Rh Center Research Articles

Interfacial Electrochemistry of Catalyst-Coordinated Graphene Nanoribbons.

The immobilization of molecular electrocatalysts on conductive electrodes is an appealing strategy for enhancing their overall activity relative to those of analogous molecular compounds. In this study, we report on the interfacial electrochemistry of self-assembled two-dimensional nanosheets of graphene nanoribbons (GNR-2DNS) and analogs containing a Rh-based hydrogen evolution reaction (HER) catalyst (RhGNR-2DNS) immobilized on conductive electrodes. Proton-coupled electron transfer (PCET) taking place at N-centers of the nanoribbons was utilized as an indirect reporter of the interfacial electric fields experienced by the monolayer nanosheet located within the electric double layer. The experimental Pourbaix diagrams were compared with a theoretical model, which derives the experimental Pourbaix slopes as a function of parameter f, a fraction of the interfacial potential drop experienced by the redox-active group. Interestingly, our study revealed that GNR-2DNS was strongly coupled to glassy carbon electrodes (f = 1), while RhGNR-2DNS was not (f = 0.15). We further investigated the HER mechanism by RhGNR-2DNS using electrochemical and X-ray absorption spectroelectrochemical methods and compared it to homogeneous molecular model compounds. RhGNR-2DNS was found to be an active HER electrocatalyst over a broader set of aqueous pH conditions than its molecular analogs. We find that the improved HER performance in the immobilized catalyst arises due to two factors. First, redox-active bipyrimidine-based ligands were shown to dramatically alter the activity of Rh sites by increasing the electron density at the active Rh center and providing RhGNR-2DNS with improved catalysis. Second, catalyst immobilization was found to prevent catalyst aggregation that was found to occur for the molecular analog in the basic pH. Overall, this study provides valuable insights into the mechanism by which catalyst immobilization can affect the overall electrocatalytic performance.

Bimetallic [Ge-Rh] complex catalyzed alkyne semihydrogenation: Theoretical investigations on the germanium promoted mechanism and selectivity

Transitional metal catalyzed alkynes functionalization is a very important method to obtain the organic intermediates. In this work, the mechanism of PhCCPh hydrogenation catalyzed by [Ge-Rh] bimetallic catalyst has been studied by means of density functional theory. The whole reaction consists of two cycles, cis-semihydrogenation of alkyne to give Z-alkene and trans-isomerization to get E-alkene. The rate-determining step is the hydrogen complexation in cis-semihydrogenation. Because the isomerization from Z-alkene to E-alkene is easy, while the reverse transformation is difficult, E-alkene is main product of alkyne semihydrogenation. The semihydrogenation would be facilitated by replacing the terphenyl ligands in the [Ge-Rh] bimetallic catalyst with the polar N-heterocyclic ligands. The addition of H2 is crucial in controlling the chemoselectivity of the reaction. The Ge acts as the Lewis acid to reduce the electron density on Rh, making Rh center becomes more electropositive than that in the monorhodium catalysts, thus increasing the selectivity of the [Ge-Rh] bimetallic catalyst. Our work elucidates the detailed mechanism of PhCCPh semihydrogenation catalyzed by the [Ge-Rh] catalyst and provides explanations for the experimental results.

Regioselective hydroformylation of propene catalysed by rhodium-zeolite.

Hydroformylation is an industrial process for the production of aldehydes from alkenes1,2. Regioselective hydroformylation of propene to high-value n-butanal is particularly important, owing to a wide range of bulk applications of n-butanal in the manufacture of various necessities in human daily life3. Supported rhodium (Rh) hydroformylation catalysts, which often excel in catalyst recyclability, ease of separation and adaptability for continuous-flow processes, have been greatly exploited4. Nonetheless, they usually consist of rotationally flexible and sterically unconstrained Rh hydride dicarbonyl centres, only affording limited regioselectivity to n-butanal5-8. Here we show that proper encapsulation of Rh species comprising Rh(I)-gem-dicarbonyl centres within a MEL zeolite framework allows the breaking of the above model. The optimized catalyst exhibits more than99% regioselectivity to n-butanal and more than99% selectivity to aldehydes at a product formation turnover frequency (TOF) of 6,500 h-1, surpassing the performance of all heterogeneous and most homogeneous catalysts developed so far. Our comprehensive studies show that the zeolite framework can act as a scaffold to steer the reaction pathway of the intermediates confined in the space between the zeolite framework and Rh centres towards the exclusive formation of n-butanal.

