Reactivity Of Rh Research Articles

New insights into thermal processes of metal deposits on h-BN/Rh(1 1 1): A comparison of Au and Rh

In this paper the thermal properties of Au and Rh deposits are compared on hexagonal boron nitride (h-BN) “nanomesh” prepared on Rh(111), applying STM, XPS, low energy ion scattering (LEIS), LEED, and DFT. At room temperature, both metals essentially follow Volmer-Weber (3D) growth. Upon subsequent annealing, agglomeration (sintering), intercalation, and desorption are competing surface processes for both metals. For the more reactive Rh, we suggest an additional encapsulation mechanism: between 600 K and 750 K, fragments of decomposed h-BN diffuse locally from the bottom onto the metal clusters covering them partially. STM data indicates that agglomeration of gold nanoparticles proceeds faster compared to rhodium. At higher temperatures (∼1050 K–1100 K), all non-desorbing gold atoms diffuse below h-BN, even for large initial coverages. On the other hand, for larger Rh deposits (≥5 ML), the outermost layer always contains Rh. Accumulation of gold at the interface between h-BN and Rh(111) significantly enhances the thermal stability of h-BN, attributed to the lower reactivity of Au in the decomposition of h-BN compared to Rh(111). At elevated substrate temperatures, intercalation of individual adatoms takes place during deposition, which requires higher temperatures for rhodium due to its slower diffusion and higher probability of nucleation.

Regulating local coordination environment of rhodium single atoms in Rh/CeO2 catalysts for N2O decomposition

Rh single atom catalysts (SACs) have been insensitively investigated recently due to the maximum utilization efficiency of Rh, one of the most expensive precious metals. Although great efforts have been made in the development and application of Rh SACs, there are few reports on the precise control of the local coordination environment of Rh single sites on CeO2 and their catalytic performance for N2O decomposition. Herein, Rh/CeO2 catalysts with different Rh–O coordination numbers (CNs) were successfully prepared using different CeO2 supports and a simple incipient wetness impregnation (IWI) method. It is observed that the Rh/CeO2 catalyst with slightly higher CN of Rh–O (Rh/CeO2-H) prepared from CeO2 shows much higher N2O decomposition activity than the catalyst with lower CN of Rh–O (Rh/CeO2-L) obtained from Ce(OH)x. The Rh species within Rh/CeO2-H are found to be more reactive than those within Rh/CeO2-L, which can better facilitate the O2 desorption once formed during N2O decomposition. In addition, more surface oxygen vacancies are present on Rh/CeO2-H than on Rh/CeO2-L, well explaining the superior N2O adsorption and activation capability on the former catalyst. It is concluded that more abundant oxygen vacancies and reactive Rh single atom sites with slightly higher CN of Rh–O and significantly higher reducibility altogether contribute to the superior N2O decomposition activity on the Rh/CeO2-H catalyst.

Triple Resonance Experiments for the Rapid Detection of 103Rh NMR Shifts: A Combined Experimental and Theoretical Study into Dirhodium and Bismuth-Rhodium Paddlewheel Complexes.

A H(C)Rh triple resonance NMR experiment makes the rapid detection of 103Rh chemical shifts possible, which were previously beyond reach. It served to analyze a series of dirhodium and bismuth–rhodium paddlewheel complexes of the utmost importance for metal–carbene chemistry. The excellent match between the experimental and computed 103Rh shifts in combination with a detailed analysis of the pertinent shielding tensors forms a sound basis for a qualitative and quantitative interpretation of these otherwise (basically) inaccessible data. The observed trends clearly reflect the influence exerted by the equatorial ligands (carboxylate versus carboxamidate), the axial ligands (solvents), and the internal “metalloligand” (Rh versus Bi) on the electronic estate of the reactive Rh(II) center.

