Polyhedron xxx (2014) xxx–xxx

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Organoiron-mediated synthesis and redox activity of organoiron-containing dendrimers Didier Astruc ⇑, Yanlan Wang, Amalia Rapakousiou, Abdou Diallo, Rodrigue Djeda, Jaime Ruiz, Catia Ornelas ISM, Univ. Bordeaux, UMR CNRS 5255, 351 Cours de la Libération, 33405 Talence Cedex, France

a r t i c l e

i n f o

Article history: Received 27 February 2014 Accepted 2 April 2014 Available online xxxx Dedicated to Dr. Claude Lapinte, in recognition of his excellent contribution to organometallic chemistry and materials science.

a b s t r a c t In this micro-review, the concepts that have led to ligand activation and redox robustness with organoiron complexes in the authors’ laboratory are developed. It is shown how these activation modes have led to the construction of redox-active metallodendrimers including various types of mixed-valent complexes with application to redox recognition, sensing and controlled nanoparticle synthesis. Ó 2014 Published by Elsevier Ltd.

Keywords: Iron Dendrimer Mixed valence Electrostatic effect Nanoparticle

1. Introduction Organoiron synthesis and redox activity of organoiron complexes are crucial to the development of organoiron materials with unusual properties, as demonstrated by Claude Lapinte and colleagues at the University of Rennes [1,2]. In this micro-review, we emphasize two key aspects of organoiron dendrimers that were essentially developed in our research group: (i) organoiron-mediated synthesis and (ii) redox properties and applications of organoiron dendrimers. These two fields are related, because organoiron activation depends on the metal oxidation state and in turn the redox properties of organoiron dendrimers induce new functions and applications of these nanomaterials. Since their seminal discovery by the groups of Newkome [3], Tomalia [4] and Denkewalter [5], dendrimers are a growing field of supramolecular nanochemistry [5–10] with multiple applications involving mostly sensors [11,12], energy-related nanomaterials [13–15], nanomedicine [16–20], and catalysis [20–22]. Their construction must be carefully designed using selective reactions, in particular involving the branching strategy [23], and their redox properties have

⇑ Corresponding author. Tel.: +33 540006271. E-mail address: [email protected] (D. Astruc).

specific connections with their biological, catalytic, and nanomaterials applications [24].

2. Redox and activation properties of organoiron sandwich complexes Besides the classic ferricenium/ferrocene redox system involving the relatively fragile and reactive 17-electron ferricenium cation, cationic organoiron sandwich complexes with the 18-electron electronic configuration are robust, which allows easy handling towards organic and organoiron syntheses [25]. Their most classic structures are the mixed ligand series [FeCp (g6-arene]+ with a large variety of arene ligands [26,27] including polymers [28] and dendrimers [29] and the symmetrical [Fe (g6-arene)2]2+ family [30] that is restricted to benzene and polymethylated arene derivatives. Both families are known with the three oxidation states, the [FeCp(g6-arene]2+/+/0 with the oxidation states Fe(III), Fe(II) and Fe(I) [27,31], and the series [Fe(g6-arene)2]2+/+/0 [32] with the oxidation states Fe(II), Fe(I) and Fe(0), although the two latter oxidation states are known only with the C6Me6 ligand. Although the [Fe(g6-arene)2]2+ family has yielded applications to aromatic and cyclohexadiene syntheses involving the different oxidation states indicated above [30,32],

http://dx.doi.org/10.1016/j.poly.2014.04.002 0277-5387/Ó 2014 Published by Elsevier Ltd.

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by far the most versatile and useful system has been the [FeCp(g6arene]0/+ series. The electron-rich neutral series provides ‘‘electron-reservoir’’ systems [33] that activate substrates by ligand substitution [34] or electron transfer [33], whereas the cationic 18electron complexes are powerful activators of deprotonation [35] (a principle that has been extended to the activation by CpCo+ [36]) and nucleophilic substitution [37] at the various arene ligands (Fig. 1).

