Design and Functions of Macromolecular Electron-Reservoir Complexes and Devices

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In this mini-review article the concept of electron reservoir materials developed in the author’s laboratory is summarized. Starting from mono- and bimetallic electron reservoir complexes and their electronic and catalytic properties this concept extends to macromolecular electron-reservoir devices including metallodendrimers, metallopolymers and macromolecule-stabilized late transition-metal nanoparticles. The electronic switch in these devices is discussed as well as the structure-dependent possibilities to stabilize both utilized redox forms. Catalytic and energy-related applications involving redox metallomacromolecules and nanoparticles and their potential extension are briefly discussed.

Graphic Abstract

Giant ferrocene and pentamethylferrocene-based dendrimers and polymers are typical electron reservoir macromolecules that have been isolated in both oxidized and reduced forms with the ability to carry a large flow of electrons at about the same potential. The dramatic structural and size changes of the metallomacromolecules upon redox switch is reminiscent of molecular machines.


Organometallic chemistry has proven to be a formidable source of activation at metal centers that now finds extension to heterogeneous catalysis with single-atom catalysis. Yet the basis of metal activation relies on the suitability of oxidation states and choice of ligands, so that addition or removal of an electron dramatically switches on and off a myriad of properties as pointed out by Taube [1] .

A major drawback in such studies, however, is that it is quite difficult to find species that are stable and isolable under both their reduced and oxidized forms. Therefore the seminal finding of stable 19-electron complexes was of interest first because of the possibility to obtain a couple of stable redox species, but also because the reduced form was a strong reducing agent. The progress of this chemistry finds a renewed potential of properties and applications upon extension of this electron reservoir concept to the macromolecular stage. These aspects are the subject of the present mini-review from mono- and binuclear complexes including the applications to metallomacromolecules including metallodendrimers, metallopolymers and macromolecule-based nanoparticles.

Concept and Definition of Electron Reservoir Complexes

Electron transfer processes are of primary importance in chemistry [1], biology [2] and physics [3]. They rely on materials that must withstand stability in at least two oxidation states. Transition metal species are especially suitable as building blocks, because transitions between their metal orbitals involve low energy accessible with visible light and modest redox potentials [4]. Inorganic and organometallic transition metal complexes can thus be designed as redox active systems upon suitable choice of ligands [5]. Their incorporation into macromolecular devices is desired for engineering purpose, however. This is the case in biology whereby the redox function of metalloenzymes is intimately connected to the protein environment [6] and in battery technology whereby ultrafast charging/discharging in the composites must occur without fatigue at constant cell voltage [7].

Electron-reservoir complexes have been proposed whereby redox systems comprise at least two robust redox forms, and the reduced form is a strong reducing agent [8,9,10]. The structure of these electron-reservoir complexes is of sandwich type in which a late transition metal is protected by the cyclic ligands. First the complexes [FeI5-C5R5)(η6-C6Me6] (1, R = H; 2, R = Me) were proposed [8,9,10], because they were the most electron-rich neutral molecules based on the very low values of their ionization potentials measured by HeI photoelectron spectroscopy, (4.221 eV for 2) [11] and, correlatively, low value of their FeII/I redox potentials [12, 13]. In these complexes, the iron center has 19 valence electrons with high metal character (80%) for the antibonding e*1 orbital [14] (Fig. 1). Bimetallic fulvalene complexes with a related structure are also known [15].

Fig. 1

Mononuclear electron-reservoir complexes.

Other electron-rich neutral complexes with slightly less negative redox potentials than this iron sandwich series are the 19-electron complexes [CoII5-C5R5) (3, R = H; 4, R = Me) [16,17,18] (although in the latter the 19th electron has only about 42% metal character[16]) and the 20-electron complex [Fe06-C6Me6)2], 5 [19, 20].