Studies on the reactivity of Rh(I) complexes towards SF5Cl

Sulfur chloride pentafluoride reacts with the rhodium(I) NHC complexes (N-heterocyclic carbene) trans-[Rh(Cl)(CO)(IMes)2] (1) and trans-[Rh(F)(CO)(IMes)2] (5) to give the cationic square-pyramidal complexes trans,trans-[Rh(Cl)2(CO)(IMes)2][SF5] (2) and trans-[Rh(Cl)(F)(CO)(IMes)2][SF5] (6) at 183 K. The compounds convert into the fluorido complexes trans,trans-[Rh(Cl)2(F)(CO)(IMes)2] (4) and cis,trans-[Rh(Cl)(F)2(CO)(IMes)2] (7) at ambient temperatures. In the reaction of 1 the rhodium(III) fluoroacyl complex trans,trans-[Rh(Cl)2(COF)(IMes)2] (3) was identified as additional product. DFT calculations were carried out in order to determine the configurations of the complexes 2, 3 and 6. For comparison the phosphine complex, trans-[Rh(F)(CO)(PEt3)2] (8) was treated with SF5Cl to yield cis,trans-[Rh(F)2(Cl)(CO)(PEt3)2] (9) at 183 K. At 213 K the sulfanyl complex cis,trans-[Rh(F)2(SF3)(CO)(PEt3)2] (10) was also identified.

Variable reactivity of Rh D antigen and its serological characterization

ABSTRACT Background: Variation in the reactivity on Rh D typing may pose challenges in interpretation and ambiguity in further patient management. Materials and Methods: A prospective study was conducted in the department of transfusion medicine for a period of 18 months. Blood grouping was performed by fully automated equipment employing column agglutination technique. All the samples with Rh D negative or discrepant reactions were subjected to weak D testing by the antihuman globulin testing method. Samples that tested positive were categorized as serological weak D type or Variant D and were further phenotyped with Partial D typing set with 6 monoclonal anti D antisera. Results: A total of 82,824 samples were tested for Rh D type during the study period. Of the study population, 65.7% were males. On Rh D type majority were Rh D positive (93%), 6.9% were negative, and the result was discrepant in 0.1% (70) samples. The overall prevalence of variant D was 1.28% (75) of the Rh D negative population and 0.09% of the total study population. The detection rate of variant D phenotype was significantly higher by the Column agglutination technique. Upon testing with Partial D kit, the partial D variant in the majority reacted wil all the 6 antisera and hence we could not rule out DIII(60%), in rest it was inconclusive. In 43% of subjects with Rh D discrepancy ‘C’ antigen was found in a homozygous state. Conclusion: The introduction of partial D typing kit alone may not help in the absolute characterization of variant D. Extended serological testing and selective integration of molecular testing is the need of the hour.

Characterization of a Reactive Rh2 Nitrenoid by Crystalline Matrix Isolation.

The fleeting lifetimes of reactive intermediates in C-H functionalization chemistry often prevent their direct characterization. For example, the critical nitrenoid intermediates that mediate Rh2-catalyzed C-H amination have eluded characterization for more than 40 years. In the absence of structural characterization of these species, methodological development is often computationally guided. Here we report the first X-ray crystal structure of a reactive Rh2 nitrenoid, enabled by N2 elimination from an organic azide ligand within a single-crystal matrix. The resulting high-resolution structure displays metrical parameters consistent with a triplet nitrene complex of Rh2. The demonstration of facile access to reactive metal nitrenoids within a crystalline matrix provides a platform for structural characterization of the transient species at the heart of C-H functionalization.

Peptide-Driven Fabrication of Catalytically Reactive Rhodium Nanoplates

Here we present the use of a well-known materials binding peptide for the generation of highly catalytically reactive Rh nanoplates. To this end, the A3 peptide, originally isolated with affinity f...

Transmitting metal–oxide interaction by solitary chemical waves: H2 oxidation on ZrO2 supported Rh

The interaction of catalytically active metal particles with supporting oxide surfaces is still an actual topic in catalysis. Particularly, the exact role of metal-support boundary effects, caused by the active sites along the metal–oxide interface, is not entirely clear. Here we demonstrate a long-range effect of metal–oxide boundaries in a Rh/ZrO2 model system on the reactivity of Rh in H2 oxidation. The atomically narrow (<1 nm) interface between a Rh particle and ZrO2 causes a remarkable shift in the kinetic behavior of the entire µm-sized particle, i.e. affecting metal sites thousands of nanometers away from the interface. The effect was directly observed by photoemission electron microscopy (PEEM) using the kinetics by imaging approach which allows the evaluation of reaction kinetics for individual supported particles within the resolution limit of PEEM. The observed effect is attributed to the critical role of the perimeter sites for the initiation of activation and deactivation fronts (chemical waves) during H2 oxidation, which transmit the metal–oxide interface effect over the entire Rh particle.