Extension of this chemistry to the arene ligand in [FeCp(g6-pEtO-C6H4CH3)] [PF6], 8, easily available from [FeCp(g6-p-ClC6 H4CH3)] [PF6] by reaction with ethanol in the presence of t-BuOK, provided in good yield the very useful dendron p-HO-C6H4C (CH2–CH@CH2)3, 9, resulting from 8 steps in a single pot reaction [37a].

4. Dendrimer constructions 3. Organoiron mediated syntheses of dendritic cores and dendrons Both organoiron activation of arenes in the isostructural cationic 18-electron and neutral 19-electron configurations lead to benzylic C–H activation of the arene ligands in the mixed-sandwich complexes that allows further functionalization of these arene ligands leading to carbon–carbon or carbon-element bond formation. Such a reaction in the cationic 18-electron series requires the action of a base such as KOH or t-BuOK under ambient conditions, whereas the reaction of the 19-electron analogues requires simple contact with dioxygen or air at or below room temperature (Scheme 1). For single C–H activation (Scheme 1), the 19-electron complexes provide somewhat cleaner reactions, whereas for multiple C–H activation of benzylic hydrogens, the cationic route is preferred, because it can be conducted in situ. This cationic route is in fact so remarkable that, in the presence of an electrophile such as allyl bromide, it leads to 9 consecutive C–H activation reactions in situ under ambient conditions with the mesitylene complex, each deprotonation being followed by a nucleophilic substitution of the halide in the organic halide. This iterative arene activation reaction was first disclosed in 1979 with the complex [FeCp(g6-C6Me6)][PF6] and methyl iodide in the presence of t-BuOK and occurred spontaneously in about one minute to yield the hexaethylbenzene complex [38]. Later this reaction was also extended to allyl bromide and led to the extremely bulky dodecaallylbenzene complex upon 12 deprotonation-allylation sequences in situ [35b], whereas with pentamethylcobalticenium decasubstitution in the C5Me5 ligand in the presence of methyl iodide or benzyl bromide was easier because of the reduced congestion [36]. In 1979, however, dendrimer chemistry was not yet born, although these iteration reactions were premices as well as Vögtle’s iteration [39]. The CpFe+-induced nona-allylation of mesitylene in 1D was choosen, however, to develop dendrimer syntheses because of its easy completeness under ambient conditions, and the facile visible-light-induced arene exchange of the nonaallylated g6-arene ligand by mesitylene in 6 recycling the starting material in this way (Scheme 2).

+

Fe 1A

Rn

+n

+2

+n

Fe

Fe

Fe

1B

2A

2B

Fig. 1. Iron sandwich complexes that are useful for both redox processes and arene ligand activation (the counter anion of the salts usually is PF6 ): 1A is known with various Rn groups (n = 1: large variety of R; n = 1–6: R = Me) and undergoes nucleophilic reactions and deprotonation in benzylic position; 1B is stable with n = 0–2; in particular the redox system 1B+/0 is a redox system known as electron reservoir; the neutral 19-electron form reacts with O2 yielding H-atom abstraction; 2A undergoes hydride addition followed by another nucleophilic addition to form functional cyclohexadienne complexes; 2B is stable for n = 0–2 and for n = 0 the 20electron complex reacts with O2 with double H-atom abstraction to form an 18electron Fe(0) o-xylylene complex [25].

Further dendrimer construction was achieved upon hydrosilylation of the 9 double bonds of 7 by HSiCH2Me2Cl followed by 1 ? 3 connectivity by a Williamson reaction in the presence of the dendron, iodide and potassium carbonate (Scheme 3). This sequence was iterated 9 times, which allowed reaching the 9th dendrimer generation; this set of reaction sequences achieved the largest known dendrimers, although dendritic construction proceeded essentially inside the dendrimers due to peripheral steric bulk and backfolding of the termini after the 6th generation involving much longer reaction times and an increased number of defects [37b]. Other remarkable variations involved cross metathesis of the terminal dendrimer double bonds with acrylic acid and acrylates selectively yielding trans-olefins [40], or CuAAC ‘‘click’’ reactions achieved between the polyazido dendrimer with a propargylated dendron [41]. Both of these divergent dendrimer syntheses, the metathesis and ‘‘click’’ methods, were efficiently iterated yielding large dendrimers.

