Rhodium Catalysts Research Articles

Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation.

Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O2, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O2 enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.

Theoretical Investigation of Chemoselectivity between C-H Insertion and Amide Insertion in Intramolecular Rhodium-Carbene Reactions.

C-H insertion and amide insertion reactions using metal-carbene species provide a powerful synthetic method for direct functionalization of kinetically inert or thermodynamically stable chemical bonds. Our group previously developed an amide insertion reaction using a rhodium-dimer complex, constructing an array of nitrogen-bridged heterocycles. Another research group reported C-H insertion reactions using structurally related substrates and rhodium catalysts. Detailed mechanistic studies were not provided, however, and therefore, the origin of the chemoselectivity was ambiguous. Here we describe our theoretical investigation of the chemoselectivity between the amide insertion reaction and C-H functionalization. An energy gap of the identified transition states in the reaction coordinates could support the reported experimental results and the observed chemoselectivity. Moreover, frontier molecular orbital analysis revealed that functionalities adjacent to the metal-carbene species could affect orbital populations and their energy levels, resulting in the construction of a completely distinctive ring system.

Advances in Exploring Cyclopentadienyl (Cp) Rhodium Catalysts Featuring Diastereotopic or Enantiotopic Cp Faces for Asymmetric C–H Activation

AbstractChiral cyclopentadienyl rhodium (CpRh) complexes have emerged as a class of powerful catalysts for enantioselective C–H activation reactions. In terms of Cp ligand development, the mainstream is to design chiral ligands with C 2 symmetry in order to avoid the problem of Cp face selectivity during their metalation with rhodium. In recent years, CpRh catalysts with diastereotopic or enantiotopic Cp faces were also revealed and successfully applied in asymmetric C–H activation. These advances are summarized in this short review together with perspectives for their future development.1 Introduction2 Cp Ligands with Diastereotopic Cp Faces3 Cp Ligands with Enantiotopic Cp Faces4 Conclusion and Outlook

Synthesis of 3‐Methylidene‐2,3‐dihydropyrroles via Formal 1,2‐Enamine Migration/Cyclization Cascade of Rhodium Carbenes

Abstract3‐Methylidene‐2,3‐dihydropyrroles with an ester or keto group at the C4 position were obtained in moderate to good yields from enamine‐tethered triazoles in the presence of a rhodium(II) catalyst. A formal 1,2‐enamine migration/cyclization cascade triggered by formation of an α‐amino carbene is proposed to illustrate the mechanism. The valuable exocyclic double bond showed distinctive reactivity, which is very useful for further introduction of functional groups. The dihydropyrrole is stable under neutral, weakly acidic, or weakly basic conditions.magnified image

Low-Coordination Rhodium Catalysts for an Efficient Electrochemical Nitrate Reduction to Ammonia

Ammonia is an essential bulk chemical and the main component of fertilizers. In addition, the use of ammonia (NH3) as an energy carrier and hydrogen storage material has continuously surged. Electrochemical nitrate reduction is a low-carbon, environment-friendly, and efficient method of ammonia synthesis, which has attracted extensive attention in recent years; however, the overpotential needed to produce NH3 with most catalysts is still too large. In this work, we rationally designed rhodium nanoflowers (Rh NFs) composed of ultrathin nanosheets and explored their performance for the electrocatalytic nitrate reduction to ammonia (NITRR). With a high faradic efficiency of 95% at 0.2 V vs reversible hydrogen electrode (RHE) for ammonia production, the overpotential required for the NH3 formation on an Rh NF catalyst is much lower than on most previously reported catalysts. X-ray absorption spectroscopy (XAS) analysis shows that there are low-coordination atoms in the Rh NF catalyst, which can promote the adsorption of NO3– ions and stabilize intermediates as revealed by the density functional theory (DFT) calculation, resulting in efficient NITRR performance.

