Roberto Ciganda

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.

Precise localization of metal nanoparticles in dendrimer nanosnakes or inner periphery and consequences in catalysis.

Understanding the relationship between the location of nanoparticles (NPs) in an organic matrix and their catalytic performances is essential for catalyst design. Here we show that catalytic activities of Au, Ag and CuNPs stabilized by dendrimers using coordination to intradendritic triazoles, galvanic replacement or stabilization outside dendrimers strongly depends on their location. AgNPs are found at the inner click dendrimer periphery, whereas CuNPs and AuNPs are encapsulated in click dendrimer nanosnakes. AuNPs and AgNPs formed by galvanic replacement are larger than precursors and only partly encapsulated. AuNPs are all the better 4-nitrophenol reduction catalysts as they are less sterically inhibited by the dendrimer interior, whereas on the contrary CuNPs are all the better alkyne azide cycloaddition catalysts as they are better protected from aerobic oxidation inside dendrimers. This work highlights the role of the location in macromolecules on the catalytic efficiency of metal nanoparticles and rationalizes optimization in catalyst engineering.

Redox-Robust Pentamethylferrocene Polymers and Supramolecular Polymers, and Controlled Self-Assembly of Pentamethylferricenium Polymer-Embedded Ag, AgI, and Au Nanoparticles.

We report the first pentamethylferrocene (PMF) polymers and the redox chemistry of their robust polycationic pentamethylferricenium (PMFium) analogues. The PMF polymers were synthesized by ring-opening metathesis polymerization (ROMP) of a PMF-containing norbornene derivative by using the third-generation Grubbs ruthenium metathesis catalyst. Cyclic voltammetry studies allowed us to determine confidently the number of monomer units in the polymers through the Bard-Anson method. Stoichiometric oxidation by using ferricenium hexafluorophosphate quantitatively and instantaneously provided fully stable (even in aerobic solutions) blue d(5) Fe(III) metallopolymers. Alternatively, oxidation of the PMF-containing polymers was conducted by reactions with Ag(I) or Au(III) , to give PMFium polymer-embedded Ag and Au nanoparticles (NPs). In the presence of I2 , oxidation by using Ag(I) gave polymer-embedded Ag/AgI NPs and AgNPs at the surface of AgI NPs. Oxidation by using Au(III) also produced an Au(I) intermediate that was trapped and characterized. Engineered single-electron transfer reactions of these redox-robust nanomaterial precursors appear to be a new way to control their formation, size, and environment in a supramolecular way.

Living ROMP Synthesis and Redox Properties of Diblock Ferrocene/Cobalticenium Copolymers.

Using the third-generation Grubbs catalyst, the living ring-opening metathesis polymerization of ferrocene/cobalticenium copolymers is conducted with theoretical numbers of 25 monomer units for each block, and their redox and electrochemical properties allow using the Bard-Anson electrochemical method to determine the number of metallocenyl units in each block.

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

The design of redox-robust metallocopolymers is expected to produce new nanomaterials with multiple applications. Here the ROMP syntheses using Grubbs’ 3rd generation catalyst of three new living diblock copolymers each containing two distinct redox-stable iron sandwich complexes of the ferrocene, pentamethylferrocene and cyclopentadienyl-iron-arene families in the side chain are reported. The electrochemical properties of these diblock metallocopolymers investigated by cyclic voltammetry show complete chemical and electrochemical reversibilities of the redox waves, which also allow using the Bard–Anson method to determine the number of monomer units. The selective and reversible “chemical” oxidation of one of the blocks in two of these metallocopolymers afforded the syntheses of mixed-valent FeIIFeIII polymers.

Tunneling Dendrimers. Enhancing Charge Transport through Insulating Layer Using Redox Molecular Objects

Charge transport through an insulating layer was probed using ferrocenyl-terminated dendrimers and scanning electrochemical microscopy. Experiments show that the passage through the layer is considerably enhanced when the transferred charges are brought globally to the surface by the ferrocenyl dendrimer instead of a single ferrocene molecule. This result shows that charge tunneling through an insulator could be promoted by a purely molecular nano-object.

From Mono to Tris-1,2,3-triazole-Stabilized Gold Nanoparticles and Their Compared Catalytic Efficiency in 4-Nitrophenol Reduction

Mono-, bis-, and tris-1,2,3-triazole ligands are used for the stabilization of gold nanoparticles (AuNPs), and the catalytic activities of these AuNPs in 4-nitrophenol reduction by NaBH4 in water are compared as well as with polyethylene glycol 2000 (PEG)- and polyvinylpyrrolidone (PVP)-stabilized AuNPs. The excellent catalytic results specifically obtained with the tris-triazolate ligand terminated by a PEG tail are taken into account by the synergy between the weakness of the tris-triazole-AuNP bond combined with the stabilizing ligand bulk.

