Rh(0) Nanoclusters in Benzene Hydrogenation Catalysis: Kinetic and Mechanistic Evidence that a Putative [(C8H17)3NCH3]+[RhCl4]- Ion-Pair Catalyst Is Actually a Distribution of Cl- and [(C8H17)3NCH3]+ Stabilized Rh(0) Nanoclusters

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water

Molecular hydrogen (H₂) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H₂ production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [Rh^III(tpy)(CH₃CN)Cl₂](CF₃SO₃) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H₂ in water in the presence of the [Ru(bpy)₃]Cl₂ (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of Ru, the Rh-tpy catalyst produces H₂ with turnover number versus catalyst (TON_Cat*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru/Rh-tpy/HA^-/H₂A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh^III(dmbpy)₂Cl₂]⁺ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh^I(tpy)Cl] (Rh^I-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep_c(Rh^III/Rh^I) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [Ru^II(bpy)(bpy^•-)]⁺ (Ru^-) (E_1/2(Ru^II/Ru^-) = -1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern-Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru^-) were then able to activate the Rh^III-tpy H₂-evolving catalyst by reduction generating Rh^I-tpy, which can react with a proton on a sub-nanosecond time scale to form a Rh^III(H)-tpy hydride, the key intermediate for H₂ evolution.

Brønsted Acid Catalyzed Dearomatization by Intramolecular Hydroalkoxylation/Claisen Rearrangement: Diastereo- and Enantioselective Synthesis of Spirolactams

Described herein is a novel Brønsted acid catalyzed intramolecular hydroalkoxylation/Claisen rearrangement, allowing the practical and atom-economic synthesis of a range of valuable spirolactams from readily available ynamides in generally good to excellent yields with excellent diastereoselectivities and broad substrate scope. Importantly, an unexpected dearomatization of nonactivated arenes and heteroaromatic compounds is involved in this tandem sequence. Moreover, an asymmetric version of this tandem cyclization was also achieved by efficient kinetic resolution by chiral phosphoric acid catalysis. In addition, the [3,3]-rearrangement is shown to be kinetically preferred over the related [1,3]-rearrangement by theoretical calculations.

Rhodium nanoparticles inside well-defined unimolecular amphiphilic polymeric nanoreactors: synthesis and biphasic hydrogenation catalysis

Rhodium nanoparticles (Rh NPs) embedded in different amphiphilic core-crosslinked micelle (CCM) latexes (RhNP@CCM) have been synthesized by [RhCl(COD)(TPP@CCM)] reduction with H₂ (TPP@CCM = core-anchored triphenylphosphine). The reduction rate depends on temperature, on the presence of base (NEt₃) and on the P/Rh ratio. For CCMs with outer shells made of neutral P(MAA-co-PEOMA) copolymer chains (RhNP@CCM-N), the core-generated Rh NPs tend to migrate toward the hydrophilic shell and to agglomerate depending on the P/Rh ratio and core TPP density, whereas the MAA protonation state has a negligible effect. Conversely, CCMs with outer shells made of polycationic P(4VPMe⁺I^-) chains (RhNP@CCM-C) maintain core-confined and well dispersed Rh NPs. All RhNP@CCMs were used as catalytic nanoreactors under aqueous biphasic conditions for acetophenone, styrene and 1-octene hydrogenation. Styrene was efficiently hydrogenated by all systems with high selectivity for vinyl reduction. For acetophenone, competition between benzene ring and carbonyl reduction was observed as well as a limited access to the catalytic sites when using CCM-C. Neat 1-octene was also converted, but the activity increased when the substrate was diluted in 1-nonanol, which is a better core-swelling solvent. Whereas the molecular Rh^I center was more active than the Rh⁰ NPs in 1-octene hydrogenation, the opposite trend was observed for styrene hydrogenation. Although Rh NP migration and agglomeration occurred for RhNP@CCM-N, even at high P/Rh, the NPs remained core-confined for RhNP@CCM-C, but only when toluene rather than diethyl ether was used for product extraction before recycling.

Phosphine-Built-in Porous Organic Cage for Stabilization and Boosting the Catalytic Performance of Palladium Nanoparticles in Cross-Coupling of Aryl Halides

Herein, we report first a novel phosphine-containing porous organic cage (PPOC) from a [2 + 3] self-assembly of triphenyl phosphine-based trialdehyde and (S,S)-1,2-diaminocyclohexane via dynamic imine chemistry, which was employed as a porous material for the controlled growth of palladium nanoparticles (NPs) due to the strong affinity of Pd to the phosphine ligand based on the principle of hard and soft acids and bases. Comprehensive characterizations including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, NMR, and X-ray absorption spectroscopy reveal that ultrafine Pd NPs with narrow size distribution (1.7 ± 0.3 nm) and enhanced surface electronic density via a strong interaction between NPs and phosphine were homogeneously dispersed in the PPOC. The resultant catalyst Pd@PPOC exhibits remarkably superior catalytic activities for various cross-coupling reactions of aryl halides, for example, Sonogashira, Suzuki, Heck, and carbonylation. The catalytic activity of Pd@PPOC outperforms the state-of-the-art Pd complexes and other Pd NPs supported on N-containing porous cages under identical conditions, owing to the enhanced surface electronic density of Pd NPs and their high stability and dispersibility in solution. More importantly, Pd@PPOC is highly stable and easily recycled and reused without loss of their catalytic activity. This work provides a new functional POC with extended potentials in catalysis and material science.