5. Organoiron dendrimers and their redox activity Dendrimers terminated by ferrocenyl groups are the prototype of organoiron-containing dendrimers [42–45]. They were shown to serve as biosensors for instance for NADPH using the Fe(II)/Fe(III) redox interchange with good electrocatalytic performances [45]. With amidoferrocenyl termini, dendrimers were shown early on to be excellent sensors for the redox recognition of oxo-anions such as H2PO-4 and HSO-4 with positive dendritic effects, i.e. better recognition as the dendrimer generation increases. As such, they proved to be excellent exoreceptors [45]. Again, this redox recognition was based on the Fe(II)/Fe(III) interchange, electrostatic interaction between the ferricenium cation and the recognized oxo-anion being one of the key sensing factors in synergy with the dendrimer topological effect [46]. With dendrimers terminated by the cationic redox groups [Fe(g5-C5Me5)(g6-C6H5NHR)] [PF6] and linked to the core by amino groups, the oxidation states that are involved in redox sensing are Fe(II) and Fe(I), because the latter is much more reasily accessible upon cathodic reduction than the far Fe(III) state due to the cationic charge [47]. This situation parallels that encountered with cobaltocenium-terminated dendrimers that are more easily reducible than the above cationic iron complexes to the 19-electron species [44b]. Giant ferrocenyl-terminated dendrimers were constructed with up to 15 000 ferrocenyl termini upon lengthening the terminal tethers in order to minimize bulk at the periphery (Fig. 2) as models of molecular batteries. Such large ferrocenyl dendrimers were shown to ‘‘breath’’ between their Fe(II) and larger (Fe(III) forms due to repulsion among charged redox-active termini, and this process was demonstrated using Atom Force Microscopy (AFM) and Electron Force Microscopy (EFM) [48]. Another interest of the redox activity of ferrocenyl-terminated dendrimers is their ability to reduce HAuCl4 to dendrimer-stabilized Au nanoparticles, a property that they share with other dendrimers [49].

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Scheme 1. Compared C–H activation in the 18e vs. 19e complex [FeCp(g6-toluene)] followed by C–C or C-element bond formation upon reaction with RX. This chemistry is general for the series of complexes [FeCp(g6-C6H6 nMen)]+/0 (n = 1–6; counter anion: PF6 ) [25–27a].

Scheme 2. One-pot nona-allylation of [FeCp(g6-mesitylene)][PF6], 1D, under ambient conditions followed by visible light-induced regeneration of the starting mesitylene complex 1D together with liberation of the nonaallylated dendritic arene core 7 [35].

6. Mixed-valent dendrimers and the stabilization of gold nanoparticles The oxidation of giant ferrocenyl dendrimers to their ferricenium analogue allowed to synthesize mixed-valent dendrimers upon mixing the ferrocenyl and ferricenium dendrimers , and therein the distribution of Fe(II) and Fe(III) termini was given by the statistics of a binomial law (Fig. 4) [48]. With diferrocenylsilyl-terminated such as 12 (or its diamagnetic precursor), cyclic voltammetry shows two well-separated ferrocenyl waves related to the electronically and electrostatically interacting ferrocenyl centers [50]. With small dendritic frameworks containing alternating ferrocenyl and pentamethylferrocenyl dendrimers, precise mixed-

valent [51] Fe(II)–Fe(III) dendrimers such as 13 were isolated as well as the all-Fe(II) and all-Fe(III) dendritic complexes, although the ferrocenyl and pentamethylferrocenyl groups were not interacting with one another (Fig. 6) [52]. In the star-shaped hexa(ethynylferrocenyl)benzene 14a (R = H), the three distinct two-electron waves observed in cyclic voltammetry essentially result from strong electrostatic interaction among redox centers in the presence of a perfluorinated tetraaryl borate electrolyte salt (Fig. 7) [53], a situation completely minimized (but strictly not totally absent) in dendrimers terminated by isolated redox-active termini [54]. In 1,2,3-triazolylbiferrocenyl-terminated dendrimers, however, the two ferrocenyl units of the biferrocenyl groups are interacting with each other, which allowed isolating class-II mixed-valent

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Scheme 3. Reaction sequence allowing dendrimer growth from the zeroth generation to the first generation dendrimer (11) containing 27 allyl branches. The sequence was repeated, and this dendrimer growth with 1 ? 3 connectivity was conducted until the 9th generation and monitored by 1H, 13C and 29Si NMR, MALTI TOF mass spectrometry, size exclusion chromatography (SEC) and atomic force microscopy (AFM) [37b].

