Jaime Ruiz

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.

Giant dendritic molecular electrochrome batteries with ferrocenyl and pentamethylferrocenyl termini.

Giant redox dendrimers were synthesized with ferrocenyl and pentamethylferrocenyl termini up to a theoretical number of 3(9) tethers (seventh generation). Lengthening of the tethers proved to be a reliable strategy to overcome the bulk constraint at the dendrimers periphery. These redox metallodendrimers were characterized by (1)H, (13)C, and (29)Si NMR; MALDI-TOF mass spectrometry (for the low generations); elemental analysis; UV-vis spectroscopy; dynamic light scattering (DLS); atomic force microscopy (AFM); electron-force microscopy (EFM) for half- or fully oxidized dendrimers; cyclic voltammetry; and coulometry. UV-vis spectroscopy, coulometry, and analytical data are consistent with an increasing amount of defects as the generation number increases, with this amount remaining relatively weak up to G(5). AFM shows that the dendrimers form aggregates of discrete size on the mica surface, recalling the agglomeration of metal atoms in monodisperse nanoparticles. Cyclic voltammetry reveals full chemical and electrochemical reversibility up to G(7), showing that electron transfer is fast among the flexible peripheral redox sites. Indeed, the redox stability of these new electrochromic dendrimers, i.e., a battery behavior, was established by complete chemical oxido-reduction cycles, and the blue 17-electron ferrocenium and deep-green mixed-valence Fe(III)/Fe(II) dendritic complexes were isolated and characterized. AFM studies also show the reversible dendrimer size changes from upon redox switching between Fe(II) and Fe(III), suggesting a breathing mechanism controlled by the redox potential. Considerable adsorption of high-generation dendrimers on Pt electrodes such as G(7)-Fc allows the easy formation of modified electrodes that sense the ATP anion only involving the electrostatic factor even in the absence of any other type of interaction with the redox tethers.

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

A family of rigid ferrocenyl-terminated redox stars has been synthesized--by Negishi coupling, including hexa(ferrocenethynyl)benzene complexes, a dodecaferrocenyl star, and stars with extended rigid tethers--and fully characterized. Cyclic voltammetry (CV) studies of the parent complex hexa(ferrocenylethynyl)benzene, 1, show a single wave for the six-electron oxidation of 1 using Nn-Bu(4)PF(6) as the supporting electrolyte on a Pt anode in CH(2)Cl(2), whereas three distinct two-electron reversible CV waves are observed using Nn-Bu(4)BAr(F)(4) (Ar(F) = 3,5-C(6)H(3)-(CF(3))(2)). The CV of 1,3,5-tris(ferrocenylethynyl)benzene, 11, also shows only one wave for the three-electron transfer with Nn-Bu(4)PF(6) and three one-electron waves using Nn-Bu(4)BAr(F)(4). This confirms the lack of electronic communication between the ferrocenyl groups and a significant electrostatic effect among the oxidized ferrocenyl groups. This effect is not significant between para-ferrocenyl groups in 1,4-bis(ferrocenylethynyl)benzene for which only a single wave is observed even with Nn-Bu(4)BAr(F)(4) as the supporting electrolyte. The para-ferrocenyl substituents are quite independent, which explains that two para-ferrocenyl groups are oxidized at about the same potential in a single CV wave of 1. With the additional steric bulk introduced with a methyl substituent on the ferrocenyl group, however, even the para-methylferrocenyl groups are submitted to a small electrostatic effect splitting the six-electron transfer into six single-electron waves, probably because of the overall steroelectronic constraints. Contrary to 11, 1,3-bis(ferrocenylethynyl)benzene and related complexes with a third, different substituent in the remaining meta position different from a ferrocenylethynyl only show a single two-electron wave using Nn-Bu(4)BAr(F)(4), which is attributed to the transoïd conformation of the ferricinium groups minimizing the electrostatic effect. This shows that, in 11, it is the steric frustration that is responsible for the electrostatic effect, and the same occurs in 1. In several cases, ΔE(p) is much larger than the expected 60 mV value, characterizing a quasi-reversible (i.e., relatively slow) redox process. It is suggested that this slower electron transfer be attributed to conformational rearrangement of the ferrocenyl groups toward the transoïd position in the course of electron transfer. Thus both the thermodynamic and kinetic aspects of the electrostatic factor (isolated from the electronic factor), including the frustration effect, are characterized. The distinction between the electronic communication and through-space electrostatic effect was made possible in all of these complexes in which the absence of wave splitting using a strongly ion-pairing electrolyte shows the absence of significant electronic communication, and was confirmed by the new frustration phenomenon.

How Do Redox Groups Behave around a Rigid Molecular Platform? Hexa(ferrocenylethynyl)benzenes and Their Electrostatic Redox Chemistry

A new family of hexakis(ferrocenylethynyl)benzenes was synthesized by Negishi coupling from ethynylferrocenes and C(6)Br(6) and can be reversibly oxidized to stable hexaferrocenium salts (see picture, Ar(F)=[3,5-C(6)H(3)(CF(3))(2)]). Their cyclic voltammograms show a single six-electron wave, three distinct two-electron waves, or a cascade of six single-electron waves, depending on the electrolyte counterion and number of methyl substituents on the ferrocenyl groups.

