Rh Metal Center Research Articles

CO2 hydrogenation over rhodium cluster catalyst nucleated within a manganese oxide framework

Rhodium-based manganese oxide frameworks were explored as a prototype for carbon dioxide reactive capture and conversion. Three-dimensional frameworks of MnOx were utilized as support structures to isolate Rh metal centers. V, Na, and Zn were introduced as counterions to stabilize the structure and for their beneficial effect as promoters. With this multicomponent catalyst, Rh active centers with MnOxs and varied counterions, we were able to selectively tune the catalytic performance of the material via the choice of counterion and structure of the host material. With cryptomelane-type tunnel manganese oxides octahedral molecular sieve (OMS2), we found that Rh-V-OMS2 was highly stable even after 48 hours on stream with a reaction rate of around 1.5×10−4 mol CO2/gRh/s, surpassing the net reactivity of other initially more active combinations. Furthermore, during CO2 hydrogenation, in situ XAFS showed that single Rh atoms nucleated into nanoparticles/ sub-nanometer clusters with a coordination number of 5.5 or less. Our finding of the correlation between the reaction rate and particle size offers the potential for enhanced control over the reaction rate by tuning particle size. Our activity study with control experiments demonstrates that the activities of the catalysts are proved due to the unique metal support interaction offered by the Rh-X-MnO.

The tunable effect of the second metal site on the hydroformylation of propylene over Rh-M-BTC (M= Fe, Ru, Mn, Rh, Pt, Pd, Re, Cu, Ni)

Hydroformylation is one of the most important homogeneous reactions in industry. Cu-BTC, as a kind of MOFs material with coordinate unsaturated metal sites, has strong interaction with adsorption molecules and is widely used in adsorption and catalytic research fields. The open metal sites in it can be adjusted according to the demand. In this work, Rh-BTC was selected as the catalyst for the hydroformylation of propylene. In order to regulate the electronic properties of Rh metal center, eight Rh-M-BTC materials (M= Fe, Ru, Mn, Rh, Pt, Pd, Re, Cu, Ni) were investigated. The Mulliken charge of Rh-M metal nodes and the reaction energy barrier of the key steps of propylene hydroformylation on Rh-M-BTC were calculated, including the step of propylene insertion, CO insertion, and aldehyde formation. The results showed that on Rh-Pd-BTC, the energy barrier difference between linear and branched path for the regional decisive step propylene insertion step is the lowest, only 0.04 eV. The reaction energy barrier of the linear path for CO insertion is only half of that of the branched path. The linear path has obvious advantages. Therefore, Rh-Pd-BTC was suggested as the most suitable model studied in the work for hydroformylation of propylene.

Influence of the metal center of metalloprotoporphyrins on the electrocatalytic CO2 reduction to formic acid

Electrocatalytic conversion of carbon dioxide has gained much interest for the synthesis of value-added chemicals and solar fuels. Important issues such as high overpotentials and competition of hydrogen evolution still need to be overcome for deeper insight into the reaction mechanism in order to steer the selectivity towards specific products. Herein we report on several metalloprotoporphyrins immobilized on a pyrolytic graphite electrode for the selective reduction of carbon dioxide to formic acid. No formic acid is detected on Cr-, Mn-, Co- and Fe-protoporphyrins in perchloric acid of pH 3, while Ni-, Pd-, Cu- and Ga-protoporphyrins show only a little formic acid. Rh, In and Sn metal centers produce significant amounts of formic acid. However, the faradaic efficiency varies from 1% to 70% depending on the metal center, the pH of the electrolyte and the applied potential. The differentiation of the faradaic efficiency for formic acid on these metalloprotoporphyrins is strongly related to the activity of the porphyrin for the hydrogen evolution reaction. CO2 reduction on Rh-protoporphyrin is shown to be coupled strongly to the hydrogen evolution reaction, whilst on Sn- and In-protoporphyrin such strong coupling between the two reactions is absent. The activity for the hydrogen evolution increases in the order In<Sn<Rh metal centers, leading to faradaic efficiency for formic acid increasing in the order Rh<Sn<In metal centers. In-protoporphyrin is the most stable and shows a high faradaic efficiency of ca. 70%, at a pH of 9.6 and a potential of −1.9V vs RHE. Experiments in bicarbonate electrolyte were performed in an attempt to qualitatively study the role of bicarbonate in formic acid formation.