Properties and Functions of Electron-Reservoir Complexes

Simple mononuclear electron-reservoir complexes perform several functions related to their properties of redox robustness and strong reducing power of the reduced form [9]. The prototypal complex currently used is 1. Stoichiometric electron transfer involved reduction of dioxygen to superoxide radical anion [21] involving caged intermediates [22] that were further used to deprotonate precursors of N-heterocyclic carbene (NHC) [23]. Tetracyanoquinodimethane (TCNQ) was reduced by 1 to its anion, and cationic organometallic complexes were reduced to neutral derivatives [24]. In the case of C60, reduction to its mono, bi, or trianion was shown to depend on the stoichiometry, one, two or three of the complex 1 vs. C60 [25]. A large variety of cationic and neutral organic and inorganic substrates with lower reduction potentials than the oxidation potential of 1 can be stoichiometrically and cleanly reduced. When the redox potential of the substrate is more negative than the oxidation potential of 1, reduction is also possible, if it is followed by a fast reaction. This is for instance the case of CO2 that is reduced to carbonate and CO in spite of the large potential difference between the oxidation potential of 1 and the potential of the irreversible reduction of CO2 [26]. The latter reaction is driven by the fast reaction of the radical anion \( {\text{CO}}_{2}^{ - } \) [27].

Catalytic reactions of two types can be conducted with thermally stable electron-reservoir complexes such as 1: electron-transfer chain catalysis and redox catalysis. Electron-transfer chain (ETC) catalysis usually does not involve an overall redox change from the substrate to the products: this is the case for nucleophilic substitution (SRN1) in arene chemistry [28], ligand substitution [29] and many other reactions in organometallic chemistry [30] and atom transfer reactions in inorganic chemistry [31]. In these reactions, the catalyst is the electron carried either by an electrode [32] or by an electron reservoir compound [10]. The latter must have a redox potential negative (cathodic) enough in order to induce electron transfer onto the substrate in the initiation step [10]. A library of electron-reservoir complexes was designed to transfer electrons to substrates at various potentials to bring about reactions selectivity as demonstrated in fulvalene-transition–metal-carbonyl complexes [33]. Reactions can analogously be catalyzed by an electron hole carried by an anode or a compound reservoir of electron hole, i.e. a monoelectronic oxidant. For instance the dication of 2 bears 17 valence electrons on the central iron with a redox potential of 0.92 V vs. Cp2Fe+/0 in liquid SO2 and behave as an electron hole to catalyze various reactions [34].

According to Savéant, the definition of redox catalysis is the catalysis of redox reactions by various molecular compounds or materials [10, 35]. These reactions are broadly encountered in biological [36], photochemical [37] and energy-related systems [38], industrial processes [39] and electrochemistry [35]. An example is provided for the redox catalysis of reduction of nitrites and nitrates to ammonia in water by an electron-reservoir compound derived from 1. The water-soluble carboxylate [FeI5-C5H5CO2)(η6-C6Me6] [Na], 6, was generated upon cathodic or Na/Hg reduction of the carboxylic acid [FeII5-C5H5CO2H)(η6-C6Me6][PF6], 7, Fig. 2) in basic aqueous medium or in THF respectively.

Fig. 2

Functional organoiron electron-reservoir complexes

In the electrochemical cell, this carboxylate catalyzes cathodic nitrite and nitrate reduction to ammonia; whereas, in the absence of catalyst, water is reduced at the cathode, because nitrate is not electroactive under these conditions [40]. Kinetic studies using a variety of water-soluble iron sandwich complexes of this family [41] demonstrated that the mechanism involved interaction of the nitrate or nitrite oxygen atom onto the partly decoordinated paramagnetic (17-electron FeI) iron complex during the catalytic process [42]. This subject is a matter of challenge in modern times in view of soil denitrification [43].

Another interest of the electron-reservoir compounds is their ability to serve as references in electrochemistry for the determination of standard redox potentials. The current IUPAC reference, ferrocene, is generally used, but its redox potential varies with the nature of the solvent, because nucleophilic solvents interact with the Fe(III) center of the ferricinium cation. In the electron reservoir family many of the complexes possess permethylated cyclic ligands, so that the approach of substrates to the metal center is not possible due to the steric inhibition of the methyl group. In addition, a variety of fully methylated compounds including decamethyl ferrocene, decamethyl cobaltocene and complex 2 have been designed in order to offer several possibilities to use a redox system that does not perturb the substrate under study by overlapping redox waves [44,45,46].