Mechanistic Understanding of the Heterogeneous, Rhodium-Cyclic (Alkyl)(Amino)Carbene-Catalyzed (Fluoro-)Arene Hydrogenation

Recently, chemoselective methods for the hydrogenation of fluorinated, silylated, and borylated arenes have been developed providing direct access to previously unattainable, valuable products. Herein, a comprehensive study on the employed rhodium-cyclic (alkyl)(amino)carbene (CAAC) catalyst precursor is disclosed. Mechanistic experiments, kinetic studies, and surface-spectroscopic methods revealed supported rhodium(0) nanoparticles (NP) as the active catalytic species. Further studies suggest that CAAC-derived modifiers play a key role in determining the chemoselectivity of the hydrogenation of fluorinated arenes, thus offering an avenue for further tuning of the catalytic properties.

CuMoO4 Bimetallic Nanoparticles, An Efficient Catalyst for Room Temperature C-S Cross-Coupling of Thiols and Haloarenes

Cu^II catalyst is less efficient at room temperature for C-S cross-coupling. C-S cross-coupling by Cu^II catalyst at room temperature is not reported; however, doping of copper with molybdenum metal has been realized here to be more efficient for C-S cross-coupling in comparison to general Cu^II catalyst. The doped catalyst CuMoO₄ nanoparticle is found to be more efficient than copper. The catalyst works under mild conditions without any ligand at room temperature and is recyclable and effective for a wide range of thiols and haloarenes (ArI, ArBr, ArF) from milligram to gram scale. The copper-based bimetallic catalyst is developed and recognized for C-S cross-coupling of haloarenes with alkyl and aryl thiols.

Bimetallic BaMoO4 nanoparticles for the C–S cross-coupling of thiols with haloarenes

The C–S cross-coupling of alkyl/aryl thiols with haloarenes using BaMoO₄ nanoparticles was accomplished with high yields, and good functional group tolerance and selectivity. Unactivated bromo and 4-acetyl fluoroarenes can afford thioethers in high yields.

Selective Arene Hydrogenation for Direct Access to Saturated Carbo- and Heterocycles

Arene hydrogenation provides direct access to saturated carbo- and heterocycles and thus its strategic application may be used to shorten synthetic routes. This powerful transformation is widely applied in industry and is expected to facilitate major breakthroughs in the applied sciences. The ability to overcome aromaticity while controlling diastereo-, enantio-, and chemoselectivity is central to the use of hydrogenation in the preparation of complex molecules. In general, the hydrogenation of multisubstituted arenes yields predominantly the cis isomer. Enantiocontrol is imparted by chiral auxiliaries, Brønsted acids, or transition-metal catalysts. Recent studies have demonstrated that highly chemoselective transformations are possible. Such methods and the underlying strategies are reviewed herein, with an emphasis on synthetically useful examples that employ readily available catalysts.

Recyclable bimetallic CuMoO4 nanoparticles for C–N cross-coupling reaction under mild conditions

New bimetallic CuMoO₄ nanocatalysts for C–N cross-coupling.

Morphology-Controlled Synthesis of Rhodium Nanoparticles for Cancer Phototherapy

Rhodium nanoparticles are promising transition metal nanocatalysts for electrochemical and synthetic organic chemistry applications. However, notwithstanding their potential, to date, Rh nanoparticles have not been utilized for biological applications; there has been no cytotoxicity study of Rh reported in the literature. In this regard, the absence of a facile and controllable synthetic strategy of Rh nanostructures with various sizes and morphologies might be responsible for the lack of progress in this field. Herein, we have developed a synthetic strategy for Rh nanostructures with controllable morphology through an inverse-directional galvanic replacement reaction. Three types of Rh-based nanostructures-nanoshells, nanoframes, and porous nanoplates-were successfully synthesized. A plausible synthetic mechanism based on thermodynamic considerations has also been proposed. The cytotoxicity, surface functionalization, and photothermal therapeutic effect of manufactured Rh nanostructures were systematically investigated to reveal their potential for in vitro and in vivo biological applications. Considering the comparable behavior of porous Rh nanoplates to that of gold nanostructures that are widely used in nanomedicine, the present study introduces Rh-based nanostructures into the field of biological research.

