Noble Metal Complexes Research Articles

Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes

Homogeneous catalysis of organic transformations by metal complexes has been mostly based on complexes of noble metals. In recent years, tremendous progress has been made in the field of base-metal catalysis, mostly with pincer-type complexes, such as iron, cobalt, nickel, and manganese pincer systems. Particularly impressive is the explosive growth in the catalysis by Mn-based pincer complexes, the first such complexes being reported as recently as 2016. This review covers recent progress in the field of homogeneously catalyzed reactions using pincer-type complexes of cobalt and manganese. Various reactions are described, including acceptorless dehydrogenation, hydrogenation, dehydrogenative coupling, hydrogen borrowing, hydrogen transfer, H–X additions, C–C coupling, alkene polymerization and N2 fixation, including their scope and brief mechanistic comments.

Preparation of noble metal/polymer nanocomposites via in situ polymerization and metal complex reduction

Abstract Noble metal/polymer nanocomposites were prepared with homogeneous distribution of the metal nanoparticles in the polymer via in situ polymerization and subsequent thermal reduction. First, noble metal complexes having good compatibility with methyl methacrylate (MMA) were dissolved in MMA. The mixture was pre-polymerized by adding an initiator, and then transferred to a mold to complete the polymerization. The composites obtained were reduced in situ by post-heating at different temperatures, depending on the metal. X-ray CT and transmission electron microscopy were utilized to prove the uniform distribution of noble metal nanoparticles in the poly (methyl methacrylate) (PMMA). The noble metal nanoparticles have average diameters of ∼3.0, 5.9, 12.7, and 24.1 nm for Pt, Pd, Cu, and Au, respectively. Thermogravimetric analysis shows that the weight of the Pd/PMMA nanocomposites increases with increasing initial molar concentration of metal complexes. UV/Visible transmission spectra also show plasmon absorptions characteristic of metals. The tensile strength of Pd/PMMA nanocomposites increased by 18.8% by adding Pd nanoparticles. These results suggest that uniform distribution of noble metal nanoparticles in PMMA can be achieved through in situ polymerization and thermal reduction.

3D-macroporous chitosan-based scaffolds with in situ formed Pd and Pt nanoparticles for nitrophenol reduction

3D-macroporous chitosan-based scaffolds (cryogels) were produced via growth of metal-polymer coordinated complexes and electrostatic interactions between oppositely charged groups of chitosan and metal ions under subzero temperatures. A mechanism of reduction of noble metal complexes inside the cryogel walls by glutaraldehyde is proposed, which produces discrete and dispersed noble metal nanoparticles. 3D-macroporous scaffolds prepared under different conditions were characterised using TGA, FTIR, nitrogen adsorption, SEM, EDX and TEM, and the distribution of platinum nanoparticles (PtNPs) and palladium nanoparticles (PdNPs) in the material assessed. The catalytic activity of the in situ synthesised PdNPs, at 2.6, 12.5 and 21.0 μg total mass, respectively, was studied utilising a model system of 4-nitrophenol reduction. The kinetics of the reaction under different conditions (temperature, concentration of catalyst) were examined, and a decrease of catalytic activity was not observed over 17 treatment cycles. Increasing the temperature of the catalytic reaction from 10 to 22 and 35 °C by PdNPs supported within the cryogel increased the kinetic rate by 44 and 126%, respectively. Turnover number and turnover frequency of the PdNPs catalysts at room temperature were in the range 0.20–0.53 h−1. The conversion degree of 4-nitrophenol at room temperature reached 98.9% (21.0 μg PdNPs). Significantly less mass of palladium nanoparticles (by 30–40 times) was needed compared to published data to obtain comparable rates of reduction of 4-nitrophenol.

The Discovery of an Iridium(III) Dimer Complex as a Potent Antibacterial Agent against Non-Replicating Mycobacterium smegmatis.

Novel agents are urgently needed to rapidly kill drug-resistant Mycobacterium tuberculosis. Noble metal complexes, particularly polypyridyl iridium complexes serving as therapeutic agents, have attracted considerable interest recently, due to their significant cytotoxic or antimicrobial activities. Here, we reported an polypyridyl iridium dimer complex [Ir(ppy)2Cl]2 (3), with ppy = phenylpyridine, which was found to be active against both exponential growing and non-replicating M. smegmatis, with minimum inhibitory concentration values of 2 μg/mL, and exhibited rapid bactericidal kinetics, killing pathogens within 30–60 min. Moreover, 3 was demonstrated to generate a large amount of reactive oxygen species and to be effective in drug-resistant strains. Taken together, the selectively active iridium(III) dimer complex showed promise for use as a novel drug candidate for the treatment of M. tuberculosis infection.