Dendritic Electron Reservoir Complexes

Metallodendrimers that contain redox active termini are of interest for a variety of functions of electron-reservoir complexes including redox recognition and sensing, catalysis and molecular batteries. Dendrimers were pioneered with organic frameworks in the early 1980s [47,48,49,50] and the introduction of their decoration with peripheral redox groups around small dendrimers appeared in the following decade in particular with Ru–Os polypyridine [51] and electron-reservoir systems of the [CpFe(η6-arene]+ type [52]. The latter family resulted from an original strategy of iteration reactions involving a series of deprotonation-allylation sequences of polymethylarene iron complexes [53]. Subsequent dendrimer construction from mesitylene was based on 1 → 3 branching connectivity of the tethers, often followed by “click” (CuAAC) reaction [54] (Fig. 3). This type of triple-branching connectivity was pioneered by Newkome with his 27-arborol [47, 50].

Fig. 3

Planar representation of the structures of “click” (1,2,3-triazolylferrocenyl dendrimers 9-Fc, 27-Fc, 81-Fc and 243-Fc [54].

Ferrocene is the easiest and most flexible redox system for dendrimer decoration, and a large number of ferrocene-terminated dendrimers are known [55, 56]. Loading of large dendrimers with 1 → 3 connectivity was achieved with ferrocene and pentamethylferrocene up to the 7th generation containing a theoretical number of 19,083 termini and an actual number around 15,000 ferrocenyl groups (Eq. 1) [57, 58].

Eq. 1

Electrochrome metallodendritic system based on pentamethylferrocene-terminated dendrimers of the 7th generation. Orange metallodendrimer with a theoretical number of 19,683 pentamethylferrocenyl termini (actual number around 15,000 termini) oxidized anodically or using ferricinium hexafluorophosphate to the green pentamethylferricinium dendrimer; back reduction to the Fe(II) form was carried out cathodically or using decamethylferrocene [58].

At this stage the plots obtained in AFM were heterogeneously distributed, but until the 5th generation containing a theoretical number of 2187 ferrocene termini (actually around 2000 ferrocene groups), the AFM plots were homogeneously distributed. Each plot represented a package of dendrimers in the same way as nanoparticles containing a package of atoms. This signified that dendrimers behaved as atoms and could be considered as quantized. Although ferricinium suffers from some instability in solution, especially in air, pentamethylferrocene is well protected by the cyclopentadienyl permethylation, and therefore pentamethylferricinium derivatives can be used for instance in molecular batteries. The orange metallodendrimer with about 2000 pentamethylferrocene termini was reversibly oxidized by a ferricinium salt to its dark-blue pentamethylferricinium metallodendrimer analogue that was reduced back to the ferrocene form upon reaction with decamethylferrocene. AFM measurements indicated that the size increase upon oxidation was from 4.5 ± 0.4 nm in the neutral Fe(II) form to 6.5 ± 0.6 nm in the polycationic Fe(III) form, i.e. of the order of 50% mostly due to charge repulsion between the cationic Fe(III) termini (Fig. 4). In cyclic voltammetry, the redox process appears fully chemically and electrochemically reversible at the electrochemical time scale of about 0.1 s, which means that the structural transformations upon redox change are faster than the time scale [58]. This fast giant redox breathing-type transformation recalls that of molecular lungs [59] and may find use in the context of molecular batteries [60] (Eq. 2).

Eq. 2

AFM shows reversible size expension upon redox change of about 50% for the G5 ferrocene dendrimer [58].

Not only stable robust systems with pentamethylferrocene-terminated dendrimers were designed, but also analogues with other more electron-rich electron-reservoir systems, such as cobaltocene [61] and the robust [FeCp(η6-C6Me6] termini [62]. In these cationic metallodendrimers, the reversibility was preserved, but significant adsorption was also observed by cyclic voltammetry.