Atomically Adjustable Rhodium Catalyst Synthesis with Outstanding Mass Activity via Surface‐Limited Cation Exchange

Rh has been widely studied as a catalyst for the promising hydrazine oxidation reaction that can replace oxygen evolution reactions for boosting hydrogen production from hydrazine‐containing wastewater. Despite Rh being expensive, only a few studies have examined its electrocatalytic mass activity. Herein, surface‐limited cation exchange and electrochemical activation processes are designed to remarkably enhance the mass activity of Rh. Rh atoms were readily replaced at the Ni sites on the surface of NiOOH electrodes by cation exchange, and the resulting RhOOH compounds were activated by the electrochemical reduction process. The cation exchange‐derived Rh catalysts exhibited particle sizes not exceeding 2 nm without agglomeration, indicating a decrease in the number of inactive inner Rh atoms. Consequently, an improved mass activity of 30 A mgRh−1 was achieved at 0.4 V versus reversible hydrogen electrode. Furthermore, the two‐electrode system employing the same CE‐derived Rh electrodes achieved overall hydrazine splitting over 36 h at a stable low voltage. The proposed surface‐limited CE process is an effective method for reducing inactive atoms of expensive noble metal catalysts.

Controlling selectivity for dechlorination of poly(vinyl chloride) with (xantphos)RhCl.

Reaction of poly(vinyl chloride) (PVC) with 5 equiv. of triethyl silane in THF, in the presence of in situ generated (xantphos)RhCl catalyst, results in partial reduction of PVC via hydrodechlorination to yield poly(vinyl chloride-co-ethylene). Increasing catalyst loading or using N,N-dimethylacetamide (DMA) as a solvent both diminished selectivity for hydrodechlorination, promoting competitive dehydrochlorination reactions. Reaction of PVC with 2 equiv. of sodium formate in THF in the presence of (xantphos)RhCl affords excellent selectivity for hydrodechlorination along with complete PVC dechlorination, yielding polyethylene-like polymers. Higher catalyst loadings were necessary to activate PVC towards reduction in this case. In contrast, reaction of PVC with 1 equiv. of NaH in DMA, in the presence of (xantphos)RhCl, exhibited good selectivity for dehydrochlorination, as well as much higher reaction rates. These results combined shed light on the interplay between critical reaction parameters that control PVC's mode of reactivity.

β-(Z)-Selective alkyne hydrosilylation by a N,O-functionalized NHC-based rhodium(I) catalyst.

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-tBuImCH2PyCH2OMe)] and [Rh(cod)(κ2C,N-tBuImCH2PyCH2OMe)]PF6, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)2(κ2C,N-tBuImCH2PyCH2OMe)]PF6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)2(κC-tBuImCH2PyCH2OMe)] and [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe2Ph with a marked selectivity towards the β-(Z)-vinylsilane product. Catalyst [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMe2Ph, HSiMePh2, HSiPh3 and HSiEt3, showing excellent β-(Z) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn-addition vinylsilane products. The β-(Z) selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be β-(E) selective. An energy barrier ΔG‡ of 19.8 ± 2.0 kcal mol-1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe2Ph. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed via a classical Si-H oxidative addition.

Rh-catalysed divergent synthesis of polysubstituted pyrroles from α,β-unsaturated ketones via selective single or double insertion of isocyanides

A novel divergent synthetic method for polysubstituted pyrroles from isocyanides and α,β-unsaturated ketones in the presence of a rhodium catalyst and bis(pinacolato)diboron (B2pin2) is described here.

Operando characterization of rhodium catalyst degradation in hydroformylation

Kinetic studies combined with DFT calculations revealed degradation of catalyst to Rh-carbonyl clusters is responsible for the loss of regioselectivity in hydroformylation.

A recyclable rhodium catalyst anchored onto a bipyridine covalent triazine framework for transfer hydrogenation of N-heteroarenes in water

Bipyridine covalent triazine framework (bpyCTF) was used as a support material for the development of a highly efficient Rh catalyst. The heterogeneous catalyst was applied for both batch and flow transfer hydrogenation of various N-heteroarenes.

Rh-Catalyzed Environmentally Benign Selective Hydrogenation of a Broad Variety of Functional Groups Using Al-Water as a Hydrogen Source

Supported rhodium catalysts were screened to catalyze the one-step hydrogenation of a broad variety of functional groups. The results show that 5% Rh/Al2O3 and 5% Rh/C performed well in controlling selective hydrogenation under the desired amount of time and temperature. In this regard, partial and full hydrogenation were achieved by controlling reaction time or temperature. In addition to aliphatic C–C, C–N, C–O, and N–O multiple bonds, the applicability of this method was demonstrated by the hydrogenation of C=C double bonds of arenes, which is considered challenging. Importantly, the Al-H2O system producing hydrogen in situ and the high, controllable selectivity make this protocol environmentally benign and highly efficient.