Dicyclopentadiene Hydroformylation to Value-Added Fine Chemicals over Magnetically Separable Fe3O4-Supported Co-Rh Bimetallic Catalysts: Effects of Cobalt Loading

Six Co-Rh/Fe3O4 catalysts with different cobalt loadings were prepared by the co-precipitation of RhCl3, Co(NO3)2, and Fe(NO3)3 using Na2CO3 as the precipitant. These catalysts were tested for dicyclopentadiene (DCPD) hydroformylation to monoformyltricyclodecenes (MFTD) and diformyltricyclodecanes (DFTD). The results showed that the MFTD formation rate increased with increasing cobalt loading, whereas the DFTD formation rate initially increased and then decreased when the cobalt loading was greater than twice that of Rh. The DFTD selectivity was only 21.3% when monometallic Rh/Fe3O4 was used as the catalyst. In contrast, the selectivity was 90.6% at a similar DCPD conversion when the bimetallic 4Co-2Rh/Fe3O4 catalyst was employed. These catalysts were characterized by temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), and thermogravimetric and differential thermal analyses (TG-DTA). The results obtained by these complimentary characterization techniques indicated that adding cobalt to the Rh/Fe3O4 catalyst enhanced the Rh reducibility and dispersion; the Rh reducibility was easily altered, and increasing the cobalt loading improved the Rh dispersion. It was concluded that the enhanced catalytic performance with increasing cobalt loading might be due to the formation of a more reactive Rh species with a different Rh–phosphine interaction strength on the catalyst surface.

Rh-based catalysts supported on MCM-41-type mesoporous silica for dicyclopentadiene hydroformylation

Impregnation method was used in this study to prepare five different Rh based catalysts supported on MCM-type mesoporous silica materials. These catalysts were evaluated based on their performance for the hydroformylation of dicyclopentadiene (DCPD) to produce diformyltricyclodecanes (DFTD). While a selectivity of DFTD was only 8.7% using monometallic Rh/MCM-41 as catalyst, the selectivity was enhanced to 76.2% at a similar DCPD conversion using bimetallic Co–Rh/MCM-41 catalyst, even when the Co loading was very low(25mol% relative to Rh). Extensive catalyst characterization including TPR, TPD, XRD, XPS, BET and TEM was performed to study the nature of metal modification on Rh/MCM-41. It was found that modification on the Rh/MCM-41 by Co addition could substantially enhance the selectivity towards DFTD. This improvement was likely attributed to the formation of a more reactive Rh species on the catalyst surface.

Rh and Au deposited on ultrathin TiO~1.2 film formed on Rh(111) facets and the effects of CO exposure

Abstract This STM study deals with the structure and reactivity of Rh and Au deposited on a “pinwheel” TiO~ 1.2 ultrathin film (w-TiO-UTO) formed on Rh nanoparticles supported on TiO2(110). The deposition of a monolayer (ML) Au at 500 K disturbed the order of the w-TiO-UTO layer and led to the formation of 3D adparticles. The deposition of Rh (~ 1 ML) at 500 K and annealing up to 900 K led exclusively to the formation of 2D adparticles. It was found that 3D Au nanoparticles are gradually sintering in the range of 500–800 K. Upon a further raise of the temperature, a 3D→2D transformation takes place. The main difference is that on the top facet of the 2D Rh particles a w-TiO-UTO film forms, while the top facets of the 2D Au particles exhibit a clean gold metal structure. CO exposure of max. 10 min at 10 Pa causes no remarkable change of the encapsulated Rh particles, however, the 2D gold nanoparticles change significantly. The disintegration of Au particles and the spillover of Au atoms to the UTO surface accelerate at a CO pressure above 10− 1 Pa. This morphological process is reversible as an effect of annealing in UHV.