Dendritic ferrocene and biferrocene complexes of various generations have been synthesized in particular by “click” (CuAAC) chemistry upon reaction of azido-terminated dendrimers with ethynylferrocene or ethynylbiferrocene [63,64,65]. Click chemistry is indeed a practical synthetic route for the extension of dendrimer tethers and construction that also allows the introduction of useful intradendritic triazole groups [66,67,68,69,70,71,72]. Coordination of these intradendritic triazoles by transition-metal cations is very productive in catalysis upon reduction of these coordinated metal cations into catalytically very efficient transition-metal nanoparticles [73,74,75,76,77,78]. With ferrocene-terminated dendrimers of various generations, Au(III) cations coordinated to the intradendritic triazoles are reduced by the terminal ferrocene groups of the 9, 27, 81 and 243-Fc dendrimers (Fc = ferrocenyl) by bringing about a flow of electrons reducing Au(III) to Au(0) nanoparticles (AuNPs) [54, 79]. The ferricinium dendrimer-stabilized AuNPs are reduced by cobaltocene back to ferrocene dendrimer-weakly stabilized AuNPs. In this case the metallodendrimers serve as electron-reservoir macromolecules transferring a flow of a large number of electrons back and forth at about the same redox potential. These AuNPs formed are larger than those produced using NaBH4 as a reductant and are between 30 and 47 nm, which is adequate for nanomedicine.

Abd-El-Aziz et al. have also recently developed iron-sandwich based metallopolymers that present a variety of interesting and applicable properties [80,81,82,83,84].

Polymeric Electron Reservoir Complexes

Ferrocene-containing metallopolymers have long been a popular field [85,86,87,88,89,90,91,92] especially in the last decades with Masters’ leading developments in ring-opening polymerization (ROP) [89,90,91,92]. Recently, studies have focused on the biomedical potential of supramolecular properties of the ferrocene redox switch in ferrocene polymers, and applications have been proposed [79, 93,94,95,96,97,98,99,100,101,102]. Our interest for ferrocene-containing polymers started a decade ago with the “click” (CuAAC) reaction between poly(p-azidomethyl)-substituted derivatives with ethynylferrocene yielding 1,2,3-triazolylferrocene-containing dendronized polymers [103]. Dendronized polymers [103], which are an important class of polymers [104], intermediate between dendrimers and polymers, were pioneered in the mid’90 s by the groups of Schlüter [105] and Percec [106]. Recently we have developed a class of living polymers and dendronized polymers containing ferrocene groups. Using the illustrious Bard-Anson’s empirical equation, the number of ferrocene groups in these metallomacromolecules was estimated [107]. This method is marred by the adsorption onto the electrode, but it can be applied to relatively small polymers with good results, especially if a suitable solvent such as DMF is used for the electrochemical experiments. The living polymerization strategy is of great interest to synthesize block co-polymers for instance using ring opening metathesis polymerization (ROMP) [108,109,110,111]. We have exploited this technique using Grubb’s 3rd generation olefin metathesis catalyst [112,113,114] for the synthesis of block metallopolymers containing ferrocene [115] pentamethylferrocene [116], cobalticinium [117] or [FeCp(C6Me6)]+ groups [118] or a mixture of two, [117,118,119] three [120, 121] or even four distinct such metal sandwich groups [122] (Fig. 4).

Fig. 4

Tetrablock metallopolymer electrochrome. Living ROMP polymerization was conducted using Grubb’s 3rd generation olefin metathesis catalyst in order to synthesize these polyblock metallopolymer electron-reservoir electrochrome systems [122].

The introduction of a tetraethylene glycol block improves the polymer solubilities [115] and dendronized polymers with metallocene termini at the end of the dendronic tethers allows to stabilize Au and Ag NPs as indicated above with the dendrimers [123,124,125].

In sum, ferrocene polymers and dendronic polymers serve as electron reservoirs only for the reduction of the strong oxidants Au(III) and Ag(I) to NPs due to the weak electron donor properties of the ferrocene group. On the other hand cobaltocene polymers might be of interest for battery applications together with ferrocene polymers, because of the advantages of fast redox kinetics of the first-row late transition-metal metallocenes and stability of both redox forms [126]. Recent work on ferrocene-based polymers has been discussed [79, 126] and may find applications with future improvements.