Rh(II)-Catalyzed C-N Bond Formation Using Enynones and N-H Imines: An Approach to Diarylmethylamines.

A Rh(II)-catalyzed furyl carbene N-H insertion reaction involving an N-sp2-hybridized imine is described, which represents an atom-economical route by which to access diarylmethylamine derivatives with high efficiency and broad substrate scope. An unusual reaction mechanism is proposed, in which the rhodium catalyst plays a dual role in facilitating enynone cyclization via activation and enhancement of the nucleophilicity of the NH imine.

Rhodium‐Catalyzed Dynamic Kinetic Asymmetric Hydrosilylation to Access Silicon‐Stereogenic Center

AbstractStrategies on the construction of enantiomerically pure silicon‐stereogenic silanes generally relies on desymmetrization of prochiral and symmetric substrates. However, dynamic kinetic asymmetric transformations of organosilicon compounds have remained underdeveloped and unforeseen owing to a lack of an effective method for deracemization of the static silicon stereocenters. Here we report the first Rh‐catalyzed dynamic kinetic asymmetric intramolecular hydrosilylation (DyKAH) with “silicon‐centered” racemic hydrosilanes that enables the facile preparation of silicon‐stereogenic benzosiloles in good yields and excellent enantioselectivities. The special rhodium catalyst controlled by non‐diastereopure‐type mixed phosphine‐phosphoramidite ligand with axial chirality and multiple stereocenters can induce enantioselectivity efficiently in this novel DyKAH reaction. Density functional theory (DFT) calculations suggest that the amide moiety in chiral ligand plays important role in facilitating the SN2 substitution of chloride ion to realize the chiral inversion of silicon center.

Rhodium-Catalyzed Dynamic Kinetic Asymmetric Hydrosilylation to Access Silicon-Stereogenic Center.

Strategies on the construction of enantiomerically pure silicon-stereogenic silanes generally relies on desymmetrization of prochiral and symmetric substrates. However, dynamic kinetic asymmetric transformations of organosilicon compounds have remained underdeveloped and unforeseen owing to a lack of an effective method for deracemization of the static silicon stereocenters. Here we report the first Rh-catalyzed dynamic kinetic asymmetric intramolecular hydrosilylation (DyKAH) with "silicon-centered" racemic hydrosilanes that enables the facile preparation of silicon-stereogenic benzosiloles in good yields and excellent enantioselectivities. The special rhodium catalyst controlled by non-diastereopure-type mixed phosphine-phosphoramidite ligand with axial chirality and multiple stereocenters can induce enantioselectivity efficiently in this novel DyKAH reaction. Density functional theory (DFT) calculations suggest that the amide moiety in chiral ligand plays important role in facilitating the SN 2 substitution of chloride ion to realize the chiral inversion of silicon center.

Enantioselective Intermolecular Hydroselenation of Alkenes Using a Cationic Rhodium Catalyst

Enantioselective Intermolecular Hydroselenation of Alkenes Using a Cationic Rhodium Catalyst

Ru-, Rh- and Ir-Catalyzed Enantioselective sp3 C-H Functionalization.

The development of novel methodologies that enable the construction of complex chiral molecular scaffolds in an atom and step-economic manner has always been an area of immense interest and significant synthetic value. The transition metal-catalyzed enantioselective sp3 C-H activation and functionalization reactions serve as an expeditious means for introducing diverse functionalities in a straightforward manner. However, controlling the chemo- and stereoselectivity of these reactions is a significant challenge owing to their ubiquity and low reactivity. In this review, we present a comprehensive account on the recent advances in inter- and intramolecular asymmetric sp3 C-H bond functionalization utilizing ruthenium, rhodium, and iridium catalysts and get insights into the mechanistic aspects of these transformations.