Isolierte Rhodium(II)‐Metallradikalzentren Katalysieren die Hydrofunktionalisierung von Olefinen

AbstractRh(II)‐Porphyrinkomplexe weisen einen ausgeprägten metallzentrierten Radikalcharakter auf, und die Fähigkeit kleine Moleküle unter milden Bedingungen zu aktivieren. Katalyse mit Rh(II)‐Porphyrinen ist jedoch äußerst selten. Zusätzlich zu Dimerisierung gehen Rh(II)‐Porphyrine leicht kinetisch und thermodynamisch einfache Reaktionen ein, an denen zwei Rh(II)‐Zentren beteiligt sind, um stabile Rh(III)‐X Zwischenprodukte zu erzeugen, die den Umsatz in der thermischen Katalyse behindern. Hier berichten wir über die geometrische Isolierung von Rh(II)‐Metallradikalen in einem MOF‐Wirt, die Rh(II)‐Metallradikale nicht nur vor Dimerisierung schützt, sondern ihnen auch die Teilnahme an der thermischen Katalyse ermöglicht. Der Zugang zu PCN‐224 oder PCN‐222, bei denen die Porphyrin‐Linker vollständig durch Rh(II) metalliert sind, aber keine begleitenden Rh(0)‐Nanopartikel erzeugt werden, gelang durch die erste direkte MOF‐Synthese mit einem Linker der ein Übergangsmetall‐Alkyl Motiv enthält, gefolgt von Photolyse der Rh(III)−C Bindung.

Site-Isolated Rhodium(II) Metalloradicals Catalyze Olefin Hydrofunctionalization.

Rh(II) porphyrin complexes display pronounced metal-centered radical character and the ability to activate small molecules under mild conditions, but catalysis with Rh(II) porphyrins is extremely rare. In addition to facile dimerization, Rh(II) porphyrins readily engage in kinetically and thermodynamically facile reactions involving two Rh(II) centers to generate stable Rh(III)-X intermediates that obstruct turnover in thermal catalysis. Here we report site isolation of Rh(II) metalloradicals in a MOF host, which not only protects Rh(II) metalloradicals against dimerization, but also allows them to participate in thermal catalysis. Access to PCN-224 or PCN-222 in which the porphyrin linkers are fully metalated by Rh(II) in the absence of any accompanying Rh(0) nanoparticles was achieved via the first direct MOF synthesis with a linker containing a transition-metal alkyl moiety, followed by Rh(III)-C bond photolysis.

(4-Butyl-1-ethyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane)rhodium(I) tetra-fluorido-borate.

In the title triazole-based N-heterocyclic carbene rhodium(I) cationic complex with a tetra-fluorido-borate counter-anion, [Rh(C8H12)(C8H15N3)(C18H15P)]BF4, which crystallizes with two cations and two anions in the asymmetric unit, the Rh center has a distorted square-planar coordination geometry with expected bond distances. Several nonclassical C-H⋯F hydrogen-bonding inter-actions help to consolidate the packing. Two of the F atoms of one of the anions are disordered over adjacent sites in a 0.814 (4):0.186 (4) ratio.

Twinned versus linked organometallics - bimetallic "half-baguette" pentalenide complexes of Rh(I).

