Coinage Metal Complexes as Chiral Catalysts for 1,3-Dipolar Cycloadditions

In this account, we describe the experience of our research group in the implementation of chiral coinage metal complexes into the efficient enantioselective 1,3-DC of azomethine ylides derived from α-amino acids and azlactones with different dipolarophiles. The corresponding chiral metallodipoles were generated in situ and next focused on the synthesis of highly substituted prolines. For this purpose, privileged ligands such as phosphoramidites and binap with silver(I), gold(I) and copper(II) salts are described. Depending from the ligand and mainly from the metal salt it can be possible to control the facial endo/exo-diasteroselectivity and the enantioselectivity of these types of processes. The synthetic processes are also supported by DFT calculations in order to elucidate the most plausible mechanism and the stereochemical results.

Metal-complex ionosilicas: Cationic mesoporus silica with Ni(II) and Cu(II) complexes in their framework

Metal-complex ionosilicas with cationic complexes into the mesoporous silica framework were prepared using anionic surfactants. The electrostatic interaction between the anionic surfactant and the cationic metal complexes incorporated in the silica framework allows for the fine tuning of the mesoporous structure. The gentle procedure of synthesis developed and mild ion-exchange extraction of the surfactant, allowed a cleaner route for the immobilization of homogeneous cationic catalysts in mesoporous silica, while protecting the structural and chemical integrity of the metal complexes.

Mesoporous Metal Complex–Silica Aerogels for Environmentally Friendly Amination of Allylic Alcohols

Two series of mesoporous hybrid iron(III) complex–silica aerogels were prepared in one-pot synthesis by using the sol–gel coordination chemistry approach. The use of the ligands 3-(2-aminoethylamino)propyltrimethoxysilane and 2-(diphenylphosphino)ethyltriethoxysilane, both with terminal triethoxysilyl groups, were used to incorporate metal complexes in situ into the framework of silica, through their co-condensation with a silicon alkoxide during the aerogel formation. This methodology yielded optically translucent hybrid mesoporous gels with homogeneous metal incorporation and excellent textural properties. The catalytic performance of these materials was tested in the direct amination of allylic alcohols in water as a target reaction, with activities comparable or even higher than those corresponding to the homogeneous iron(III) complex. Furthermore, these catalysts were stable and maintained their catalytic activity after six reaction cycles.

Heterocyclic carbene–metal-catalyzed Csp2–Csp2 and Csp–Csp2 couplings using nonmetallic substrates

The use of carbene ligands for transition-metal complexes has been developed in the last decades, being of special interest those carbenes derived from a nitrogen-containing heterocyclic system. An interesting variety of carbene-metal complexes has been tested in the Mizoroki-Heck reaction. In comparison, few examples can be found for the Matsuda-Heck version of this coupling reaction. Additionally, the Sonogashira coupling has been also catalyzed with different carbene-metal catalysts.

Sol–Gel Coordination Chemistry: Building Catalysts from the Bottom-Up

The development of synthetic routes for the tailoring of efficient silica-based heterogeneous catalysts functionalized with coordination complexes or metallic nanoparticles has become a important goal in chemistry. Most of these techniques have been based on postsynthetic treatments of preformed silicas. Nevertheless, there is an emerging approach, so-called sol–gel coordination chemistry, based on co-condensation during the sol–gel preparation of the hybrid material of the corresponding complex or nanoparticle modified with terminal trialkoxysilane groups with a silica source (such as tetraethoxysilane) and in the presence of an adequate surfactant. This method leads to the production of new mesoporous metal complex-silica materials, with the metallic functionality incorporated homogeneously into the structure of the hybrid material, improving the stability of the coordination complex (which is protected by the silica network) and reducing the leaching of the active phase. This technique also offers the actual possibility of functionalizing silica or other metal oxides for a wider range of applications, such as photonics, sensing, and biochemical functions.

Conformational changes of dissolved humic and fulvic superstructures with progressive iron complexation

Conformational changes of a humic acid (HA) and a fulvic acid (FA) induced by iron complexation were followed by high-performance size exclusion chromatography (HPSEC) with both UV–vis and refractive index (RI) detectors. Molecular size distribution was reduced for HA and increased for FA with progressive iron complexation. Since interactions of Fe with humic components are electrostatic, it is likely that the triple-charged Fe ions formed stronger complexes with the more acidic hydrophilic and hydrated FA than with the less acidic and more hydrophobic HA. The large content of ionized carboxyl groups in FA, thus favored the formation of intra- or intermolecular bridges between the negatively charged fulvic acid molecules, and led to more compact and larger size network than for HA. Conversely, iron complexation with HA disrupted the humic conformational arrangements stabilized by only weak hydrophobic bonds into smaller-size aggregates of greater conformational stability due to formation of strong metal complexes. These results confirmed that humic molecules in solution were organized in supramolecular associations of relatively small molecules loosely bound together by dispersive interactions and hydrogen bonds, and they specifically responded to chemical changes brought about by metal additions. The present study revealed the molecular changes occurring in superstructures of natural organic matter when in metal complexes and contributed to understand and predict the environmental behavior in waters and soil of metal complexes with natural organic matter.