Recent Developments on Processes for Recovery of Rhodium Metal from Spent Catalysts

Rhodium (Rh) catalyst has played an indispensable role in many important industrial and technological applications due to its unique and valuable properties. Currently, Rh is considered as a strategic or critical metal as the scarce high-quality purity can only be supplemented by refining coarse ores with low content (2–10 ppm) and is far from meeting the fast-growing market demand. Nowadays, exploring new prospects has already become an urgent issue because of the gradual depletion of Rh resources, incidental pressure on environmental protection, and high market prices. Since waste catalyst materials, industrial equipment, and electronic instruments contain Rh with a higher concentration than that of natural minerals, recovering Rh from scrap not only offers an additional source to satisfy market demand but also reduces the risk of ore over-exploitation. Therefore, the recovery of Rh-based catalysts from scrap is of great significance. This review provides an overview of the Rh metal recovery from spent catalysts. The characteristics, advantages and disadvantages of several current recovery processes, including pyrometallurgy, hydrometallurgy, and biosorption technology, are presented and compared. Among them, the hydrometallurgical process is commonly used for Rh recovery from auto catalysts due to its technological simplicity, low cost, and short processing time, but the overall recovery rate is low due to its high remnant Rh within the insoluble residue and the unstable leaching. In contrast, higher Rh recovery and less effluent discharge can be ensured by a pyrometallurgical process which therefore is widely employed in industry to extract precious metals from spent catalysts. However, the related procedure is quite complex, leading to an expensive hardware investment, high energy consumption, long recovery cycles, and inevitable difficulties in controlling contamination in practice. Compared to conventional recovery methods, the biosorption process is considered to be a cost-effective biological route for Rh recovery owing to its intrinsic merits, e.g., low operation costs, small volume, and low amount of chemicals and biological sludge to be treated. Finally, we summarize the challenges and prospect of these three recovery processes in the hope that the community can gain more meaningful and comprehensive insights into Rh recovery.

Rhodium(III)-Catalyzed Intramolecular Benzylic C(sp3)–H Amidation for the Synthesis of Isoindolinones

AbstractThe synthesis of N-unprotected isoindolinones via rhodium(III)-catalyzed intramolecular C(sp3)–H amidation of various 2-methyl-N-substituted benzamides is developed. This protocol employs a free amino acid l-alanine as a ligand, which could significantly increase the reactivity of rhodium catalyst.

Catalytic N-H Bond Formation Promoted by a Ruthenium Hydride Complex Bearing a Redox-Active Pyrimidine-Imine Ligand.

The synthesis of a piano-stool ruthenium hydride, [(η5-C5Me5)Ru(PmIm)H] (PmIm = (N-(1,3,5-trimethylphenyl)-1-(pyrimidin-2-yl)ethan-1-imine), for the dual purpose of catalytic dihydrogen activation and subsequent hydrogen atom transfer for the formation of weak chemical bonds is described. The introduction of a neutral, potentially redox-active PmIm supporting ligand was designed to eliminate the possibility of deleterious C(sp2)-H reductive coupling and elimination that has been identified as a deactivation pathway with related rhodium and iridium catalysts. Treatment of [(η5-C5Me5)RuCl2]n with one equivalent PmIm ligand in the presence of zinc and sodium methoxide resulted in the isolation of the diruthenium complex, [(η5-C5Me5)Ru(PmIm)]2, arising from the C-C bond formation between two PmIm chelates. Addition of H2 to the ruthenium dimer under both thermal and blue light irradiation conditions furnished the targeted hydride, [(η5-C5Me5)Ru(PmIm)H], which has a relatively weak DFT-calculated Ru-H bond dissociation free energy (BDFE) of 47.9 kcal/mol. Addition of TEMPO to [(η5-C5Me5)Ru(PmIm)H] generated the 17-electron metalloradical, [(η5-C5Me5)Ru(PmIm)], which was characterized by EPR spectroscopy. The C-C bond forming process was reversible as the irradiation of [(η5-C5Me5)Ru(PmIm)]2 generated [(η5-C5Me5)Ru(PmIm)H] and a piano-stool ruthenium complex containing an enamide ligand derived from H-atom abstraction from the PmIm chelate. Equilibration studies were used to establish an experimental estimate of the effective Ru-H BDFE, and a value of 50.8 kcal/mol was obtained, in agreement with the observed loss of H2 and the DFT-computed value. The ruthenium hydride was an effective catalyst for the thermal catalytic hydrogenation of TEMPO, acridine, and a cobalt-imido complex and for the selective reduction of azobenzene to diphenylhydrazine, highlighting the role of this complex in catalytic weak bond formation using H2 as the stoichiometric reductant.