The application of Mg[Ph4Pn] and Li·K[Ph4Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of "half-baguette" anti-[RhI(L)n]2[μ:η5:η5Ph4Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] complexes as well as the related iridium complex anti-[IrI(COD)]2[μ:η5:η5Ph4Pn]. With CO exclusive syn metalation was obtained even when using mono-nuclear Rh(I) precursors, indicating an electronic preference for syn metalation. DFT analysis showed this to be the result of π overlap between the adjacent M(CO)2 units which overcompensates for dz2 repulsion of the metals, an effect which can be overridden by steric clash of the auxiliary ligands to yield anti-configuration as seen in the larger olefin complexes. syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] is a rare example of a twinned organometallic where the two metals are held flexibly in close proximity, but the two d8 Rh(I) centres did not show signs of M-M bonding interactions or exhibit Lewis basic behaviour as in some related mono-nuclear Cp complexes due to the acceptor properties of the ligands. The ligand substitution chemistry of syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] was investigated with a series of electronically and sterically diverse donor ligands (P(OPh)3, P(OMe)3, PPh3, PMe3, dppe) yielding new mono- and bis-substituted complexes, with E-syn-[RhI(CO)(P{OR})3]2[μ:η5:η5Ph4Pn] (R = Me, Ph) characterised by XRD.

Stereoselectivity control in Rh-catalyzed β-OH elimination for chiral allene formation

Stereoselectivity control and understanding in the metal-catalyzed reactions are fundamental issues in catalysis. Here we report sterically controlled rhodium-catalyzed SN2’-type substitution reactions of optically active tertiary propargylic alcohols with arylmetallic species affording the non-readily available enantioenriched tetrasubstituted allenes via either exclusive syn- or anti-β-OH elimination, respectively, under two sets of different reaction parameters. Detailed mechanistic experiments and density functional theory (DFT) studies reveal that the exclusive anti-Rh(I)-OH elimination is dictated by the simultaneous aid of in situ generated boric acid and ambient water, which act as the shuttle in the hydroxy relay to facilitate the Rh(I)-OH elimination process via a unique ten-membered cyclic transition state (anti-TS2_u). By contrast, the syn-Rh(III)-OH elimination in C–H bond activation-based allenylation reaction is controlled by a four-membered cyclic transition state (syn-TS3) due to the steric surroundings around the Rh(III) center preventing the approach of the other assisting molecules. Under the guidance of these mechanistic understandings, a stereodivergent protocol to construct the enantiomer of optically active tetrasubstituted allenes from the same starting materials is successfully developed.

Three-coordinate monoanions of rhodium(1–) and iridium(1–): Isolable examples of coordinatively-unsaturated metalate anions

Metalate complexes – those that possess metal centers in formally negative oxidation states – are typically coordinatively and electronically saturated organometallic species. The reduced nature of the metal centers within these complexes is well understood to be responsible for their high reactivity as metal-based nucleophiles. While the chemistry of such complexes is extensive, there are very few examples where coordinative or electronic unsaturation accompanies highly-reduced d-orbital manifolds as a means to augment reactivity. Reported here is the isolation of the 16e–, d10 rhodium and iridium metalate anions, [M(CNArDipp2)3]–, where coordinative unsaturation is achieved through the use of encumbering m-terphenyl isocyanide ligands. Reactivity studies with electrophiles demonstrate that the Rh and Ir centers in these metalates retain significant nucleophilic character, which enables the formation of unusual metal/main-group-element species on account of low coordination numbers.

A theoretical study of the mechanisms, regio- and enantioselectivities associated with rhodium-catalyzed allylic alkylation reactions using hard benzylzinc nucleophiles

A systematic theoretical study of rhodium-catalyzed allylic alkylation reactions using hard benzylzinc nucleophiles was performed. The mechanisms were investigated in detail together with the regioselectivity associated with the branched and linear pathways and the intriguing anti- (as opposed to syn-) diastereoselectivity of this process. The preferred mechanism was found to be initiated by the oxidative addition of the rhodium(I) complex followed by migration of the benzyl nucleophile to the Rh center. The subsequent nucleophilic addition of a benzyl group to the η3-allyl moiety takes place via the formation of a new C–C bond to afford the final allylic benzylation product. This proposed mechanism readily explains the experimental observation that the products obtained from such reactions are branched (as opposed to linear) with an anti-configuration (as opposed to a syn-configuration). This work also provides reasons for the lower regioselectivity observed for a OCO2tBu leaving group or cyanophenyl/secondary amide substituents at the 4-position of the substrate. The present study is expected to assist in the rational design of potential substrates for analogous reactions.

