Atomically precise pieces of matter of nanometer dimensions composed of noble metals are new categories of materials with many unusual properties. Over 100 molecules of this kind with formulas such as Au25(SR)18, Au38(SR)24, and Au102(SR)44 as well as Ag25(SR)18, Ag29(S2R)12, and Ag44(SR)30 (often with a few counterions to compensate charges) are known now. They can be made reproducibly with robust synthetic protocols, resulting in colored solutions, yielding powders or diffractable crystals. They are distinctly different from nanoparticles in their spectroscopic properties such as optical absorption and emission, showing well-defined features, just like molecules. They show isotopically resolved molecular ion peaks in mass spectra and provide diverse information when examined through multiple instrumental methods. Most important of these properties is luminescence, often in the visible–near-infrared window, useful in biological applications. Luminescence in the visible region, especially by clusters prot...

Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles

Large-scale applications of transition metal-catalyzed couplings for the synthesis of pharmaceuticals.

We present a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the presence of metal nanoparticles. The nanoparticles are embedded in spherical polyelectrolyte brushes, which consist of a polystyrene core onto which a dense layer of cationic polyelectrolyte brushes are grafted. The average size of the nanoparticles is approximately 2 nm. The kinetic data obtained by monitoring the reduction of 4-nitrophenol by UV/vis-spectroscopy could be explained in terms of the Langmuir−Hinshelwood model: The borohydride ions transfer a surface-hydrogen species in a reversible manner to the surface. Concomitantly 4-nitrophenol is adsorbed and the rate-determining step consists of the reduction of nitrophenol by the surface-hydrogen species. The apparent reaction rate can therefore be related to the total surface S of the nanoparticles, to the kinetic constant k related to the rate-determining step, and to the adsorption constants KNip and KBH4 of nitrophenol and of borohydride, respectively. In...

Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes

5.4. Miscellaneous Examples 2295 5.4.1. Couplings with Other Catalysts 2295 5.4.2. The Use of Other Boron Reagents 2295 5.4.3. Related Coupling Reactions 2296 5.5. Summary 2297 6. Sonogashira Coupling 2297 6.1. Couplings with Pd Particles 2297 6.2. Studies with Immobilized Palladium Complexes 2299 6.3. Miscellaneous Examples 2301 6.4. Carbonylative Sonogashira Couplings 2302 6.5. Summary 2302 7. Stille Couplings 2302 7.1. Stille Coupling 2302 7.2. Carbonylative Stille Coupling 2304 7.3. Summary 2304 8. Miscellaneous Cross-Coupling Reactions 2304 9. Allylations 2305 9.1. Tsuji-Trost Allylation 2305 9.2. Allylation of Boron Reagents 2306 10. Homocouplings 2307 10.1. Summary 2308 11. C-C Couplings in Continuous Systems 2308 12. Conclusions and Outlook 2311 Author Information 2314 Biography 2314 References 2314

Efficient, selective, and recyclable palladium catalysts in carbon-carbon coupling reactions.

Catalysis by Supported Gold Nanoparticles: Beyond Aerobic Oxidative Processes

The noble metal nanoparticles of Pd, Au, and Ag embedded in the shell layer of core−shell poly(styrene-co-4-vinylpyridine) micospheres were synthesized, and the catalytic activity of the shell-embedded Pd nanoparticles was investigated. To increase the accessible active site and therefore increase the catalytic activity of noble metal nanoparticles, the in situ synthesized noble metal nanoparticles are selectively immobilized in the outer shell layer of the core−shell poly(styrene-co-4-vinylpyridine) microspheres, which are synthesized by one-stage soap-free emulsion polymerization in water and contain a core of polystyrene and a coordinative shell of poly(4-vinylpyridine). It is found the Pd nanoparticles embedded in the shell layer of the core−shell micospheres are an efficient and easily reusable catalyst for Suzuki reactions performed in water.

Synthesis of Noble Metal Nanoparticles Embedded in the Shell Layer of Core−Shell Poly(styrene-co-4-vinylpyridine) Micospheres and Their Application in Catalysis

The block copolymer poly(N-isopropylacrylamide)-b-poly(4-vinyl pyridine) (PNIPAM-b-P4VP) self-assembled into core-corona micelles with the P4VP block as core and the thermoresponsive PNIPAM block as corona in water. The diameter of the micelles was about 40 nm and the lower critical solution temperature (LCST) was about 32 °C. Gold nanoparticles of size ranging from 2 to 4 nm were loaded in the micelles to form a responsive catalyst, the activity of which could be modulated due to the thermoresponsive PNIPAM. Below LCST, the PNIPAM chains were hydrophilic and the reactants could easily diffuse through the PNIPAM corona to reach the surface of the gold nanoparticles. Within this temperature range, the catalytic activity of the micelle-supported gold nanoparticles increased with the increase in temperature. Above LCST, the PNIPAM chains collapsed to form a hydrophobic barrier on the gold nanoparticles, which decelerated diffusion of the reactants. Within this temperature range, the activity of the micelle-supported gold nanoparticles decreased with the increase in temperature until to a minimum constant at about 38 °C.