Molecular Batteries: Ferrocenylsilylation of Dendrons, Dendritic Cores, and Dendrimers: New Convergent and Divergent Routes to Ferrocenyl Dendrimers with Stable Redox Activity

The ferrocenylsilylation of the phenol triallyl dendron 2, of the phenol nonaallyl dendron 4, and of the 9-, 27-, 81-, and 243-allyl dendrimers 7-10 (monitored by the disappearance of the signals of the olefinic protons in 1H NMR spectra) has been achieved using ferrocenyldimethylsilane 1 and Karstedts catalyst in diethyl ether at 40 degrees C, yielding the corresponding ferrocenyl dendrons and dendrimers. An alternative convergent synthesis of the nonaferrocenyl dendron 5 was carried out by reaction of the triferrocenyl dendron 2 with a protected triododendron followed by deprotection. Reaction of the nonaferrocenyl dendron 5 with hexakis(bromomethyl)benzene gave the 54-ferrocenyl dendron 6. All the ferrocenyl dendron and dendrimers produce a chemically and electrochemically reversible ferrocenyl oxidation wave at seemingly the same potential. Stable platinum electrodes modified with the high ferrocenyl dendrimers were fabricated. The soluble orange-red ferrocenyl dendrimers can also be oxidized in CH2Cl2 by [NO][PF6] to the insoluble deep blue polyferrocenium dendrimers. For instance, the 243-ferrocenium dendrimer has been characterized by its Mossbauer spectrum, which is of the same type as that of ferrocenium itself. The ferrocenium dendrimers can be reduced without any decomposition back to the ferrocenyl dendrimer, indicating that these multielectronic redoxstable dendrimers behave as molecular batteries.

A Polycationic Metallodendrimer with 24 [Fe(η5‐C5Me5)(η6‐N‐Alkylaniline)]+ Termini That Recognizes Chloride and Bromide Anions

Pronounced dendritic effects are shown by the title compound 1 in the recognition of Cl- and Br- -this is shown by the comparison with a mononuclear compound with one dendrimer arm and a trinuclear compound with a tripodal dendrimer branch. Thus, 1 differs distinctly from 24-ferrocene dendrimers of comparable topology.

New examples of dinuclear copper complexes with ferromagnetic interactions mediated by μ-1,1 bridging azido and nitrito groups: structures and magnetic properties of [L2(N3)2Cu2] and [L2(NO2)2Cu2]·H2O (L=7-amino-4-methyl-5-aza-3-hepten-2-onato(1−))

Abstract The reaction of L 2 Cu 2 OAc 2 ( L = 7amino-4-methyl-5-aza-3-hepten-2-onato(−1); OAc = acetate ion) with NaN 3 and NaNO 2 yields [L 2 (N 3 ) 2 Cu 2 ] ( 1 ) and [L 2 (NO 2 ) 2 Cu 2 ]·H 2 O ( 2 ), respectively. The azido complex crystallizes in the triclinic space group P 1 with a = 8.261(1), b = 9.176(1), c = 7.915(1) A , α = 113.06(1), γ = 108.90(1)°, Z = 1 . The nitrito complex also crystallizes in the triclinic space group P 1 with a = 11.523(1), b = 11.532(1), c = 112.22(1), β = 91.33(1), γ = 92.06(1)°, Z = 2 . In both cases the structure consists of neutral dinuclear entities resulting from the pairing of two mononuclear units via single-atom bridges connecting an equatorial position of a copper centre to an axial position of the other one. The copper ions adopt a (4 + 1) square-based geometry in the azido complex while (4 + 1) and (4 + 2) environments are present in the nitrito complex. Analysis of the thermal variation of the magnetic susceptibility shows that in both complexes, the two spins are ferromagnetically coupled within the dimer while an antiferromagnetic coupling is operative between dinuclear entities. Both types of interactions are larger in the case of the azido complex ( J = 24 cm −1 , J ′ = 1.6 cm −1 ) than for the nitrito complex ( J = 8 cm −1 , J ′ = −0.2 cm −1 ).

Highly Efficient Transition Metal Nanoparticle Catalysts in Aqueous Solutions

A ligand design is proposed for transition metal nanoparticle (TMNP) catalysts in aqueous solution. Thus, a tris(triazolyl)-polyethylene glycol (tris-trz-PEG) amphiphilic ligand, 2, is used for the synthesis of very small TMNPs with Fe, Co, Ni, Cu, Ru, Pd, Ag, Pt, and Au. These TMNP-2 catalysts were evaluated and compared for the model 4-nitrophenol reduction, and proved to be extremely efficient. High catalytic efficiencies involving the use of only a few ppm metal of PdNPs, RuNPs, and CuNPs were also exemplified in Suzuki-Miyaura, transfer hydrogenation, and click reactions, respectively.

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).

Dendrimers and gold nanoparticles as exo-receptors sensing biologically important anions

Dendrimers, alkylthiol-gold nanoparticles and gold-nanoparticle-cored dendrimers containing tethers terminated by a redox group (typically an iron sandwich) attached to a hydrogen-bonding group (amido, amino, silyl) are selective and efficient exo-receptors for the recognition, sensing and titration of oxo-anions, including ATP(2-), or halogens, mostly using cyclic voltammetry. Various positive dendritic effects were disclosed (in contrast to catalysis), and large gold-nanoparticle-cored redox dendrimers of this type that contain several hundred equivalent ferrocenyl groups readily adsorb on Pt electrodes, providing useful regenerable electrochemical sensors.