Liquid–Liquid Interfacial Electron Transfer from Ferrocene to Gold(III): An Ultrasimple and Ultrafast Gold Nanoparticle Synthesis in Water under Ambient Conditions

Ferrocene (Fc) in ether reduces HAuCl4 in water within seconds under ambient conditions in air upon stirring, forming ferricinium chloride stabilized water-soluble 20 nm gold nanoparticles (AuNPs) that are redispersible in the presence of poly(N-vinylmethylpyrrolidone) or NaBH4 + thiol. After reduction with NaBH4 yielding Fc and 26 nm sodium poly(hydroxyborate) stabilized AuNPs, the core size no longer changes following reactions with thiols providing (RS)nAuNPs.

Electrostatic Assembly of Functional and Macromolecular Ferricinium Chloride-Stabilized Gold Nanoparticles

Substituted ferrocenes with various stereoelectronic effects including a ferrocene-terminated dendrimer in ether reduce aqueous HAuCl4 to gold nanoparticles (AuNPs) by interfacial electron transfer. The dependence on the stirring speed plays a crucial role, and the stereoelectronic influences on the reaction rates are dramatic. With a ferrocene-containing polymer, the reaction is conducted using an homogeneous THF/water medium, also forming AuNPs. Fully stable functional, dendritic and polymeric ferricinium chloride-stabilized AuNPs are obtained with core sizes between 13 and 35 nm, an optimal size range for potential biomedical applications. Finally the ferricinium coating of the Au nanoparticles is replaced by a more electron-rich ferricinium derivative by exergonic redox reaction with the corresponding ferrocene derivative.

Reactivity of hydridoirida-β-diketones with bases: the selective formation of new di-μ-acyl-μ-hydridodiiridium(III) or dihydridoirida-β-diketone complexes and heterometallic Ir(III)–Rh(I) derivatives

The hydridoirida-β-diketone [IrHCl{(PPh2(o-C6H4CO))2H}] (1a) reacts with bases such as KOH or NaHCO3 in methanol to undergo dehydrodechlorination and acyl-bridge formation affording [Ir2H2(PPh2(o-C6H4CO))2(μ-PPh2(o-C6H4CO))2] (2) with two acylphosphine chelate-bridging ligands, in a head-to-tail disposition, and terminal hydrides. The acyl bridges can be broken by pyridine, PPh3, CO or dimethylsulfoxide affording selectively mononuclear diacylhydrido neutral derivatives [IrH(PPh2(o-C6H4CO))2L] (3–6). 1a reacts with KOH or NaHCO3 in refluxing methanol to afford a novel dihydridoirida-β-diketone [IrH2{(PPh2(o-C6H4CO))2H}] (7), via dehydrodechlorination to afford 2, which then undergoes hydrogenation and protonation. The reaction of 1a with NEt3 affords 2 and [NHEt3]Cl. Further reaction affords [Ir2(μ-H){μ-PPh2(o-C6H4CO)}2(PPh2(o-C6H4CO))2]+ (8), with two acylphosphine chelate-bridging ligands and a bridging hydride. Neutral or cationic hydridoirida-β-diketone complexes react with [Rh(cod)(OMe)]2 (cod = 1,5-cyclooctadiene) to afford hydridoirida-β-diketonaterhodium(I) complexes [IrHCl(μ-PPh2(o-C6H4CO))2Rh(cod)] (9) or [IrHL(μ-PPh2(o-C6H4CO))2Rh(cod)]ClO4 (L = py, 10; CO, 11), respectively that isomerise to the thermodynamically stable isomers of [IrCl(PPh2(o-C6H4CO))(μ-H))(μ-PPh2(o-C6H4CO))Rh(cod)] (12) or [Ir(py)(PPh2(o-C6H4CO))(μ-H))(μ-PPh2(o-C6H4CO))Rh(cod)]ClO4 (13). The reaction of 7 with [Rh(cod)(OMe)]2 affords [Ir(PPh2(o-C6H4CO))2(μ-H)2Rh(cod)] (14). All the complexes were fully characterised spectroscopically. Single-crystal X-ray diffraction analysis was performed on 2, 4, 7, [8]ClO4 and 9.