Ion pair interactions in noble metal complexes with halogen atoms

Ion pair interactions in noble metal complexes with halogen atoms

(Invited) Silicon Nanowires Photocathodes Combined with Mn-Based Molecular Complex Catalysts for the Efficient Light-Driven Electrocatalytic Reduction of CO2 to CO

Proton-assisted multielectron reduction potentials for CO2 lie within the bandgap of several semiconductors, in such a way that their use as photocathodes for this reaction allows a substantial decrease in the energy input required [1-3]. In the present work, hydrogen-terminated silicon nanowires (SiNWs-H) are used as a photocathode for the electrocatalytic reduction of CO2 in the presence of different metallic bipyridyl complexes. Due to their nanostructured surface, SiNWs are highly interesting as light-harvesting electrodes, being efficient in converting solar to electrical energy, easy to make and stable under reduction conditions [4]. Among the tested complexes, we have used Mn non-noble metal based complexes to catalyze CO2-to-CO photoelectroreduction, which is a less expensive approach versus the traditionally used based on rare and noble metal complexes [5]. The photoelectrochemical reduction of CO2 at p-type SiNWs-H was achieved at potentials significantly lower than those required with a glassy carbon electrode (GCE) and planar Si-H [6]. Thus, for intermediate reduction potentials the photocurrent density values provided by silicon were noticeably higher than the current densities provided by GCE. Besides, impedance spectroscopic measurements were carried out and combined with cyclic voltammetry data to obtain the quantitative complete characterization of the energetics of all the systems studied. All the experiences were carried out at p-type flat silicon for comparison purposes and key electrocatalytic parameters such as fill factor (FF) and energy conversion efficiency (η) were determined. Finally, in order to develop technologically viable electrocatalytic devices, the elaboration of SiNWs−H photoelectrodes modified with a Mn-based complex has been successfully achieved from an electropolymerizable catalyst, and it was shown that the electrocatalytic activity of the complex was retained after immobilization. [1] Aresta, M. et al., Chem. Rev. 2014, 114, 1709. [2] Costentin, C. et al., Chem. Soc. Rev. 2013, 42, 2423. [3] Schmidt-Mende, L. et al., Angew. Chem. Int. Ed. 2013, 52, 7372. [4] Wang, D. et al., Angew. Chem. Int. Ed. 2012, 51, 6709. [5] Kubiak, C. P. et al., J. Phys. Chem. C 2010, 114, 14220. [6] Torralba-Penalver, E. et al., ACS Catal. 2015, 5, 6138.

Designing Highly Efficient CuI Photosensitizers for Photocatalytic H2 Evolution from Water.

A series of CuI photosensitizers was synthesized, characterized, and investigated for photocatalytic H2 evolution from water. A structure-property correlation was established for their catalytic activity and photophysical properties, which was further elaborated by DFT calculations. A new CuI photosensitizer (Cu-TPAPhen) with triphenylamine-substituted phenanthroline ligands showed unprecedentedly high turnover numbers of 19 000 when tested in combination with triethylamine as a sacrificial reagent and colloidal Pt as a H2 evolution catalyst. This work paves the way toward cheap metal-based photosensitizers which can replace noble-metal complexes in photocatalytic systems.

Preparation of conjugated linoleic acid enriched derivatives by conventional and biphasic isomerisation

The preparation of conjugated linoleic acid (CLA)-enriched free fatty acids by industrial processes compared with our biphasic isomerisation experiments in a special designed reactor enabling the preparation of CLA esters was evaluated. Our experiments further revealed the main disadvantage of semi-synthetic alkali isomerisation to be the formation of conjugated E,E-octadecadienoic acid isomers (2.92–3.44%) and the bioavailability of free fatty acid products. Urea fractionation technology improved the quality of the reaction mixture, but at the same time the yield of rumenic acid was decreased on purification. Therefore, we decided to apply complexes of noble metals in order to isomerise linoleic acid ester derivatives. The known Wilkinson’s hydrogenation catalyst, RhCl (PPh<sub>3</sub>)<sub>3</sub>, was found to be the most effective. We investigated the preparation of bioavailable CLA-enriched triacylglycerols. Special attention was paid to recycling of Wilkinson’s catalyst.

Abiological catalysis by artificial haem proteins containing noble metals in place of iron.

