s^-1. For both Pd NPs and AuNPs, the kinetic results are shown to strongly depend on the method of preparation of these NPs that influences the NP size and thus their catalytic efficiency. The FcMTPD-stabilized PdNPs are easily recovered and reused at least 13 times, and their catalytic performance displays only a slight decrease during the first seven runs.

Multiple applications of polymers containing electron-reservoir metal-sandwich complexes

Ferrocene-containing polymers have been investigated for more than six decades, and more recently modern synthetic methods have allowed the fabrication of precise polymers that contain a variety of transition-metal complexes. Trends are now oriented towards applications, such as optics, energy conversion and storage, electrochemistry, magnetics, electric conductors and biomedicine. Metal-sandwich complexes such as those of ferrocene type and other related complexes that present redox-robust groups in polymers, i.e. that are isolable in both their oxidized and reduced forms, are of particular interest, because it is possible to address them using electronic or photonic redox stimuli for application. Our research groups have called such complexes Electron-Reservoirs and introduced them in the main chain or in the side chains of well-defined polymers. For instance, polymers with ferrocene in the main chain or in the side chain are oxidized to stable polycationic polyelectrolytes only if ferrocene is part of a biferrocene unit, because biferrocene oxidation leads to the biferrocenium cation that is stabilized by the mixed valency. Then a group of several redox-robust iron sandwich complexes were fabricated and incorporated in precise polymers including multi-block copolymers whose controlled synthesis and block incorporation was achieved for instance using ring-opening-metathesis polymerization. Applications of this family of Electron-Reservoir-containing polymers includes electrochemically induced derivatization of electrodes by decorating them with these polymers, molecular recognition and redox sensing, electrochromics with multiple colours, generation of gold and silver nanoparticles of various size by reduction of gold(iii) and silver(i) precursors and their use for nanocatalysis towards depollution and biomedicine.

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

Stabilization of a new nanocomposite family by reduction of gold nanoclusters with electron-reservoir complexes

Four stable 19-electron Fe and Co sandwich electron-reservoirs and three hydride reservoir complexes reduce the prototypal nanocluster Au25(SR)18 (R = CH2CH2Ph) leading to a largely increased kinetic stabilization of the anionic nanocluster form compared to previously known nanocluster anion salts; these salts represent new and robust nanocomposite anionic gold clusters with 18-electron sandwich complexes as counter cations.

High catalytic activity of Rh nanoparticles generated from cobaltocene and RhCl3 in aqueous solution

Cobalticinium chloride-stabilized RhNPs are very efficient catalysts for hydrolysis of H₃N-BH₃, reduction of 4-NP, hydrogenation of benzene and transfer hydrogenation.

Diblock metallocopolymers containing various iron sandwich complexes: living ROMP synthesis and selective reversible oxidation

New diblock copolymers containing two iron-sandwich complexes in the side chain have been synthesized and oxidized to obtain mixed-valent Fe^II–Fe^III copolymers.

Synthesis and redox activity of "clicked" triazolylbiferrocenyl polymers, network encapsulation of gold and silver nanoparticles and anion sensing

The design of redox-robust polymers is called for in view of interactions with nanoparticles and surfaces toward applications in nanonetwork design, sensing, and catalysis. Redox-robust triazolylbiferrocenyl (trzBiFc) polymers have been synthesized with the organometallic group in the side chain by ring-opening metathesis polymerization using Grubbs-III catalyst or radical polymerization and with the organometallic group in the main chain by Cu(I) azide alkyne cycloaddition (CuAAC) catalyzed by [Cu(I)(hexabenzyltren)]Br. Oxidation of the trzBiFc polymers with ferricenium hexafluorophosphate yields the stable 35-electron class-II mixed-valent biferrocenium polymer. Oxidation of these polymers with Au(III) or Ag(I) gives nanosnake-shaped networks (observed by transmission electron microscopy and atomic force microscopy) of this mixed-valent Fe(II)Fe(III) polymer with encapsulated metal nanoparticles (NPs) when the organoiron group is located on the side chain. The factors that are suggested to be synergistically responsible for the NP stabilization and network formation are the polymer bulk, the trz coordination, the nearby cationic charge of trzBiFc, and the inter-BiFc distance. For instance, reduction of such an oxidized trzBiFc-AuNP polymer to the neutral trzBiFc-AuNP polymer with NaBH4 destroys the network, and the product flocculates. The polymers easily provide modified electrodes that sense, via the oxidized Fe(II)Fe(III) and Fe(III)Fe(III) polymer states, respectively, ATP(2-) via the outer ferrocenyl units of the polymer and Pd(II) via the inner Fc units; this recognition works well in dichloromethane, but also to a lesser extent in water with NaCl as the electrolyte.