Fig. 2. Lengthening of terminal tethers of large dendrimers in order to minimize bulk at the periphery and introduce numerous (up to 6000) ferrocenyl groups at the periphery [48].

Fig. 3. ‘‘Breathing of dendrimers terminated by 2000 ferrocenyl groups between their orange Fe(II) form and their blue Fe(III)form. It was found by AFM that the average height obtained for the G5–Fc+ (6.5 ± 0.6 nm) is much larger than that of its neutral form (4.5 ± 0.4 nm). The long tethers between the dendritic core and the ferrocenyl redox termini prevent any significant interaction between these redox centers that all appear in a single cyclovoltammetry wave [48]. (Color online.)

dendrimers (Fig. 5) [55]. Moreover, in triazolylbiferrocenyl dendrimers the electron-withdrawing triazolyl linker renders the inner ferrocenyl group more difficult to oxidize than the terminal ferrocenyl group, thus oxo anions such as ATP are recognized by the latter, whereas metal cations such as Pd(II) interacting with the triazolyl ligands are recognized by the inner ferrocenyl group [55]. A related situation is encountered in arene-terminated dendrimers in which the two electronically interacting meta positions are functionalized by Lapinte-type [1,2,56] piano-stool organoiron

Fig. 4. Statistical distribution of the FeII and FeIII species in the mixed-valent dendrimers. For instance, with G0 (9 terminal ferrocenyl branches with equal amounts of Fe(II) and Fe(III)) the statistical distribution obtained with a simple binomial law gives 24.6% of 4FeII–5FeIII, 16.4% 3FeII–6FeIII, 7% 2FeII–7FeIII and 1.7% 1FeII–8FeIII and 0.2% 0FeII–9FeIII, etc. [48].

groups linked to the arene by an ethynyl group. In the latter case, the three oxidation states Fe(III), Fe(III) and the class-II mixedvalent Fe(II)–Fe(III) of the dendrimers were synthetically accessible. Such dendrimers were also synthesized with and without 1,2,3-triazolyl linkers, which allowed comparing the dendrimer stabilization of Au nanoparticles. The triazolyl ligand coordinated the Au(III) precursor intradendritically leading to the intradendritic

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Fig. 5. 27-Branch class-II mixed-valent triazolylbiferrocenyl-terminated dendrimer 12 synthesized by reaction of an azido terminated dendrimer with ethynylbiferrocene followed by stoichiometric oxidation using a ferricenium salt. The outer ferrocenyl groups are more easily oxidized than the inner ferrocenyl group because of the electronwithdrawing character of the 1,2,3-triazolyl linkers. Consequently, the outer ferrocenyl group recognize oxoanions such as ATP, whereas the inner ferrocenyl group recognize Pd2+ than coordinate to the triazole nitrogen atoms [55].

14 (a: R = H; b: R = CH3 ) Fig. 7. In hexa(ferrocenylethynyl)benzene 14a (R = H) and hexa (pentamethylferrocenyl-ethynyl)benzene 14b (R = CH3), a single cyclic voltammetry (CV) wave is observed if [Nn-Bu4][PF6] is used as the supporting electrolyte indicating the absence of significant electronic interaction through the core, but three distinct CV wave are observed (R = H) with [Nn-Bu4][BArF4] (ArF = 3,5-C6H3(CF3)2) due only to the electrostatic interactions between the neighboring ferricenium groups [53,54]. With R = CH3, shielding of this electrostatic effect by the methyl substituents provokes the appearance of a broad multi-electron wave with the latter electrolyte.

13 Fig. 6. Mixed-valent Fe(II)–Fe(III) dendritic ethylnylferrocenyl complexes in which the ferrocenyl (in red) and pentamethylferricenium (in blue) centers have no significant electronic or electrostatic interaction [52]. (color online.)

formation of small Au nanoparticles whereas analogous dendrimers that do not contain triazolyl ligand interdendritically stabilize large Au nanoparticles outside the dendrimers [57].

