Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine Oxides as Catalyst for the Chemoselective Hydrogenation of Substituted Aldehydes: a Remarkable Ligand Effect

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Strategies to improve hydrogen activation on gold catalysts

Catalytic reactions involving molecular hydrogen are at the heart of many transformations in the chemical industry. Classically, hydrogenations are carried out on Pd, Pt, Ru or Ni catalysts. However, the use of supported Au catalysts has garnered attention in recent years owing to their exceptional selectivity in hydrogenation reactions. This is despite the limited understanding of the physicochemical aspects of hydrogen activation and reaction on Au surfaces. A rational design of new improved catalysts relies on making better use of the hydrogenating properties of Au. This Review analyses the strategies utilized to improve hydrogen-Au interactions, from addressing the importance of the Au particle size to exploring alternative mechanisms for H2 dissociation on Au cations and Au-ligand interfaces. These insights hold the potential to drive future applications of Au catalysis.

Acute Bite Angle POP- and PSP-Type Ligands and Their Trinuclear Copper(I) Complexes: Synthesis and Photo-Luminescence Properties

In the current work, the rational synthesis of trinuclear copper complexes, incorporating acute bite angle POP- and PSP-type ligands, is reported. The in situ formation of POP (Ph2P-O-PPh2) or PSP (Ph2P-S-PPh2) ligands in the presence of a copper(I) precursor gave access to various trinuclear copper complexes of the form [Cu3(μ3-Hal)2(μ-PXP)3]PF6 [X = O; Hal = Cl (1), Br (2), I (3) and X = S; Hal = Cl (5), Br (6), I (7)]. Related iodide-containing complexes and clusters, such as [Cu4(μ3-I)4(Ph2PI)4] (4) and [Cu3(μ3-I)2(μ-I)(μ-PSP)2] (8), could also be obtained via the variation of the reaction stoichiometry. The investigation of the photo-optical properties by photo-luminescence spectroscopy has demonstrated that the phosphorescence in the visible region can be switched off through the mere change of the heteroatom in the ligand backbone (POP vs PSP ligand scaffold). Theoretical studies have been conducted to complement the experimental photo-optical data with detailed insights into the occurring electronic transitions. Consequently, this systematic study paves the way for tuning the photo-optical properties of transition metal complexes in a more rational way.

Electron-Poor Phosphines Enable the Selective Semihydrogenation Reaction of Alkynes with Pd on Carbon Catalysts

An alternative to the Lindlar catalyst for the semihydrogenation reaction of alkynes to alkenes is of high interest. Here we show that palladium on carbon (Pd/C), i.e., a widely available supported Pd catalyst, is converted from an unselective to a chemoselective catalyst during the semihydrogenation reaction of alkynes, after the addition of catalytic amounts of commercially available electron-poor phosphines. The catalytic activity is ≤7 times greater, and the selectivity is comparable to that of the industrial benchmark Lindlar catalyst.

Phosphinous Acid-Phosphinito Tetra-Icosahedral Au52 Nanoclusters for Electrocatalytic Oxygen Reduction

While the formation of superatomic nanoclusters by the three-dimensional assembly of icosahedral units was predicted in 1987, the synthesis and structural determination of such clusters have proven to be incredibly challenging. Herein, we employ a mixed-ligand strategy to prepare phosphinous acid-phosphinito gold nanocluster Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ with a tetra-icosahedral kernel. Unlike expected, each icosahedral Au₁₃ unit shares one vertex gold atom with two adjacent units, resulting in a "puckered" ring shape with a nuclearity of 48 in the kernel. The phosphinous acid-phosphinito ligand set, which consists of two phosphinous acids and one phosphinito motif, has strong intramolecular hydrogen bonds; the π-π stacking interactions between the phosphorus- and sulfur-based ligands provide additional stabilization to the kernel. Highly stable Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ serves as an effective electrocatalyst in the oxygen reduction reaction. Density functional theory calculations suggest that the phosphinous acid-phosphinito ligands provide the most active sites in the electrochemical catalysis, with O* formation being the rate-determining step.