Increased photocatalytic activity in Ru(II),Rh(III) supramolecular bimetallic complexes with terminal ligand substitution

Three new Ru(II),Rh(III) supramolecular bimetallic complexes of the design [(Ph2phen)2Ru(dpp)RhCl2(R2-bpy)](PF6)3 (R=CH3 (Ru-Rh(Me2bpy)), H (Ru-Rh(bpy)), or COOCH3 (Ru-Rh(dmeb)); Ph2phen=4,7-diphenyl-1,10-phenanthroline; dpp=2,3-bis(2-pyridyl)pyrazine; dmeb=4,4′-dimethyl ester-2,2′-bipyridine; bpy=2,2′-bipyridine; Me2bpy=4,4′-dimethyl-2,2′-bipyridine) have been synthesized and analyzed to determine the impact that the polypyridyl terminal ligand (TL) coordinated to the cis-dihalide rhodium(III) metal center has on the photocatalytic activity for water reduction. The bimetallic complexes demonstrate that a correlation exists between the σ-donating ability of the substituted bipyridine ligand, the rate of chloride dissociation upon electrochemical reduction and the activity towards photocatalytic hydrogen production. The weaker σ-donating –COOCH3 substituent in Ru-Rh(dmeb) increases the rate constant for Cl− dissociation (k−Cl=0.7s−1) and the amount of H2 produced photocatalytically (37±4μmol H2, 63±7 turnovers after 20h; turnovers=mol H2/mol photocatalyst) when compared to –H and –CH3 substituted complexes Ru-Rh(bpy) (k−Cl=0.2s−1, 21±2μmol H2, 35±3 turnovers) and Ru-Rh(Me2bpy) (k−Cl=0.2s−1, 18±2μmol H2, 30±4 turnovers), respectively. Varied catalytic activity with respect to the σ-donor capacity of the Rh-TL is attributed to the relative ease of ligand dissociation and the ability to afford rapid electron collection at the Rh metal center.

Nonchromophoric halide ligand variation in polyazine-bridged Ru(II),Rh(III) bimetallic supramolecules offering new insight into photocatalytic hydrogen production from water.

The new bimetallic complex [(Ph2phen)2Ru(dpp)RhBr2(Ph2phen)](PF6)3 (1) (Ph2phen = 4,7-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine) was synthesized and characterized to compare with the Cl(-) analogue [(Ph2phen)2Ru(dpp)RhCl2(Ph2phen)](PF6)3 (2) in an effort to better understand the role of halide coordination at the Rh metal center in solar H2 production schemes. Electrochemical properties of complex 1 display a reversible Ru(II/III) oxidation, and cathodic scans indicate multiple electrochemical mechanisms exist to reduce Rh(III) by two electrons to Rh(I) followed by a quasi-reversible dpp(0/-) ligand reduction. The weaker σ-donating ability of Br(-) vs Cl(-) impacts the cathodic electrochemistry and provides insight into photocatalytic function by these bimetallic supramolecules. Complexes 1 and 2 exhibit identical light-absorbing properties with UV absorption dominated by intraligand (IL) π → π* transitions and visible absorption by metal-to-ligand charge transfer (MLCT) transitions to include a lowest energy Ru(dπ) → dpp(π*) (1)MLCT transition (λ(abs) = 514 nm; ε = 16 000 M(-1) cm(-1)). The relatively short-lived, weakly emissive Ru(dπ) → dpp(π*) (3)MLCT excited state (τ = 46 ns) for both bimetallic complexes is attributed to intramolecular electron transfer from the (3)MLCT excited state to populate a low-energy Ru(dπ) → Rh(dσ*) triplet metal-to-metal charge transfer ((3)MMCT) excited state that allows photoinitiated electron collection. Complex 1 outperforms the related Cl(-) bimetallic analogue 2 as a H2 photocatalyst despite identical light-absorbing and excited-state properties. Additional H2 experiments with added halide suggest ion pairing plays a role in catalyst deactivation and provides new insight into observed differences in H2 production upon halide variation in Ru(II),Rh(III) supramolecular architectures.

Rhodium-Catalyzed [6 + 2] Cycloaddition of Internal Alkynes with Cycloheptatriene: Catalytic Study and DFT Calculations of the Reaction Mechanism

A rhodium-catalyzed [6 + 2] cycloaddition of internal alkynes with cycloheptatriene is described. A series of substituted alkynes were cycloadded to cycloheptatriene through a [6 + 2] addition to give a variety of substituted bicyclic compounds in excellent yields. The optimal catalytic system for these transformations was a [Rh(COD)Cl]2 (5.0 mol %) catalyst in combination with CuI (10 mol %) and PPh3 (10 mol %). The proposed mechanism for this system includes an initial oxidative coupling reaction between the coordinated cycloheptatriene and the internal alkyne, followed by a [1,3]-shift of the Rh metal center and a reductive elimination from the Rh(III)–allyl complex to give the final product. Calculations using a model Rh(I) catalyst were also carried out to further understand this mechanism.