Metalation of polyamine dendrimers with ethynylcobalticenium for the construction of mono- and heterobimetallic polycationic metallodendrimers

The introduction of robust redox groups at the periphery of common amine-terminated dendrimers is of interest in the design of dendritic nanobatteries, sensors, and redox catalysts. Here we are applying the recently discovered uncatalyzed hydroamination of ethynylcobalticenium, a mild "green" reaction that quantitatively yields trans-enamines without the formation of any byproduct, to functionalize dendrimers that are terminated with primary or secondary amino groups. Poly(amido amine) (PAMAM) dendrimers terminated by primary amino groups and arene-centered dendrimers terminated by secondary amino groups yield dendrimers that contain up to 81 trans-enamine-cobalticenium termini using this reaction. The hydroamination reaction was also conducted with dendrimers that contained ferrocenylmethylamino groups, which yielded dendrimers that contained both ferrocenyl and cobalticenium termini. The size of the dendrimers was investigated using both dynamic light scattering and diffusion-ordered spectroscopy (DOSY) (1)H NMR spectroscopy, and the number of electrons involved in heterogeneous multielectron transfers at electrodes was searched by cyclic voltammetry. The latter works well up to the 27-branch dendrimer, whereas the 81-dendrimer yielded a result in an excess amount (110 electrons) owing to adsorption onto the cathode that becomes all the more significant as the metallodendrimer size increases.

Click dendrimers and triazole-related aspects: catalysts, mechanism, synthesis, and functions. A bridge between dendritic architectures and nanomaterials

One of the primary recent improvements in molecular chemistry is the now decade-old concept of click chemistry. Typically performed as copper-catalyzed azide-alkyne (CuAAC) Huisgen-type 1,3-cycloadditions, this reaction has many applications in biomedicine and materials science. The application of this chemistry in dendrimer synthesis beyond the zeroth generation and in nanoparticle functionalization requires stoichiometric use of the most common click catalyst, CuSO(4)·5H(2)O with sodium ascorbate. Efforts to develop milder reaction conditions for these substrates have led to the design of polydentate nitrogen ligands. Along these lines, we have described a new, efficient, practical, and easy-to-synthesize catalytic complex, [Cu(I)(hexabenzyltren)]Br, 1 [tren = tris(2-aminoethyl)amine], for the synthesis of relatively large dendrimers and functional gold nanoparticles (AuNPs). This efficient catalyst can be used alone in 0.1% mol amounts for nondendritic click reactions or with the sodium-ascorbate additive, which inhibits aerobic catalyst oxidation. Alternatively, catalytic quantities of the air-stable compounds hexabenzyltren and CuBr added to the click reaction medium can provide analogously satisfactory results. Based on this catalyst as a core, we have also designed and synthesized analogous Cu(I)-centered dendritic catalysts that are much less air-sensitive than 1 and are soluble in organic solvents or in water (depending on the nature of the terminal groups). These multivalent catalysts facilitate efficient click chemistry and exert positive dendritic effects that mimic enzyme activity. We propose a monometallic CuAAC click mechanism for this process. Although the primary use of click chemistry with dendrimers has been to decorate dendrimers with a large number of molecules for medicinal or materials purposes, we are specifically interested in the formation of intradendritic [1,2,3]-triazole heterocycles that coordinate to transition-metal ions via their nitrogen atoms. We describe applications including molecular recognition of anions and cations and the stabilization of transition metal nanoparticles according to a principle pioneered by Crooks with poly(amido amine) (PAMAM) dendrimers, and in particular, the control of structural and reactivity parameters in which the intradendritic [1,2,3]-triazoles and peripheral tripodal tri(ethylene glycol) termini play key roles in the click-dendrimer mediated synthesis and stabilization of gold nanoparticles (AuNPs). By varying these parameters, we have stabilized water-soluble, weakly liganded AuNPs between 1.8 and 50 nm in size and have shown large differences in behavior between AuNPs and PdNPs. Overall, the new catalyst design and the possibilities of click dendrimer chemistry introduce a bridge between dendritic architectures and the world of nanomaterials for multiple applications.