Enzymes that contain metal ions--that is, metalloenzymes--possess the reactivity of a transition metal centre and the potential of molecular evolution to modulate the reactivity and substrate-selectivity of the system. By exploiting substrate promiscuity and protein engineering, the scope of reactions catalysed by native metalloenzymes has been expanded recently to include abiological transformations. However, this strategy is limited by the inherent reactivity of metal centres in native metalloenzymes. To overcome this limitation, artificial metalloproteins have been created by incorporating complete, noble-metal complexes within proteins lacking native metal sites. The interactions of the substrate with the protein in these systems are, however, distinct from those with the native protein because the metal complex occupies the substrate binding site. At the intersection of these approaches lies a third strategy, in which the native metal of a metalloenzyme is replaced with an abiological metal with reactivity different from that of the metal in a native protein. This strategy could create artificial enzymes for abiological catalysis within the natural substrate binding site of an enzyme that can be subjected to directed evolution. Here we report the formal replacement of iron in Fe-porphyrin IX (Fe-PIX) proteins with abiological, noble metals to create enzymes that catalyse reactions not catalysed by native Fe-enzymes or other metalloenzymes. In particular, we prepared modified myoglobins containing an Ir(Me) site that catalyse the functionalization of C-H bonds to form C-C bonds by carbene insertion and add carbenes to both β-substituted vinylarenes and unactivated aliphatic α-olefins. We conducted directed evolution of the Ir(Me)-myoglobin and generated mutants that form either enantiomer of the products of C-H insertion and catalyse the enantio- and diastereoselective cyclopropanation of unactivated olefins. The presented method of preparing artificial haem proteins containing abiological metal porphyrins sets the stage for the generation of artificial enzymes from innumerable combinations of PIX-protein scaffolds and unnatural metal cofactors to catalyse a wide range of abiological transformations.

Bimetallic systems of mesoporous ordered silica supports and noble metals nanoparticles

This paper describes of preparation and characterization of new bimetallic materials based on platinum/silver, platinum/palladium and palladium/silver systems of nanoparticles supported on mesoporous ordered silica composite. In all investigated materials unusual ammine complexes of noble metals ([Pt (NH3)4]Cl2, [Ag(NH3)2]OH and [Pd(NH3)4]Cl2) were used as nanoparticles sources. Bimetallic materials were synthesized by one-pot procedure or post synthesis modification via impregnation method with metal precursors on surface of mesoporous ordered silica. The obtained materials were characterized by various techniques including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), nitrogen physisorption and FTIR/PAS spectroscopy.Proposed system of noble metal nanoparticles allows determining subtle changes in the interaction between metallic phases and porous carrier with particular emphasis of spectroscopic properties. In the current study, we have attempted to relate the synergistic effect of components for understanding relationships between type of noble metal ammine complexes as metal precursors and final material properties. We found that significant interactions occur between supported metals and relatively inert mesoporous ordered silica. What is more, noble metal crystallites in bimetallic systems exhibit different sizes depending on the kind of metal source. Such a difference in sizes of individual components may be responsible for lack or presence of interactions between metal nanoparticles and solid support. Specific distribution of platinum nanoparticles on the support surface and their very small sizes in Pt,Ag/SBA-15 systems causes intrinsic electron deficiency due to the interactions between a small platinum crystallite and the silica surface. The possible interaction effect between palladium and the silicon atoms was also observed. This indicates that the metal-support effect should be considered during designing and production of new functional materials, models of catalysts and adsorbents.

Preparation and Structural Properties of Bimetallic Noble Metals Nanoparticles in SBA-15 Systems

New noble bimetallic materials composed of platinum/silver, platinum/palladium and palladium/silver arrangements of nanoparticles supported on mesoporous ordered silica support were obtained and characterized. Ammine complexes of noble metals ([Pt(NH3)4]Cl2), [Ag(NH3)2]OH and ([Pd(NH3)4]Cl2)) were used as nanoparticles sources. The selected ammine complexes of noble metals were proven to be an easy solution for obtaining good quality materials in comparison with other sources. Bimetallic materials were synthesized by traditional impregnation technique, making the proposed procedure simple and reproducible. Our studies demonstrated that there are significant differences between types of materials and thermal stability of their metal precursor and their decomposition mechanisms. The X-ray diffraction results showed that the obtained metal crystallites exhibited different sizes depending on the kind of noble metal, as indicated by significant differences in intensity and width of diffraction profiles for met...