Transition metal complexes of the PPO/POP ligand: variable coordination chemistry and photo-luminescence properties

In the current work the tautomeric equilibrium between tetraphenyldiphosphoxane (Ph₂P-O-PPh₂, POP) and tetraphenyldiphosphine monoxide (Ph₂P-P(O)Ph₂, PPO) in the absence and presence of transition metal precursors is investigated. Whereas with hard transition metal ions, such as Fe(II) and Y(III), PPO-type complexes, such as [FeCl₂(PPO)₂] (1) and [YCl₃(THF)₂(PPO)] (2), are formed, softer transition metals ions tend to form so-called coordination stabilised tautomers of the POP ligand form, such as [Cu₂(MeCN)₃(μ₂-POP)₂](PF₆)₂ (3), [Au₂Cl₂(μ₂-POP)] (4), and [Au₂(μ₂-POP)₂](OTf)₂ (5). The photo-optical properties of the PPO- and POP-type transition metal complexes are investigated experimentally using photo-luminescence spectroscopy, whereby the presence of metallophillic interactions was found to play a crucial role. The dinuclear copper complex [Cu₂(MeCN)₃(μ₂-POP)₂](PF₆)₂ (3) shows a very interesting thermochromic behavior and intense photo-luminescence with remarkable phosphoresence lifetimes at 77 K, which can probably be attributed to short intramolecular Cu-Cu distances.

Functionalization of graphene oxide with a hybrid P, N ligand for immobilizing and stabilizing economical and non-toxic nanosized CuO: an efficient, robust and reusable catalyst for the C–O coupling reaction in O-arylation of phenol

A new graphene oxide based heterogeneous catalytic system holding CuO nanoparticles through P and N donor sites for the C–O coupling reaction.

Ruthenium nanoparticles canopied by heptagon-containing saddle-shaped nanographenes as efficient aromatic hydrogenation catalysts

Ruthenium nanoparticles stabilized with non-planar polycyclic aromatic hydrocarbons (PAHs) are active catalysts in the hydrogenation of aromatic substrates under mild conditions.

Highly efficient and selective aqueous phase hydrogenation of aryl ketones, aldehydes, furfural and levulinic acid and its ethyl ester catalyzed by phosphine oxide-decorated polymer immobilized ionic liquid-stabilized ruthenium nanoparticles

Phosphine oxide-decorated polymer immobilized ionic liquid stabilized RuNPs catalyse the hydrogenation of aryl ketones with remarkable selectivity for the CO bond, complete hydrogenation to the cyclohexylalcohol and hydrogenation of levulinic acid to γ-valerolactone.

Secondary Phosphine Oxide Functionalized Gold Clusters and Their Application in Photoelectrocatalytic Hydrogenation Reactions

Ligands in ligand-protected metal clusters play a crucial role, not only because of their interaction with the metal core, but also because of the functionality they provide to the cluster. Here, we report the utilization of secondary phosphine oxide (SPO), as a new family of functional ligands, for the preparation of an undecagold cluster Au11-SPO. Different from the commonly used phosphine ligand (i.e., triphenylphosphine, TPP), the SPOs in Au11-SPO work as electron-withdrawing anionic ligands. While coordinating to gold via the phosphorus atom, the SPO ligand keeps its O atom available to act as a nucleophile. Upon photoexcitation, the clusters are found to inject holes into p-type semiconductors (here, bismuth oxide is used as a model), sensitizing the p-type semiconductor in a different way compared to the photosensitization of a n-type semiconductor. Furthermore, the Au11-SPO/Bi₂O₃ photocathode exhibits a much higher activity toward the hydrogenation of benzaldehyde than a TPP-protected Au₁₁-sensitized Bi₂O₃ photocathode. Control experiments and density functional theory studies point to the crucial role of the cooperation between gold and the SPO ligands on the selectivity toward the hydrogenation of the C═O group in benzaldehyde.

Clean protocol for deoxygenation of epoxides to alkenes via catalytic hydrogenation using gold

Au NP catalyst combined with triethylphosphite, P(OEt)₃, is remarkably more reactive than solely Au NPs for the selective deoxygenation of epoxides to alkenes.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H₂ splitting by a cooperative mechanism with metal nanoparticles.

Chelating Phosphine Ligand Stabilized AuNPs in Methane Detection

The capping reagent plays an essential role in the functional properties of gold nanoparticles (AuNPs). Multiple stimuli-responsive materials are generated via diverse surface modification. The ability of the organic ligand shell on a gold surface to create a porous shell capable of binding small molecules is demonstrated as an approach to detect molecules, such as methane, that would be otherwise difficult to sense. Thiols are the most studied capping ligands of AuNPs used in chemiresistors. Phosphine capping groups are usually seen as stabilizers in synthesis and catalysis. However, by virtue of the pyramidal shape of triarylphosphines, they are natural candidates to create intrinsic voids within the ligand shell of AuNPs. In this work, surface-functionalized (capped) AuNPs with chelating phosphine ligands are synthesized via two synthetic routes, enabling chemiresistive methane gas detection at sub-100 ppm levels. These AuNPs are compared to thiol-capped AuNPs, and studies were undertaken to evaluate structure-property relationships for their performance in the detection of hydrocarbons. Polymer overcoatings applied to the conductive networks of the functionalized AuNP arrays were shown to reduce resistivity by promoting the formation of conduction pathways with decreased core-core distance between nanoparticles. Observations made in the context of developing methane sensors provide insight relevant to applications of phosphine or phosphine-containing surface groups in functional AuNP materials.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