Heterobimetallic Catalysis: Platinum-Gold-Catalyzed Tandem Cyclization/C-X Coupling Reaction of (Hetero)Arylallenes with Nucleophiles

Heterobimetallic catalysis offers new opportunities for reactivity and selectivity but still presents challenges, and only a few metal combinations have been explored so far. Reported here is a Pt-Au heterobimetallic catalyst system for the synthesis of a family of multi-heteroaromatic structures through tandem cyclization/C-X coupling reaction. Au-catalyzed 6-endo-cyclization takes place as the first fast step. Pt-Au clusters are proposed to be responsible for the increased reactivity in the second step, that is, the intermolecular nucleophilic addition which occurs through an outer-sphere mechanism by hybrid homogeneous-heterogeneous catalysis.

Aromatic ring hydrogenation catalysed by nanoporous montmorillonite supported Ir(0)-nanoparticle composites under solvent free conditions

Ir nanoparticles supported on nanoporous montmorillonite clay showing efficient catalytic activity for hydrogenation of aromatic compounds.

Eco-friendly Suzuki–Miyaura coupling of arylboronic acids to aromatic ketones catalyzed by the oxime-palladacycle in biosolvent 2-MeTHF

Oxime-palladacycle catalyzed aerobic cross-coupling reaction of arylboronic acids and acyl chlorides to yield aryl ketones was developed. The reaction proceeds efficiently in 2-MeTHF, which is derived from corncobs, sugar cane, and fulfils the 3rd, 5th and 7th principles of Green Chemistry.

Design and development of POCN-pincer palladium catalysts for C-H bond arylation of azoles with aryl iodides

Well-defined and efficient POCN-ligated palladium complexes have been developed for the direct C-H bond arylation of azoles with aryl iodides. The phosphinite-amine pincer ligands 1-(R2PO)-C6H4-3-(CH2N(i)Pr2) [(R2)POCN(iPr2)-H; R = (i)Pr (), R = (t)Bu ()] and corresponding palladium complexes {2-(R2PO)-C6H3-6-(CH2N(i)Pr2)}PdCl [((R2)POCN(iPr2))PdCl; R = (i)Pr (), R = (t)Bu ()] were synthesized in good yields. Treatment of palladium complex with KI and AgOAc afforded the complexes ((iPr2)POCN(iPr2))PdI () and ((iPr2)POCN(iPr2))Pd(OAc) (), respectively. Similarly, the reaction of with benzothiazolyl-lithium produces the ((iPr2)POCN(iPr2))Pd(benzothiazolyl) () complex in a quantitative yield. The pincer palladium complex efficiently catalyzes the C-H bond arylation of benzothiazole, substituted-benzoxazoles and 5-aryl oxazoles with diverse aryl iodides in the presence of CuI as a co-catalyst under mild reaction conditions. This represents the first example of a pincer palladium complex being applied for the direct C-H bond arylation of any heterocycle with low catalyst loading. A preliminary mechanistic investigation reveals that palladium nanoparticles are presumably not the catalytically active form of and supports the direct involvement of the catalyst , with complex being a probable key intermediate in the catalytic reaction.

Microwave-assisted synthesis of 3-nitroindoles from N-aryl enamines via intramolecular arene-alkene coupling

A variety of N-aryl β-nitroenamines were effectively transformed into 3-nitroindoles in good yields and with complete regioselectivity via a rapid microwave (μW) assisted intramolecular arene-alkene coupling reaction. This report further demonstrates the versatility of this method by constructing 3-carboalkoxy- and 3-cyanoindoles. Optimization data, substrate scope, and applications are discussed.

Solar fuels generation and molecular systems: is it homogeneous or heterogeneous catalysis?

Catalysis is a key enabling technology for solar fuel generation. A number of catalytic systems, either molecular/homogeneous or solid/heterogeneous, have been developed during the last few decades for both the reductive and oxidative multi-electron reactions required for fuel production from water or CO(2) as renewable raw materials. While allowing for a fine tuning of the catalytic properties through ligand design, molecular approaches are frequently criticized because of the inherent fragility of the resulting catalysts, when exposed to extreme redox potentials. In a number of cases, it has been clearly established that the true catalytic species is heterogeneous in nature, arising from the transformation of the initial molecular species, which should rather be considered as a pre-catalyst. Whether such a situation is general or not is a matter of debate in the community. In this review, covering water oxidation and reduction catalysts, involving noble and non-noble metal ions, we limit our discussion to the cases in which this issue has been directly and properly addressed as well as those requiring more confirmation. The methodologies proposed for discriminating homogeneous and heterogeneous catalysis are inspired in part by those previously discussed by Finke in the case of homogeneous hydrogenation reaction in organometallic chemistry [J. A. Widegren and R. G. Finke, J. Mol. Catal. A, 2003, 198, 317-341].