7. Conclusion and prospect Sophisticated organoiron engineering results in both remarkable ligand activation modes (Schemes 1–3) and redox flexibility. These properties involve a minimized structural transformation that is characteristic of late first-raw transition-metal complexes with p ligands (Fig. 1) [58]. Such designs result in unique physical properties together with robustness of the complexes under various oxidation states including mixed-valences. These mixed-valent

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Fig. 8. Metallodendrimers terminated by 54 [Fe(g5-C5Me5)(PPh2CH2CH2PPh2)]-ethynyl units [56] constructed either by Sonogashira (15, left) or ‘‘click’’ (16, right) coupling [57]. These dendrimers were synthesized in their red Fe(II) (for instance left), blue Fe(III), and stable brown mixed-valent (for instance right) forms resulting from the electronic interaction between the two terminal iron groups located in related meta position [56]. Only the ‘‘click’’ dendrimer allows coordinating Au3+ intradendritically on triazole ligands, which allows forming small dendrimer-encapsulated Au nanoparticles upon Au3+ reduction, whereas a group of Sonogashira dendrimers interdendritically stabilizes large Au nanoparticles formed outside the dendrimer [57]. (Color online.)

dendrimers can result, in giant ferrocenyl dendrimers terminated by a large number of redox groups (Fig. 2 and Fig. 3), from statistical distribution of the two oxidation states (Fig. 5), slightly different structure upon introducing methyl groups on a Cp ligand in the absence of inter-site interaction (Fig. 6), linkage to the core de-symmetrizing the two electronically interacting redox centers, or using electronically interacting centers (Fig. 4). Interactions between two redox centers in arene-cored structures can be not only electronic (Fig. 8), but also electrostatic, and sometimes such as in hexa(ferrocenylethynyl)benzene it is possible to isolate the electrostatic interaction, when the electronic interaction is not significant (Fig. 7). These concepts have applications in nanosciences [59] and nanomaterials chemistry of metal-containing macromolecules [60,61] including transition-metal nanoparticles that are of interest in catalysis [19], sensing [24] and nanomedicine [20,59]. Acknowledgements Financial support from the Institut Universitaire de France (DA, IUF), the Agence Nationale pour la Recherche (ANR), the Université of Bordeaux, the Centre Nationale de la Recherche Scientifique (CNRS) and L’Oréal is gratefully acknowledged. References [1] For an approach using the organoiron (dppe)Cp ⁄ Fe building block (Cp⁄ = g5C5Me5) to organometallic molecular wires and other nanoscale-sized devices, see: F. Paul, C. Lapinte, Coord. Chem. Rev. 178–180 (1998) 431. [2] For charge delocalization vs. localization in carbon-rich iron mixed-valence complexes involving a subtle interplay between the carbon spacer and the (dppe)Cp ⁄ Fe organometallic electrophore, see: J.-F. Halet, C. Lapinte, Coord. Chem. Rev. 257 (2013) 1584. [3] (a) G.R. Newkome, Z. Zao, G.R. Baker, V.K. Gupta, J. Org. Chem. 50 (1985) 2003; (b) G.R. Newkome, C.N. Moorefield, Aldrichim. Acta 25 (1992) 31; (c) G.R. Newkome, Pure Appl. Chem. 70 (1998) 2337. [4] (a) D.A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roek, J. Ryder, P. Smith, Polym. J. 17 (1985) 117; (b) D.A. Tomalia, A.M. Naylor, W. Goddard III, Angew. Chem., Int. Ed. Engl. 29 (1990) 138. [5] R.G. Denkewalter, J.F. Kolc, W.J. Likasage, U.S. Patent, 4410688 53. Chem. Abstr. 100 1984, 103(907), p. 24. [6] V. Percec, P.W. Chu, G. Ungar, J. Zhou, J. Am. Chem. Soc. 117 (1995) 11441; (b) V. Percec, Pure Appl. Chem. 67 (1995) 2031.

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