Promotion and inhibition effects of TiOx species on Rh inverse model catalysts

Abstract The effect of TiO x on the bonding of CO test molecules to different Rh surfaces has been investigated in model systems under UHV conditions. The TiO x overlayers had a twofold effect both on Rh nanoparticles and Ar + -ion sputtered Rh films prepared on TiO 2 (1 1 0). They suppressed the uptake of molecular CO by a site blocking mechanism. In the other mechanism the recombinative CO desorption states went through a maximum at intermediate TiO x coverages on both kinds of rhodium surfaces, indicating the promotion of CO decomposition. These findings are in harmony with earlier high pressure studies which showed that the methanation rate of CO on rhodium is maximal at intermediate TiO x coverages. The effect of a TiO x film underneath a Rh deposit (0.4 monolayer) on its surface chemistry has also been addressed. A Rh–TiO x –Rh structure was formed at 230 K by the deposition of Rh on a continuous TiO x film supported by a 20 monolayer thick Rh multilayer. CO desorption curves indicated a highly reactive Rh overlayer at 230 K and also a dramatic structural change in the rather narrow temperature range of 230–265 K. The rearrangement is accompanied by the migration of TiO x species onto the post-deposited Rh as it can be deduced from the appearance of a CO desorption feature with T p = 360 K. The driving force of the process is the formation of strong Rh–Rh bonds and the low surface free energy of TiO x overlayer with respect to the bare Rh surface.

Theoretical study of the reactivity of Rh(I) and Rh(III) Bis(isonitrile) complexes in cycloaddition reactions with nitrones

The mechanism of 1,3-dipolar cycloaddition of nitrone (CH2=N(Me)O) to methylisonitrile coordinated to Rh(I) and Rh(III) in the [RhCl(PH3)(CNMe)2] and [RhCl3(PH3)(CNMe)2] complexes has been studied by quantum-chemical methods. The molecular and electronic structures of the cycloaddition products, the nature of transition states, the mechanism of reactions, their kinetic and thermodynamic parameters, and the solvent effect have been described. The reactions occur via the concerted strongly asynchronous mechanism involving the formation of a five-membered cyclic transition state. The use of rhodium complexes as reagents leads to a noticeable decrease in the activation barriers of the processes under consideration and an increase in the magnitudes of energy effects of the reactions. It has been demonstrated that the Rh(III) complexes are better activators of the cycloaddition of nitrone to isonitrile than the Rh(I) complex. The calculations predict that in the case of the Rh(I) complexes, only one isonitrile ligand can be involved in cycloaddition of nitrone, whereas the use of the Rh(III) complexes enables the participation of both ligands. The solvation effects inhibit the reactions.

C–CN Bond Activation of Benzonitrile with [Rh–I(dippe)]−

The complex [Rh(dippe)(μ-Cl)]2 (1) was reduced with potassium metal to produce the highly reactive Rh–I species [Rh(dippe)(μ-K·THF)]2 (2). 2 was characterized by NMR spectroscopy (31P, 1H) and IR spectroscopy after forming the derivative K[Rh(dippe)(CO)2] (2a). The C–CN bond of benzonitrile was cleaved by oxidative addition when it was reacted stoichiometrically with 2 to form the anionic Rh(I) complex K[Rh(dippe)(CN)(Ph)] (3). 3 has been characterized by NMR spectroscopy (31P, 1H, 13C), and by the use of 13C-labeled benzonitrile. C–CN bond cleavage was also attempted by reacting benzonitrile with the zwitterionic complex [Rh(dippe)(η6-Ph-BPh3)] (4). A product 5 was formed from reaction with 2 equiv of benzonitrile. Reaction with 13C-labeled benzonitrile has ruled out the possibility of C–CN cleavage in this species, and a C–H bond activation pathway is operative instead.