A Coordination Chemistry Approach to a Multieffector Enzyme Mimic

A macrocyclic complex, formed via the weak-link approach, behaves as a multieffector enzyme mimic in the context of pseudorotaxane formation. The complex has Rh metal centers as regulatory sites, which can be used to change the size and charge of the cavity responsible for pseudorotaxane formation. The first binding events on the metal center (M) involving CO (the filled squares) change the shape of the molecule, while the second binding event involving Cl anion (the filled circles) changes the charge on the molecule and its ability to recognize and sequester a hexyl viologen guest molecule (G).

Remarks on the process of homogeneous carbonylation of rhodium compounds by N, N-dimethylformamide

Reductive carbonylation of rhodium(III) chloride complexes, commercial RhCl 3 · nH 2O neutralized with BaCO 3, (Me 2NH 2) 2[RhCl 5(DMF)], (PPh 4)[RhCl 4(H 2O) 2], RhCl 3(DMF) 3, RhCl 3(CH 3CN) 3, RhCl 3(CH 3CN) 2(DMF), [Rh(CO) 2Cl 3] 2, and rhodium(I) complex, Rh(PPh 3) 3Cl, by N, N-dimethylformamide (DMF) is studied. The data obtained support the conception of direct carbonyl group transfer from DMF molecule to the Rh metal center. The mechanistic scheme of carbonylation process is refined and discussed with regard of new experimental results.

Alkane Coordination Selectivity in Hydrocarbon Activation by [Tp‘Rh(CNneopentyl)]: The Role of Alkane Complexes

The competitive activation of C-H bonds of linear, cyclic, and branched hydrocarbons using the coordinatively unsaturated 16-electron [Tp'RhL] reactive fragment have been studied (Tp' = tris-(3,5-dimethylpyrazolyl)borate; L = CNCH2CMe3). Activation of the hydrocarbons leads to the formation of Tp'Rh(L)(R)(H) alkyl complexes, which were converted to the stable chlorides immediately following the activation of the bonds via photolysis of Tp'Rh(L)(PhN=C=NCH2CMe3) in the solvent mixture. The products were analyzed by 1H NMR spectroscopy. The experiments described provide relative rates for the coordination of primary and secondary C-H bonds to the Rh metal center, indicating a 1.5x preference for the latter.

Syntheses of Polythioethers Tethered with Bulky Aryl Groups and Their Complexation with Late-Transition Metals

New types of o-phenylene-bridged polythioethers tethered with extremely bulky aryl groups at their terminal sulfur atoms, such as TbtS(o-Phen)S(o-Phen)S(o-Phen)STbt (1) and TbtS(o-Phen)S(o-Phen)SS(o-Phen)S(o-Phen)STbt (2) (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, o-Phen = o-phenylene), were synthesized and subjected to the complexation with several kinds of late-transition metals. In the case of polythioether 1, the reaction with RhCl3·3H2O in benzene/EtOH resulted in the formation of a unique bimetallic complex, in which a part of ligand 1 is lost and the resulting sulfur atom is directly bound to the other Rh metal center. Interestingly, similar treatment of 1 with IrCl3·3H2O afforded ethyl-coordinated mononuclear Ir complex. Furthermore, 1 underwent complexation with Na2PdCl4 in EtOH to give the corresponding square planar dichloropalladium complex coordinated with two inner sulfur atoms of 1, while the S6-ligand 2 reacted with excess of Pd(PPh3)4 in benzene to afford a quite interesting trinuclear Pd complex multi-step metal insertion reactions.

Regioselective Reduction of NAD+ Models with [Cp*Rh(bpy)H]+ : Structure-Activity Relationships and Mechanistic Aspects in the Formation of the 1,4-NADH Derivatives.

The hydride transfer mechanism of the NAD+ model compound 1 to its 1,4-NADH derivative 3 [Eq. (1)] is proposed to be a consequence of the critical role of the carbonyl group of the amide to coordinate to the ring-slipped η5 - to η3 -Cp*Rh metal center of the catalyst [Cp*Rh(bpy)H]+ , prepared in situ from 2, while a steric effect of a substituent in the 3 position, for example, C(O)NEt2 , was found to totally inhibit this regioselective reduction. bpy=2,2'-bipyridine, Cp*=C5 Me5 , OTf=trifluoromethanesulfanate.