Nanoparticles as Electron Reservoirs

Ferrocene itself, as a modest electron-reservoir compound, reduces Au(III) and Ag(I) to relatively large NPs in processes that therefore transform single electron transfer between Fe(II) and Fe(III) into multiple electron NP sources [127], an important biologically relevant paradigm [128]. Likewise, this property has been extended to ferrocene-containing polymers [129]. The multiple electron-reservoir properties of transition metal NPs is illustrated not only in their multi-electron electrochemistry (up to 15 single-electron waves for well-defined Au nanoclusters due to Coulomb blockade) [130, 131] but also in their multielectronic catalytic properties such as for the reduction of nitroarenes by NaBH4 to aminoarene in water [132], an hexaelectronic process [133,134,135].

Interestingly, NaBH4 is able not only to reduce Au(III) to Au(0), but also to stabilize AuNPs in water by itself in the absence of other stabilizing agent, because NaB(OH)4 formed polymerizes in water forming an inorganic polyborate layer around the AuNPs. This nanomaterial is an excellent catalyst for p-nitrophenol reduction by NaBH4 to p-aminophenol [136]. Inorganic and organometallic hydrides or hydride-rich compounds are hydride reservoirs, i.e. also electron reservoir materials, since a hydride carries two electrons [137]. Indeed single electron transfer using NaBH4 has been well characterized [138].

Ferrocene-terminated dendrimers of various generations transfer a flow of electrons to Au(III) forming Au nanoparticles that are stabilized by the formed ferricinium dendrimers. These Fe(III) dendrimers are reduced back to ferrocene dendrimers by the electron-reservoir cobaltocene as shown in Scheme 1 [54].

Scheme 1

Synthesis of AuNP-243-Fc+Cl in THF/water and its reaction with cobaltocene [54].

Another simple multi-electronic example of NP sources is the catalysis by transition-metal NPs of the hydrolysis of NaBH4 and ammonia borane to H2 via reductive elimination of bis-hydrido NP(H)2 intermediate species, a mechanism supported inter alia by significant kinetic isotope effects [139,140,141].

Conclusion and Prospects

The concept of electron reservoirs started with simple mononuclear complexes that allowed one to efficiently carry out a variety of stoichiometric and catalytic electron transfer reactions, so that these species behaved as molecular electrodes [142]. Then it moved to electronically flexible mixed valency with binuclear systems illustrating the crucial role of the bridging hydrocarbon ligand in the electronic structure [143]. With the advent of small, then larger and finally giant dendrimers, flows of electrons were demonstrated upon redox change of the iron sandwich termini. The slight instability of 17-electron ferricinium and 19-electron [FeCp(arene)] [144] in solution forced designing more robust ferrocene and [FeCp(arene)] dendrimer systems in which at least one of the cyclic ligand was permethylated in order to isolate the odd-electron state [145]. The same type of problem was encountered in ferrocene- and other metal-sandwich-containing polymers, and it was solved in the same way. Applications have been disclosed with both types of macromolecules to generate AuNPs and AgNPs from metal-sandwich termini of these macromolecules [146]. The interplay between these metallomacromolecules and transition metal nanoparticles offers perspectives of generating and using various transition metal NPs for a number of nanocatalyzed reactions [71, 147,148,149,150]. Nanocatalysts can be engineered even with dendrons of modest size that contain the very useful 1,2,3-triazolyl ligand [150,151,152] provided by click chemistry [71, 153]. Finally, other outstanding redox-active metallodendrimers or metallomacromolecules, especially those designed by Newkome [154, 155] and Yamamoto [156, 157] should prove of great interest as electronic, catalytic and sensing devices. Optimizing the dendronic or other macromolecular design in terms of synthetic simplicity, stability and catalytic efficiency should boost the fields of nanocatalysis, nanosensing and energy conversion.


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Invaluable contributions from the co-authors involved in the original works are cited in the references. Financial support from the University of Bordeaux, the Centre National de la Recherche Scientifique (CNRS), the Institut Universitaire de France (IUF), the European Community (FP7), L’Oréal Research and Innovation, and the Chinese Scientific Council (CSC) is gratefully acknowledged.

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Astruc, D. Design and Functions of Macromolecular Electron-Reservoir Complexes and Devices. J Inorg Organomet Polym 30, 111–120 (2020) doi:10.1007/s10904-019-01412-9

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