Coordination of Ethylene to a Zwitterionic Rh(III) Half-Sandwich Complex: Influence of Ambiphilic Ligands on Reactivity

The reactivity of Rh(III) half-sandwich complex Cp*RhMe2(κP-μAl-MePMe2CH2AlMe2) 2 (Cp* = η5-pentamethylcyclopentadienyl) with ethylene was investigated. Coordination of ethylene (or ethylene-d4) gave a mixture of products including as principal species zwitterionic complexes Cp*Rh+Me(C2H4)(PMe2CH2AlMe3−) 4 (or 4-d4) and [Cp*Rh+Me(C2H4)(PMe2CH2AlMe3−)][AlMe3] (4′), after the abstraction of a Rh-methyl group by the pendant Lewis acid, and Cp*RhMe2(PMe2CH2AlMe2·C2H4) (5). Heating this reaction mixture at 50 °C results in the generation of propene (or propene-d3,4), methane (or methane-d1), and trace amounts of butene, as organic materials, as well as previously characterized [Cp*RhMe(μ2-η2(P,C)-PMe2CH2)]2 (10) and rhodium(I) species. Two different pathways for the reactivity of the zwitterionic π-complex were investigated by density functional theory (DFT). It is likely that propene is formed by β-hydride elimination from a cationic Rh-propyl fragment that is generated either by insertion of ethylene into a Rh−C bond or by a nucleophilic attack of the methyl-aluminate fragment on coordinated ethylene. After release of propene, the neutral complex Cp*RhHMe(PMe2CH2AlMe2) 11 is most likely responsible for the reductive elimination of methane.

Iminophosphorane-based [P2N2] rhodium complexes: synthesis, reactivity, and application in catalysed transfer hydrogenation of polar bonds

[Rh(P2N2)X] complexes (2-X, X = Cl, BF4) featuring a tetradentate iminophosphorane phosphine ligand were synthesised and characterised. X-Ray analysis provides evidence for a square planar geometry without coordination of the chloride anion. These complexes proved to be air-sensitive, and their oxidation to Rh(III) complexes 3-X was observed in air. The controlled reaction of 2-BF4 with one equivalent of hexachloroethane yielded [Rh(P2N2)Cl2(BF4)] (3-BF4). Direct synthesis of 3-Cl can also be achieved by coordination of the [P2N2] ligand to [RhCl(THT)3] (THT = tetrahydrothiophene). The reactivity of Rh(I) complexes 2 was further investigated, and no reaction was observed with silanes, aryl halides, or pinacolborane, although decomposition was observed under 1 atm of H2 upon prolonged heating. Interestingly, reduction of complex 3-Cl was observed by NMR upon treatment with silanes or sodium isopropoxide. Therefore, complex 3-Cl was used for catalytic transfer hydrogenation of polar bonds. The reduction of aromatic and aliphatic ketones can be carried out using 1% catalyst and 10% sodium isopropoxide, while imines are partially reduced under these conditions.

Base-Catalyzed Insertion of Dioxygen into Rhodium−Hydrogen Bonds: Kinetics and Mechanism

The reaction between molecular oxygen and rhodium hydrides L(OH)RhH(+) (L = (NH(3))(4), trans-L(1), and cis-L(1), where L(1) = cyclam) in basic aqueous solutions rapidly produces the corresponding hydroperoxo complexes. Over the pH range 8 < pH < 12, the kinetics exhibit a first order dependence on [OH(-)]. The dependence on [O(2)] is less than first order and approaches saturation at the highest concentrations used. These data suggest an attack by OH(-) at the hydride with k = (1.45 +/- 0.25) x 10(3) M(-1) s(-1) for trans-L(1)(OH)RhH(+) at 25 degrees C, resulting in heterolytic cleavage of the Rh-H bond and formation of a reactive Rh(I) intermediate. A competition between O(2) and H(2)O for Rh(I) is the source of the observed dependence on O(2). In support of this mechanism, there is a significant kinetic isotope effect for the initial step, L(1)(OH(D))RhH(D)(+) + OH(D)(-) k(1)/k(-1) L(1)(OH(D))Rh(I) + H(D)(2)O, k(1H)/k(1D) = 1.7, and k(-1H)/k(-1D) = 3.0. The activation parameters for k(1) for trans-L(1)(OH)RhH(+) are DeltaH(++) = 64.6 +/- 1.3 kJ mol(-1) and DeltaS(++) = 40 +/-4 J mol(-1) K(-1).