Electron-transfer processes in dendrimers and their implication in biology, catalysis, sensing and nanotechnology

The extraordinary development of the design and synthesis of dendrimers has allowed scientists to locate redox sites at precise positions (core, focal points, branching points, termini, cavities) of these perfectly defined macromolecules, which have generation-controlled sizes and topologies matching those of biomolecules. Redox-dendrimer engineering has led to fine modelling studies of electron-transfer metalloproteins, in which the branches of the dendrimers hinder access to the active site in a manner reminiscent of that of the protein. It has also enabled the construction of remarkable catalysts, sensors and printboards, including by sophisticated design of the interface between redox dendrimers and solid-state devices - for example by functionalizing electrodes and other surfaces. Electron-transfer processes between dendrimers and a variety of other molecules hold promising applications in diverse areas that range from bio-engineering to sensing, catalysis and energy materials.

Branching the electron-reservoir complex [Fe(eta(5)-C5H5)(eta(6)-C6Me6)][PF6] onto large dendrimers: "click", amide, and ionic bonds

Several strategies have been used to functionalize 1,3,5-trisubstituted arene-cored dendrimers with the organometallic electron-reservoir moiety [Fe(eta(5)-C(5)H(5))(eta(6)-C(6)Me(6))](+), 1, to provide dendritic multielectron reservoirs. They all start from the carboxylic acid [Fe(eta(5)-C(5)H(4)COOH)(eta(6)-C(6)Me(6))][PF(6)], 2, or its acyl chloride derivative [Fe(eta(5)-C(5)H(4)COCl)(eta(6)-C(6)Me(6))][PF(6)], 3. For this purpose, a series of new polyamine dendrimers from G(0) to G(2) with 1--> 3 C connectivity of the branching to the core have been synthesized. Amide, "click" and ionic ammonium carboxylate linkage successfully provided G(0), G(1), and G(2) metallodendrimers with 9, 27, and 81 cationic terminal organoiron groups respectively. Further construction of large metallodendrimers up to G(7) with approximately 14 000 organoiron termini was only possible by combining amide, "click", and tether lengthening strategies to avoid steric bulk at the dendrimer periphery. Reduction of the 18-electron Fe(II) metallodendrimers, exemplified by a G(4)-DAB-64-Fe(II) complex, was achieved exergonically using the parent electron-reservoir complex [Fe(eta(5)-C(5)H(5))(eta(6)-C(6)Me(6))], 1a, at -30 degrees C in MeCN, which allowed further reduction of 64 equiv of C(60) to C(60)(*-) using the 19-electron Fe(I) metallodendrimer.

Dendritic molecular electrochromic batteries based on redox-robust metallocenes

In this Concept article, we summarize and discuss recent reports on dendritic molecular electrochromic batteries. Giant dendrimers containing 3(n+2) terminal tethers (n = generation number) and terminated by first-raw late-transition-metal metallocenes, permethyl metallocenes and other sandwich complexes were shown to be redox robust. Indeed, they can be oxidized and reduced without decomposition and exist under two stable oxidation states (Fe(III/II), Co(III/II)). Thus, a pre-determined number of electrons (up to 14,000) per dendrimer can be exchanged. Cyclic voltammetry showed a remarkable complete reversibility even up to 14,000 Fe and Co termini in metallodendrimers, indicating fast electron hoping among the redox sites and between dendrimers on a carbon surface covered by arylcarboxylate groups. The dendrimer sizes were measured by dynamic light scattering in solution and by AFM (subsequent to flattening in the condensed state also indicating that these metallodendrimers aggregate to form discrete nanoparticles of dendrimers, as atoms do). The metallodendrimer size varies considerably between the two redox forms due to tether extension of the cationic dendrimers upon oxidation, and a breathing mechanism was shown by atomic and electric force microscopy (AFM and EFM). When the redox potential is very negative, the reduced form is an electron-reservoir system that can deliver a large number of electrons per dendrimer to various reducible substrates. These systems are thus potential dendritic molecular batteries with two different colors for the two redox forms (electrochromic behavior).