Interventional Photothermal Therapy Enhanced Brachytherapy: A New Strategy to Fight Deep Pancreatic Cancer

Photothermal-radiotherapy (PT-RT) is an effective strategy for relieving hypoxia-related radiotherapy resistance and inducing tumor-specific cell apoptosis/necrosis. Nevertheless, limited tissue penetration of near-infrared (NIR) laser and the serious side effects of high-dose radiation severely hinder its applications for deep tumors. An interventional photothermal-brachytherapy (IPT-BT) technology is proposed here for the internal site-specific treatment of deep tumors. This technology utilizes a kind of biodegradable honeycomb-like gold nanoparticles (HGNs) acting as both internal photothermal agents and radiosensitizers. A high tumor inhibition rate of 96.6% is achieved in SW1990 orthotopic pancreatic tumor-bearing mice by HGNs-mediated IPT-BT synergistic therapy. Interestingly, this approach effectively causes double-stranded DNA damage and improves the oxygen supply and the penetration of nanoparticles inside the tumor. Therefore, it is believed that this strategy may open up a new avenue for PT-RT synergistic therapy of deep malignant tumors and has a significant impact on the future clinical translation.

Preparation of high-yield and ultra-pure Au25 nanoclusters: towards their implementation in real-world applications

Colloidal approaches allow for the synthesis of Au nanoclusters (NCs) with atomic precision and sizes ranging from a few to hundreds of atoms. In most of the cases, these processes involve a common strategy of thiol etching of initially polydisperse Au nanoparticles into atomically precise NCs, resulting in the release of Au-thiolate complexes as byproducts. To the best of our knowledge, neither the removal of these byproducts nor the mass spectra in the relevant mass region were shown in previous studies. A thorough analysis of inorganic byproducts in the synthesis of [Au25(PPh3)10(PET)5X2]2+ NC, abbreviated as Au25 NC, reveals that published protocols lead to Au25 NCs in vanishingly small quantities compared to their byproducts. Three purification methods are presented to separate byproducts from the desired Au25 NCs which are proposed to be applicable to other promising Au NC systems. Additionally, critical factors for a successful synthesis of Au25 NCs are identified and discussed including the role of residual water. An important finding is that the etching duration is very critical and must be monitored by UV-Vis spectroscopy resulting in synthetic yields as high as 40%.

Coordination chemistry and catalysis with secondary phosphine oxides

Review on synthesis, coordination chemistry and catalysis with secondary phosphine oxides.

Functionalized Ionic Liquids Sputter Decorated with Pd Nanoparticles

The fabrication of surface clean palladium nanoparticles of 3–4nm was accomplished in imidazolium-based functionalized ionic liquids (ILs) having methoxy, cyano, and thio groups by magnetron sputtering deposition. The size of the NPs was strongly dependent on the surface composition and/or organisation of the ILs. The NP growth apparently occurred preferentially in the bulk of the fluids, whereas nucleation apparently occurred preferentially at the IL surface. Smaller NPs were detected close to the methoxy containing IL surface and were covered by at least one layer of IL ion pairs, as revealed by high-sensitivity low-energy ion scattering (HS-LEIS) measurements.

Secondary phosphine oxides stabilized Au/Pd nanoalloys: metal components-controlled regioselective hydrogenation toward phosphinyl (Z)-[3]dendralenes

A series of gold/palladium nanoalloys stabilized by secondary phosphine oxides have been prepared and applied in selective hydrogenation for the first time.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Secondary Oxide Phosphines to Promote Tandem Acyl-Alkyl Coupling/Hydrogen Transfer to Afford (Hydroxyalkyl)rhodium Complexes. Theoretical and Experimental Studies