Ab InitioStudy of the Interaction of Rhodium with Dinitrogen and Carbon Monoxide

Ab initio computations at the B3LYP/ECP level have been performed on a series of charged and neutral rhodium species containing carbon monoxide and dinitrogen ligands. The computations have been employed to predict possible oxidation states of supported Rh for new surface species detected by infrared spectroscopy during the interaction of the metal with CO and N2 in the presence of ultraviolet radiation. Specifically, it has been concluded that Rh species containing N2 ligands giving rise to low-frequency N−N stretching modes most likely contain Rh with a −1 oxidation state, the source of electrons being due to photoinduced electron transfer between supported Rh metal centers in the presence of UV irradiation. Although the computations refer to gas-phase Rh species, it is believed that computations, such as those reported herein, can be useful in predicting trends in vibrational frequencies for similar surface species and thus aid in vibrational band assignments for specific structures.

Bipyrimidine-Bridged Mixed-Metal Trimetallic Complexes of Ruthenium(II) with Rhodium(III) or Iridium(III), {[(bpy)(2)Ru(bpm)](2)MCl(2)}(5+).

Bipyrimidine-bridged trimetallic complexes of the form {[(bpy)2Ru(bpm)]2MCl2}5+, where M = RhIII or IrIII, bpy = 2,2‘-bipyridine, and bpm = 2,2‘-bipyrimidine, have been synthesized and characterized. These complexes are of interest in that they couple catalytically active rhodium(III) and iridium(III) metals with light-absorbing ruthenium(II) metals within a polymetallic framework. Their molecular composition is a light absorber−electron collector−light absorber core of a photochemical molecular device (PMD) for photoinitiated electron collection. The variation of the central metal has some profound effects on the observed properties of these complexes. The electrochemical data for the title trimetallics consist of a RuII/III oxidation and sequential reductions assigned to the bipyrimidine ligands, Ir or Rh metal centers, and bipyridines. In both trimetallic complexes, the first oxidation is Ru based and the bridging ligand reductions occur prior to the central metal reduction. This illustrates that the h...

Bipyrimidine-Bridged Mixed-Metal Trimetallic Complexes of Ruthenium(II) with Rhodium(III) or Iridium(III), {[(bpy)(2)Ru(bpm)](2)MCl(2)}(5+).

Bipyrimidine-bridged trimetallic complexes of the form {[(bpy)(2)Ru(bpm)](2)MCl(2)}(5+), where M = Rh(III) or Ir(III), bpy = 2,2'-bipyridine, and bpm = 2,2'-bipyrimidine, have been synthesized and characterized. These complexes are of interest in that they couple catalytically active rhodium(III) and iridium(III) metals with light-absorbing ruthenium(II) metals within a polymetallic framework. Their molecular composition is a light absorber-electron collector-light absorber core of a photochemical molecular device (PMD) for photoinitiated electron collection. The variation of the central metal has some profound effects on the observed properties of these complexes. The electrochemical data for the title trimetallics consist of a Ru(II/III) oxidation and sequential reductions assigned to the bipyrimidine ligands, Ir or Rh metal centers, and bipyridines. In both trimetallic complexes, the first oxidation is Ru based and the bridging ligand reductions occur prior to the central metal reduction. This illustrates that the highest occupied molecular orbital (HOMO) is localized on the ruthenium metal center and the lowest unoccupied molecular orbital resides on the bpm ligand. This bpm-based LUMO in {[(bpy)(2)Ru(bpm)](2)RhCl(2)}(5+) is in contrast with that observed for the monometallic [Rh(bpm)(2)Cl(2)](+) where the Rh(III)/Rh(I) reduction occurs prior to the bpm reduction. This orbital inversion is a result of bridge formation upon construction of the trimetallic complex. Both the Ir- and Rh-based trimetallic complexes exhibit a room temperature emission centered at 800 nm with tau = 10 ns. A detailed comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these polymetallic complexes is described herein.

An infrared study of support effects on the interaction of dihydrogen and dinitrogen with supported rhodium films

Abstract A substantial support effect exists for the interaction of N 2 and H 2 with supported Rh. Infrared spectroscopy was used to show that the interaction of N 2 with Rh/TiO 2 (average ΔH = −5.0 kcal mol −1 ) was stronger than that of N 2 with Rh/Al 2 O 3 (average ΔH = −2.2 kcal mol −1 ). It was also demonstrated that H 2 dissociates to a greater extent over Rh/TiO 2 than over Rh/Al 2 O 3 to produce H 2 O on the support. Low coverages of RhH (Θ=0.1–0.2) on the supported Rh caused an enhanced interaction of N 2 with Rh for both supports which was evident in enhancement of the intensity of the NN stretching mode for the RhN 2 surface species. Both weakly bound and strongly bound forms of hydrogen were thought to be involved in the enhanced N 2 interaction with Rh. The interaction mechanism probably involves an electronic effect transmitted between H and N 2 through the Rh metal centers, causing increased surface coverage of RhN 2 and possibly an increase in the NN vibrational mode extinction coefficient.