13 Search Results

The reaction of NaBEt₄ with Cp*Rh(PMe₃)PhBr produces Cp*Rh(PMe₃)PhEt by ethyl transfer. Single crystal X-ray analysis shows a typical piano-stool geometry. This compound reacts photochemically (λ > 320 nm) to give Cp*Rh(PMe₃)PhH as the major product, plus ethylene. A secondary product was assigned as the square planar complex RhPh(PMe₃)(η⁴-C₅Me₅Et) on the basis of NMR spectroscopy. Finally, the molecule decomposes over several days at ambient temperature, releasing C₅Me₅Et.

The scalable and facile preparation of single-atom catalysts remains a critical challenge. Here, in this study, we introduce diluted atomic layer deposition (DALD), a unique approach for synthesizing supported metal catalysts with precisely tunable loadings. Unlike conventional metal deposition by ALD which uses pure metal precursors, DALD employs a diluted precursor mixture, combining organometallic precursors with the corresponding free ligand in controlled ratios. The method enables precise control over metal loadings, allowing the synthesis of structures ranging from nanoparticles to isolated single atoms, as exemplified by Ir, Rh, and Pt on high-surface-area γ-Al₂O₃. With its inherent simplicity and exceptional efficiencymore » in metal precursor utilization, DALD represents a highly scalable strategy, unlocking opportunities for integrating single-atom catalysts into industrial processes.« less

Knowledge of how trace amounts of more reactive metals influence the oxidation rate and mechanism of Cu surfaces is essential for developing strategies to optimize the performance of Cu-based catalysts. We find that the addition of 1% Rh to Cu(111) increases the initial O₂ dissociation rate by approximately 9-fold. CO poisoning experiments reveal that single Rh atoms activate O₂ and facilitate the spillover of atomic oxygen to Cu sites. Scanning tunneling microscopy (STM) and in situ X-ray photoelectron spectroscopy (XPS) support this mechanism, showing enhanced surface oxygen near Rh atoms. Here, a density functional theory (DFT)-based model demonstrates that Rhmore » binds the O₂ precursor 0.15 eV more strongly than Cu(111) and lowers the O₂ dissociation barrier by 0.02 eV. Both single-crystal and nanoparticle experiments show that at low oxygen pressures, Rh enhances Cu oxidation, whereas at higher pressures, it inhibits deeper oxidation, as evidenced by in situ ultraviolet-visible (UV-vis) spectra.« less

The catalyst precursor [(η²-C₂H₄)₂Rh(μ-OAc)]₂ and in situ oxidant Cu(OPiv)₂ (OPiv = t-BuCOO^–) convert naphthalene and olefins (i.e., ethylene and propylene) to alkenylnaphthalenes. Under all reaction conditions tested, the functionalization is selective for the β-position of naphthalene with the highest observed β:α ratio >20:1. The β-selectivity is catalyst controlled, but oxidant identity, ethylene pressure, and olefin identity influence the ratio of β-alkenylation to α-alkenylation. The concentration of HOPiv and naphthalene do not have an effect on the β:α ratio under the reaction conditions tested. Arenes similar to naphthalene (i.e., o-xylene and 1,2,3,4-tetrahydronaphthalene) give quantitative selectivity for alkenylation at the position βmore » to the substituent. Using propylene as the olefin for naphthalene alkenylation, the β:α ratio is 32(7):1. and the anti-Markovnikov to Markovnikov ratio is 16(2):1.« less

Single metal atoms dispersed on oxides are a new emerging class of catalysts owing to their unique electronic and chemical properties. Here, in this study, we have prepared a series of model single-atom catalysts possessing well-characterized Rh sites that include Rh adatoms (Rh_ad), mixed surface layers with octahedrally-coordinated Rh (Rh_oct), as well as metallic Rh clusters and nanoparticles (Rh_met) on Fe₃O₄(001). Using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we investigated the activity of such model systems towards H2 and their stability in reducing environments. Our results show that the atomically dispersed Rhad and Rhoct species do notmore » activate H₂, which would result in the formation of surface hydroxyls on Fe₃O₄(001). In contrast, the presence of Rh_met in H₂ results in the formation of hydroxyls and subsequent etching of the Fe₃O₄(001) at higher temperatures (≥ 500 K) due to water formation via the Mars-van Krevelen mechanism. Additionally, such surface etching leads to the release of the Rh_oct from the surface lattice and their sintering to Rh_met. To bridge the material gap between the surface science models and high surface area catalysts, we perform parallel studies on powder Rh/Fe₃O₄ catalysts. The XPS characterization shows remarkable similarities between these systems. Further, our surface science studies provide an atomistic picture of the behavior of high surface area catalysts in the H₂ atmosphere.« less