IPMC actuators based on metal–Nafion™ composite films prepared by thermal decomposition of noble metal complexes

A simple and highly efficient compositing process was developed by synthesizing Pt or Pd nanoparticles embedded in polymers (Nafion™) with uniform distribution by in situ thermal decomposition of noble metal complexes. The homogenous mixtures of the metal complexes (trans-Pt(DMSO)(NH2CH2CH2OH)Cl2 or trans-Pd(DMSO)(H2NCH2CH2OH)Cl2) and Nafion™ dispersion went through a film casting process, and the metal ions in the Nafion™ were reduced to metal nanoparticles by the annealing process. The in situ compositing method avoids the use of excess metal sources and any external reducing agent/stabilizing agents, and leads to a uniform distribution of nanoparticles in the polymer matrixes. The films were characterized using optical microscopy and scanning electron microscopy. The actuation behavior in response to an AC electric field was investigated by bending displacements and blocking force measurements.

Redox initiated cationic polymerization of oxetanes

ABSTRACT3,3‐Disubstituted oxetane monomers were found to undergo rapid, exothermic redox initiated cationic ring‐opening polymerization in the presence of a diaryliodonium or triarylsulfonium salt oxidizing agent and a hydrosilane reducing agent. The redox reaction requires a noble metal complex as a catalyst and several potential catalysts were evaluated. The palladium complex, Cl2(COD)PdII, was observed to provide good shelf life stability while, at the same time, affording high reactivity in the presence of a variety of hydrosilane reducing agents. A range of structurally diverse oxetane monomers undergo polymerization under redox cationic conditions. When a small amount of an alkylated epoxide was added as a “kick‐start” accelerator to these same oxetanes, the redox initiated cationic polymerizations were extraordinarily rapid owing to the marked reduction in the induction period. A mechanistic interpretation of these results is offered. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1854–1861

Synthesis and photophysical properties of GemPhos noble metal complexes

GemPhos, a diphosphine ligand with a rigid paracyclophane scaffold, was used to prepare complexes of the noble metals gold, palladium, and platinum. The reaction of GemPhos with [Au(tht)2][ClO4] (tht = tetrahydrothiophene) surprisingly yielded a mononuclear charge separated gold compound [(GemPhos)Au][ClO4], in which the gold atom exhibits an uncommon trigonal planar coordination geometry. Furthermore, similar palladium [(GemPhos) (PdCl2)] and platinum [(GemPhos)(PtCl2)] complexes were obtained in very good yields by the reaction of GemPhos with [MCl2(COD)] (M = Pd, Pt; COD = 1,5-cyclooctadiene) in hot DMSO. All compounds were fully characterized by analytical and spectroscopic techniques and their solid-state structures were established by single X-ray crystallography. Their photoluminescent properties were measured at low and ambient temperatures, revealing different behavior depending on the metal and coordination mode.

Two complexes of Pt(IV) and Au(III) with 2,2'-dipyridylamine and 2,2'-dipyridylaminide ligands.

Two noble metal complexes involving ancillary chloride ligands and chelating 2,2'-bipyridylamine (Hdpa) or its deprotonated derivative (dpa), namely [bis(pyridin-2-yl-κN)amine]tetrachloridoplatinum(IV), [PtCl4(C10H9N3)], and [bis(pyridin-2-yl-κN)aminido]dichloridogold(III), [AuCl2(C10H8N3)], are presented and structurally characterized. The metal atom in the former has a slightly distorted octahedral coordination environment, formed by four chloride ligands and two pyridyl N atoms of Hdpa, while the metal atom in the latter has a slightly distorted square-planar coordination environment, formed by two chloride ligands and two pyridyl N atoms of dpa. The difference in conjugation between the pyridine rings in normal and deprotonated 2,2'-dipyridylamine is discussed on the basis of the structural features of these complexes. The influence of weak interactions on the supramolecular structures of the complexes, providing one-dimensional chains of [PtCl4(C10H9N3)] and dimers of [AuCl2(C10H8N3)], are discussed.

Cryogelation of chitosan using noble-metal ions: in situ formation of nanoparticles.