Terminology of metal–organic frameworks and coordination polymers (IUPAC Recommendations 2013)

A set of terms, definitions, and recommendations is provided for use in the classification of coordination polymers, networks, and metal–organic frameworks (MOFs). A hierarchical terminology is recommended in which the most general term is coordination polymer. Coordination networks are a subset of coordination polymers and MOFs a further subset of coordination networks. One of the criteria an MOF needs to fulfill is that it contains potential voids, but no physical measurements of porosity or other properties are demanded per se. The use of topology and topology descriptors to enhance the description of crystal structures of MOFs and 3D-coordination polymers is furthermore strongly recommended.

Palladium(II) pincer complexes of α-amino acids: towards the synthesis of catalytically active artificial peptides

NCN palladium(II) complexes have been covalently attached to the N- and C-terminus of the dipeptide L-Phe-L-Va-OMe. Remarkably, the hydrolysis of the NCN-Pd(II) L-Val-OMe afforded the corresponding, palladated free amino acid without affecting the metal site. This deprotected amino acid could be coupled to any protein, enzyme or peptidic chain by simple peptide chemistry. This bioorganometallic systems were active as catalysts in the aldol reaction between methyl isocianate and benzaldehyde.

New hybrid materials based on the grafting of Pd(II)-amino complexes on the graphitic surface of AC: preparation, structures and catalytic properties

A novel procedure for the preparation of solid Pd(II)-based catalysts consisting of the anchorage of designed Pd(II)-complexes on an activated carbon (AC) surface is reported. Two molecules of the Ar–S–F type (where Ar is a plane-pyrimidine moiety, F a Pd(II)-ligand and S an aliphatic linker) differing in F, were grafted on AC by π–π stacking of the Ar moiety and the graphene planes of the AC, thus favouring the retaining of the metal-complexing ability of F. Adsorption of Pd(II) by the AC/Ar–S–F hybrids occurs via Pd(II)-complexation by F. After deep characterization, the catalytic activities of the AC/Ar–S–F/Pd(II) hybrids on the hydrogenation of 1-octene in methanol as a catalytic test were evaluated. 100% conversion to n-octane at T = 323.1 K and P = 15 bar, was obtained with both catalysts and most of Pd(II) was reduced to Pd(0) nanoparticles, which remained on the AC surface. Reusing the catalysts in three additional cycles reveals that the catalyst bearing the F ligand with a larger Pd-complexing ability showed no loss of activity (100% conversion to n-octane) which is assigned to its larger structural stability. The catalyst with the weaker F ligand underwent a progressive loss of activity (from 100% to 79% in four cycles), due to the constant aggregation of the Pd(0) nanoparticles. Milder conditions, T = 303.1 K and P = 1.5 bar, prevent the aggregation of the Pd(0) nanoparticles in this catalyst allowing the retention of the high catalytic efficiency (100% conversion) in four reaction cycles.

Green synthesis of polypyrrole-supported metal catalysts: application to nitrate removal in water

Pt and Pt/Sn catalysts supported on polypyrrole (PPy) have been prepared using Ar plasma to reduce the metal precursors dispersed on the polymer. The PPy support was synthesized by chemical polymerization of pyrrole with FeCl3·6H2O, this leading to the conducting form of the polymer (conductimetric measurements). The Ar plasma treatment produced a partial reduction of platinum ions, anchored as platinum chloro-complexes to the PPy chain, into metallic platinum. A homogeneous distribution of Pt and Sn nanoparticles was observed by TEM. Activity of the PPy-supported catalysts was evaluated in the reduction of aqueous nitrate with H2 at room temperature. Nitrate concentration in water below the maximum acceptable level of 50 mg L−1 was achieved with all catalysts. However, considering not only efficiency in nitrate reduction, but also minimized concentrations of undesired nitrite and ammonium, the monometallic Pt catalyst seems to be the most promising one.