The study of ambiphilic systems combining L-type and Z-type ligands within the same construct has emerged as a field of active investigation, especially in the cases of ligands containing a group 13 element as a σ-acceptor for transition metals. (1) Parallel to these developments, several groups have investigated more atypical systems in which the Z-type ligand is a group 15 element. (2) Our contributions to this area have focused on the use of phosphinostibine ligands for the generation of transition metal complexes in which the antimony moiety acts as a Z-type ligand. (1d,3) We have shown that the magnitude ofmore » the resulting M → Sb interaction can be readily modulated by the oxidation state of the antimony atom (4) as well as its charge which can be manipulated by abstraction of anionic ligands. (5) Our work has also shown that these effects can be leveraged to enhance the catalytic properties of the transition metal center. (4,5) Some of the simplest systems that we have investigated are those resulting from the reaction of platinum dichloride with the bis- or tris-phosphinostibines ClSb(o-dppp)₂ and Sb(o-dppp)₃, respectively (o-dppp = o-(Ph₂P)C₆H₄). These reactions proceed by oxidative insertion of the stibine into a Pt- Cl bond to produce complexes A and B, (6) respectively (Chart 1). Reasoning that the properties of these complexes may also be influenced by the nature of the L-type buttresses, we have now questioned whether stibines featuring nitrogen donor ligands could also display the redox noninnocence of their phosphine counterparts and support the formation of such complexes. Following up on some of our work with ambiphilic tellurium-quinoline ligands, (7) we now report on the reaction of tris-(8-quinolyl)stibines (8) toward (MeCN)₃RhCl₃.« less

Nitrous oxide N₂O reduction is a big challenge due to high global warming potential of N₂O. (~300 times higher compared with CO₂). The best known catalysts, such as Rh/ceria, require relatively high temperatures for N₂O decomposition. Herein, we report that Ru/ceria catalysts with low Ru loading of ~0.25 wt% efficiently catalyze low temperature N₂O reduction by CO starting at 100 °C (full N₂O conversion below 200 °C) under industrially relevant flow rates and gas concentrations. Further, this remarkable performance stems from maintaining isolated Ru cations even on reduced ceria surface and, simultaneously, the propensity of Ru to affect ceria surfacemore » to form labile surface oxygen thereby creating large number of oxygen vacancies (Ce⁺³ cations) in the presence of CO. In contrast, for Rh/CeO₂ catalysts with equivalent metal loading, the activity is much lower because atomically dispersed Rh sinters into metallic clusters at the onset reaction temperature (~200 °C): these clusters are much less effective than isolated single Ru ions, with lower Ce⁺³ concentration maintained on reduced Rh/CeO₂ catalyst. Our study highlights the benefits of gaining molecular-level insight into the dynamic nature of catalytically active sites under reaction conditions for preparing catalysts containing low loading of precious metals with unsurpassed low temperature activity.« less

C–N coupling is significant for the synthesis of fine chemicals toward various applications. Hydroaminoalkylation of olefins is a tandem reaction of C–N coupling involving first the formation of an aldehyde through hydroformylation of an olefin and then the production of amine through reductive amination of the aldehyde. Here we report a stable, supported catalyst of singly dispersed Rh₁ atoms anchored on TiO₂ (P25) nanoparticles designated as Rh₁/P25. Its high activity for C–N coupling was demonstrated by six hydroaminoalkylations of olefins and amines with selectivity of higher than 90% for producing tertiary amines. Furthermore, the singly dispersed Rh₁O₄ on P25 exhibitmore » activity and selectivity for hydroaminoalkylation comparable or even higher than some reported molecular catalysts. In contrast to molecular catalysts, the Rh-based single-atom Rh heterogeneous catalysis (Rh₁/P25) can be readily separated from reactants and products, reused for multiple runs of hydroaminoalkylation, and recycled with a low cost.« less

Significance Direct ethanol fuel cells are attracting growing attention as portable power sources due to their advantages such as higher mass-energy density than hydrogen and less toxicity than methanol. However, it is challenging to achieve the complete electrooxidation to generate 12 electrons per ethanol, resulting in a low fuel utilization efficiency. This manuscript reports the complete ethanol electrooxidation by engineering efficient catalysts via single-atom modification. The combined electrochemical measurements, in situ characterization, and density functional theory calculations unravel synergistic effects of single Rh atoms and Pt nanocubes and identify reaction pathways leading to the selective C–C bond cleavage to oxidizemore » ethanol to CO ₂ . This study provides a unique single-atom approach to tune the activity and selectivity toward complicated electrocatalytic reactions.« less

Trispyrazolylborate complexes of rhodium and iridium have been extensively investigated over the past 3 decades with special attention to their ability to activate C–H bonds. The rhodium complexes of tris-(3,5-dimethylpyrazolyl)borate have been the subject of numerous thermodynamic investigations that provide information about rhodium-metal carbon bond strengths. This insight arises as a result of the reversibility of C–H activation with rhodium. In the case of iridium, C–H bond activation reactions are also widespread, and some of these also show reversibility. Access to some key trispyrazolylborate iridium(I) and iridium(III) starting materials has given way to a multitude of studies of reactions withmore » small molecules in which C–H bonds are made and broken reversibly. The stability of Fischer carbenes plays a role in the observed products. Here, the reactivity of trispyrazolylborate complexes of rhodium and iridium compounds over the past decade (since 2010) are summarized here. Related reactions of X–H bonds with these trispyrazolylborate compounds are also included for completeness (X = O, N, S, B, Si).« less