Acyl(σ-norbornenyl)rhodium(III) dimer [Rh(μ-Cl)(C₉H₆NCO)(C₇H₉)L]₂ (1) (C₇H₉ = σ-norbornenyl; L = 4-picoline, isoquinoline) reacts with diphenylphosphine oxide (SPO) to undergo a one-pot reaction involving (i) cleavage of the chloride bridges and coordination of the phosphine, (ii) C-C bond coupling between acyl and norbornenyl in a 18e species, and (iii) ligand-assisted outer-sphere O(P)-to-O(C) hydrogen transfer, to afford mononuclear 16e species [RhCl{(C₉H₆NC(O)C₇H₉)(Ph₂PO)H}(L)] (2) containing a quinolinyl-(norbornenylhydroxyalkyl) fragment hydrogen-bonded to a κ¹- P-phosphinite ligand. Pentacoordinated 2, which adopt a distorted trigonal bipyramidal structure, are kinetic reaction products that transform into the thermodynamic favored isomers 3. Structures 3 contain an unusual weak η¹-C anagostic interaction involving the rhodium atom and one carbon atom of the olefinic C-H bond of the norbornenyl substituent in the chelating quinolinyl-hydroxyalkyl moiety. Their structure can be described as pseudoctahedral, through a 5 + 1 coordination, with the anagostic interaction in a trans disposition with respect to the phosphorus atom of the phosphinite ligand. Complexes were characterized in solution by NMR spectroscopy and electrospray ionization mass spectrometry. Complex [RhCl{(C₉H₆NC(O)C₇H₉)(Ph₂PO)H}(4-picoline)] (3a) was also identified by X-ray diffraction. Density functional theory calculations confirm the proposed structures by a plausible set of mechanisms that accounts for the 1 (monomer) → 2 → 3 transformation. Lowest-energy pathways involve reductive elimination of quinolinylnorbornenylketone, still coordinated in the rhodium(I) species thus formed, followed by O-to-O hydrogen transfer from κ¹- P-SPO to the sp³ hybridized carbonyl group (formal alkoxide) avoiding the otherwise expected classical release of ketone. Theoretical ¹³C NMR studies also confirm the experimental spectral data for the considered structures.

Monitoring Thiol-Ligand Exchange on Au Nanoparticle Surfaces

Surface functionalization of nanoparticles (NPs) plays a crucial role in particle solubility and reactivity. It is vital for particle nucleation and growth as well as for catalysis. This raises the quest for functionalization efficiency and new approaches to probe the degree of surface coverage. We present an (in situ) proton nuclear magnetic resonance (¹H NMR) study on the ligand exchange of oleylamine by 1-octadecanethiol as a function of the particle size and repeated functionalization on Au NPs. Ligand exchange is an equilibrium reaction associated with Nernst distribution, which often leads to incomplete surface functionalization following "standard" literature protocols. Here, we show that the surface coverage with the ligand depends on the (i) repeated exchange reactions with large ligand excess, (ii) size of NPs, that is, the surface curvature and reactivity, and (iii) molecular size of the ligand. As resonance shifts and extensive line broadening during and after the ligand exchange impede the evaluation of ¹H NMR spectra, one- and two-dimensional ¹⁹F NMR techniques (correlation spectroscopy and diffusion ordered spectroscopy) with 1H,1H,2H,2H-perfluorodecanthiol as the fluorinated thiol ligand were employed to study the reactions. The enhanced resolution associated with the spectral range of the ¹⁹F nucleus allowed carrying out a site-specific study of thiol chemisorption. The widths and shifts of the resonance signals of the different fluorinated carbon moieties were correlated with the distance to the thiol anchor group. In addition, the diffusion analysis revealed that moieties closer to the NP surface are characterized by a broader diffusion coefficient distribution as well as slower diffusion.

Selective hydrogenation of unsaturated aldehydes over Pt nanoparticles promoted by the cooperation of steric and electronic effects

The selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols can reach high selectivity and activity at room temperature using Pt nanoparticles immobilized on a non-porous Al₂O₃ support stabilized by aspartic acid.

Development of a novel secondary phosphine oxide–ruthenium(ii) catalyst and its application for carbonyl reduction

A secondary phosphine oxide–phosphine mixed tridentate ligand and its ruthenium complex have been developed.

Synthesis and coordination chemistry of enantiopure t-BuMeP(O)H

Enantioselective synthesis of t-BuMeP(O)H and its coordination chemistry towards several transition metals are described, showing a variety of coordination modes.

The role and fate of capping ligands in colloidally prepared metal nanoparticle catalysts

In this Perspective article, we highlight emerging opportunities for the rational design of catalysts upon the choice, exchange, partial removal or pyrolysis of ligands.

An iridium–SPO complex as bifunctional catalyst for the highly selective hydrogenation of aldehydes

An Ir(iii) hydride catalytic system shows very high activities and selectivities in the chemoselective hydrogenation of various substituted aldehydes.

Surprisingly high sensitivity of copper nanoparticles toward coordinating ligands: consequences for the hydride reduction of benzaldehyde

Depending on the ligand, ligand-induced leaching of copper nanoparticles may produce catalytically active species for the reduction of benzaldehyde.