One of the purposes of the project was to develop the method of preparation of 3D macroporous hydrogel with a structure of interconnected pores by the use of noncovalent interactions. The combination of chitosan and noble-metal complexes was investigated as cross-linking agents for the preparation of ionic cryogels (ICs). Furthermore, the treatment of the ICs containing gold complex by glutaraldehyde results in spontaneous formation of gold nanoparticles (AuNPs) and chemical cross-linking of the cryogel. The characterization of prepared macroporous materials was carried out by the use of FTIR, SEM, TEM techniques, and texture analyzer. A new strategy for control of size distribution of AuNPs was suggested. The size distribution of obtained AuNPs and their population inside of walls of cryogels was estimated. A method for quantifying unreacted chloroauric acid in the presence of acetic acid was proposed. The possibility of use of prepared cryogels with immobilized AuNPs as a catalytic flow through reactor is shown.

Noble Metal Complexes with 4, 4′‐ and 5, 5′‐Pyridyl‐Functionalized (S)‐2, 2′‐Bis(diphenylphosphanyl)‐1, 1′‐binaphthyl Ligands

AbstractSix metal complexes based on AuI, PdII, or PtII with functionalized (S)‐4, 4′‐ or 5, 5′‐bis(3‐pyridyl)‐2, 2′‐bis(diphenylphosphanyl)‐1, 1′‐binaphthyl ligands (L1, L2) have been prepared and fully characterized. According to single‐crystal X‐ray analysis, the AuI complexes (S)‐[Au2Cl2(L1)] (1) and (S)‐[Au2Cl2(L2)] (2) show an almost linear Cl–Au–P arrangement, and the PdII and PtII complexes (S)‐[PdCl2(L1)] (3), (S)‐[PdCl2(L2)] (4), (S)‐[PtI2(L1)] (5), and (S)‐[PtI2(L2)] (6) exhibit distorted square‐planar geometries.

Recovery of noble metals from refractory sulfide black-shale ores

The processing of refractory sulfide black-shale ores and concentrates from the Sukhoy Log deposit containing gold, silver and platinum group metals (palladium and platinum) was investigated. The noble metals were leached both under atmospheric pressure and at 700kPa at 200°C in an autoclave. It was found that the preferred leaching method is the chlorination of the ore in HCl/NaCL solutions in the presence of MnO2. The sorption recovery of noble metals on Purolite anion exchangers and carbon adsorbent Calgon GRC-22 was carried out from solution with the following initial concentrations of noble metals (in mg/L): Au — 25; Pt — 5; Pd — 5; and Ag — 10 in the presence of 1M FeCl3, 0.1 М CuCl2, 1M NaCl and 1М HCl. It was shown that the chloride complexes of noble metals can be effectively adsorbed from this solution. Then, the selective elution of noble metals from anion exchangers Purolite A170 and Purolite S992 by means of different complex-forming reagents was studied. The best elution results were obtained with Na2SO3+NaNO2 and thiourea solutions.

In situ SERS monitoring of photocatalytic organic decomposition using recyclable TiO2-coated Ag nanowire arrays

Recently, semiconductor and noble metal complex nanomaterials have attracted ever-increasing attention because of the realization of multiple functionalities in a single entity. In this study, Ag–TiO2 core–shell nanocomposites were synthesized by hydrolysis of butyl titanate on the surface of Ag nanowires. Due to efficient electron transfer at the Ag–TiO2 interface, the as-prepared nanocomposites exhibit much higher photocatalytic activity than bare TiO2 films, as demonstrated by the enhanced photodegradation rate of R6G and methyl blue molecules. In the meanwhile, such nanocomposites serve as a high-sensitivity surface-enhanced Raman scattering (SERS) substrate for in situ monitoring of the photocatalytic decomposition reaction, and the substrate is recyclable due to its self-cleaning function. Full-wave numerical calculations reveal that the improved photocatalysis and SERS efficiencies are attributed to the largely enhanced electromagnetic near-field in the nanocomposites. Our results point out that bifunctional semiconductor-metal hybrids hold great promise for simultaneously detecting and decomposing organic pollutants in the environment.

Biphasic catalysis using amphiphilic polyphenols-chelated noble metals as highly active and selective catalysts

In the field of catalysis, it is highly desired to develop novel catalysts that combine the advantages of both homogeneous and heterogeneous catalysts. Here we disclose that the use of plant pholyphenol as amphiphilic large molecule ligand/stabilizer allows for the preparation of noble metal complex and noble metal nanoparticle catalysts. These catalysts are found to be highly selective and active in aqueous-organic biphasic catalysis of cinnamaldehyde and quinoline, and can be reused at least 3 times without significant loss of activity. Moreover, the catalytic activity and reusability of the catalysts can be rationally controlled by simply adjusting the content of polyphenols in the catalysts. Our strategy may be extended to design a wide range of aqueous-organic biphasic catalysis system.