Effect of Al2O3 support on morphology and NO reactivity of Rh

The morphology and NO reactivity of Rh deposited on a thin Al2O3 film were studied by means of x-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and noncontact atomic force microscopy. The NO dissociation activity of the Rh deposited on the Al2O3 surface was higher than that of Rh(111). The NO dissociation kinetics and the NO and CO adsorption properties of the Rh/Al2O3 surface indicated that the surface structure of the supported Rh mainly consisted of a (100) face. The supported Rh existed as small particles on the step sites of the Al2O3 surface. Thus, the authors believe that the low-coordinated step sites of Al2O3 played a role in formation of the (100) surface structure.

Monolayer h-BN on lattice-mismatched metal surfaces: On the formation of the nanomesh

Electronic and atomic structure of hexagonal boron nitride (h-BN) on lattice-mismatched transition metal surfaces has been studied systematically by X-ray absorption and photoelectron spectroscopy and diffraction. The strength of interfacial chemical interaction between h-BN and the substrates is rising in the row Pt(1 1 1)–Ir(1 1 1)–Rh(1 1 1)–Ru(0 0 0 1). This rise directly correlates with a growing corrugation of the h-BN monolayer. In particular, the h-BN nanomesh has larger pores on Ru(0 0 0 1) than on the less reactive Rh(1 1 1) substrate. Our experiments provide strong evidence for the single-layer model of the nanomesh structure.

Steric and electronic effects on the reactivity of rh and ir complexes containing P-S, P-P, and P-O ligands. Implications for the effects of chelate ligands in catalysis.

Kinetic studies of the reactions of [M(CO)(L-L)I] [M = Rh, Ir; L-L = Ph(2)PCH(2)P(S)Ph(2) (dppms), Ph(2)PCH(2)CH(2)PPh(2) (dppe), and Ph(2)PCH(2)P(O)Ph(2) (dppmo)] with methyl iodide have been undertaken. All the chelate ligands promote oxidative addition of methyl iodide to the square planar M(I) centers, by factors of between 30 and 50 compared to the respective [M(CO)(2)I(2)](-) complexes, due to their good donor properties. Migratory CO insertion in [Rh(CO)(L-L)I(2)Me] leads to acetyl complexes [Rh(L-L)I(2)(COMe)] for which X-ray crystal structures were obtained for L-L = dppms (3a) and dppe (3b). Against the expectations of simple bonding arguments, methyl migration is faster by a factor of ca. 1500 for [Rh(CO)(dppms)I(2)Me] (2a) than for [Rh(CO)(dppe)I(2)Me] (2b). For M = Ir, alkyl iodide oxidative addition gives stable alkyl complexes [Ir(CO)(L-L)I(2)R]. Migratory insertion (induced at high temperature by CO pressure) was faster for [Ir(CO)(dppms)I(2)Me] (5a) than for its dppe analogue (5b). Reaction of methyl triflate with [Ir(CO)(dppms)I] (4a) yielded the dimer [[Ir(CO)(dppms)(mu-I)Me](2)](2+) (7), which was characterized crystallographically along with 5a and [Ir(CO)(dppms)I(2)Et] (6). Analysis of the X-ray crystal structures showed that the dppms ligand adopts a conformation which creates a sterically crowded pocket around the alkyl ligands of 5a, 6, and 7. It is proposed that this steric strain can be relieved by migratory insertion, to give a five-coordinate acetyl product in which the sterically crowded quadrants flank a vacant coordination site, exemplified by the crystal structure of 3a. Conformational analysis indicates similarity between M(dppms) and M(2)(mu-dppm) chelate structures, which have less flexibility than M(dppe) systems and therefore generate greater steric strain with the "axial" ligands in octahedral complexes. Ab initio calculations suggest an additional electronic contribution to the migratory insertion barrier, whereby a sulfur atom trans to CO stabilizes the transition state compared to systems with phosphorus trans to CO. The results represent a rare example of the quantification of ligand effects on individual steps from catalytic cycles, and are discussed in the context of catalytic methanol carbonylation. Implications for other catalytic reactions utilizing chelating diphosphines (e.g., CO/alkene copolymerization and alkene hydroformylation) are considered.