Responsive catalysis of thermoresponsive micelle-supported gold nanoparticles

A nanoreactor of poly(N-isopropylacrylamide)-grafted Pd nanoparticle (Pd@PNIPAM) is proposed for the Suzuki reaction performed in the sole solvent of water. The Pd@PNIPAM nanoparticle has a core of Pd nanoparticle and a corona of poly(N-isopropylacrylamide) brushes. The Pd@PNIPAM nanoparticle can act as a nanoreactor for the Suzuki reaction since the grafted poly(N-isopropylacrylamide) brushes provide a nanoenvironment for guest molecules. Both hydrophilic and hydrophobic reactants can be enriched in the nanoreactor of Pd@PNIPAM, and therefore the Suzuki reaction within the nanoreactor is performed in water at room temperature or above the phase-transition temperature of the corona-forming brushes of poly(N-isopropylacrylamide). Besides, the nanoreactor of Pd@PNIPAM can be recycled due to the reversible phase-transition of the poly(N-isopropylacrylamide) brushes.

Suzuki Reaction within the Core−Corona Nanoreactor of Poly(N-isopropylacrylamide)-Grafted Pd Nanoparticle in Water

The copolymer of poly(N-isopropylacrylamide)-co-poly(4-vinylpyridine) was synthesized by free radical copolymerization of 4-vinylpyridine and N-isopropylacrylamide. The copolymer synthesized with the feed monomer ratio of 4-vinylpyridine/N-isopropylacrylamide equal to 1/3 was associated to form thermoresponsive colloid in neutral water at room temperature, the average size and the cloud-point temperature of which were 40 nm and 32 °C, respectively. The thermoresponsive colloid was used as scaffold to load 2-nm Au nanoparticles to form the responsive catalyst of colloid-stabilizing gold nanoparticles. The catalysis in the model reduction of 4-nitrophenol with NaBH4 suggested that the catalytic reduction could be modulated due to the thermoresponsive phase-transition of the colloid-stabilizing gold nanoparticles. That was, the catalytic reduction firstly accelerated with the increase in temperature below the cloud-point temperature and then decelerated with the increase in temperature above the cloud-point temperature of the thermoresponsive colloid-stabilizing Au nanoparticles.

Synthesis of gold nanoparticles stabilized with poly(N-isopropylacrylamide)-co-poly(4-vinyl pyridine) colloid and their application in responsive catalysis

A porous, thermoresponsive and pH-responsive, and chelating hydrogel of poly(N-isopropylacrylamide)-co-poly[2-methacrylic acid 3-(bis-carboxymethylamino)-2-hydroxypropyl ester] (PNIPAM-co-PMACHE) is proposed as both a reaction medium and the Pd catalyst support for organic synthesis. Organic synthesis within the PNIPAM-co-PMACHE hydrogel has three merits. First, organic reactions such as Suzuki and Heck reactions within the hydrogel can be accelerated due to the enriched Pd catalyst and reactants within the hydrogel by the reversible deswelling/swelling. Second, organic synthesis within the PNIPAM-co-PMACHE hydrogel, which holds about ∼300 times of water and is similar to the environmentally benign reaction medium of water, can be performed efficiently without surfactant or cosolvent being added. Third, the PNIPAM-co-PMACHE hydrogel itself and the therein-immobilized Pd catalyst can be easily recycled since the hydrogel/Pd composite can reversibly swell/deswell.

Yao Wang

Papers

Synthesis of Noble Metal Nanoparticles Embedded in the Shell Layer of Core−Shell Poly(styrene-co-4-vinylpyridine) Micospheres and Their Application in Catalysis

Responsive catalysis of thermoresponsive micelle-supported gold nanoparticles

Suzuki Reaction within the Core−Corona Nanoreactor of Poly(N-isopropylacrylamide)-Grafted Pd Nanoparticle in Water

Synthesis of gold nanoparticles stabilized with poly(N-isopropylacrylamide)-co-poly(4-vinyl pyridine) colloid and their application in responsive catalysis

Pd-catalyzed C-C cross-coupling reactions within a thermoresponsive and pH-responsive and chelating polymeric hydrogel.