Photohemilabile PSP‐Rhodium Pincer Catalysts for Dehydrogenative C−H Bond Activation of Arenes

AbstractHemilabile ligands have been widely utilized for transition metal catalysis, but hemilability upon excitation, termed as “photohemilability”, is still elusive despite the recent progress of organometallic photocatalysis. In this study, it was discovered that Thioxantphos‐based PSP−Rh pincer complexes undergo dehydrogenative C−H bond activation of arenes under irradiation with visible light. Theoretical calculations indicate that a transient photodissociation of the strong S−Rh bond brought about by d‐d excitation on Rh is the key to realize oxidative addition of arenes to the Rh(I) center, which can be regarded as a photohemilabile behavior. This C−H bond activation process was successfully applied to visible light‐driven C−H borylation of simple arenes.

A dinuclear Rh(-i)/Rh(i) complex bridged by biphilic phosphinine ligands.

Bimetallic complexes have enabled precise control of catalysis by accumulating two discrete metal centres. In these complexes, bridging ligands are essential to combine multiple metals into one molecule. Among some bridging modes, an unsymmetric bridging mode will differentiate the electronic structures of the two metal centres. In this study, a dinuclear Rh(-i)/Rh(i) complex bridged by tridentate phosphine-phosphinine-phosphine ligands was prepared by reduction of the corresponding Rh(i) complex. Single-crystal X-ray analysis and DFT calculations suggest that the phosphinine ligands adopt an unsymmetric bridging mode wherein phosphinine accepts d-electrons from one Rh centre and, at the same time, donates lone pairs to the other Rh centre.

Rhodium Single‐Atom Catalyst Design through Oxide Support Modulation for Selective Gas‐Phase Ethylene Hydroformylation

AbstractA frontier challenge in single‐atom (SA) catalysis is the design of fully inorganic sites capable of emulating the high reaction selectivity traditionally exclusive of organometallic counterparts in homogeneous catalysis. Modulating the direct coordination environment in SA sites, via the exploitation of the oxide support's surface chemistry, stands as a powerful albeit underexplored strategy. We report that isolated Rh atoms stabilized on oxygen‐defective SnO2 uniquely unite excellent TOF with essentially full selectivity in the gas‐phase hydroformylation of ethylene, inhibiting the thermodynamically favored olefin hydrogenation. Density Functional Theory calculations and surface characterization suggest that substantial depletion of the catalyst surface in lattice oxygen, energetically facile on SnO2, is key to unlock a high coordination pliability at the mononuclear Rh centers, leading to an exceptional performance which is on par with that of molecular catalysts in liquid media.

Rhodium Single-Atom Catalyst Design through Oxide Support Modulation for Selective Gas-Phase Ethylene Hydroformylation.

A frontier challenge in single-atom (SA) catalysis is the design of fully inorganic sites capable of emulating the high reaction selectivity traditionally exclusive of organometallic counterparts in homogeneous catalysis. Modulating the direct coordination environment in SA sites, via the exploitation of the oxide support's surface chemistry, stands as a powerful albeit underexplored strategy. We report that isolated Rh atoms stabilized on oxygen-defective SnO2 uniquely unite excellent TOF with essentially full selectivity in the gas-phase hydroformylation of ethylene, inhibiting the thermodynamically favored olefin hydrogenation. Density Functional Theory calculations and surface characterization suggest that substantial depletion of the catalyst surface in lattice oxygen, energetically facile on SnO2 , is key to unlock a high coordination pliability at the mononuclear Rh centers, leading to an exceptional performance which is on par with that of molecular catalysts in liquid media.

Highly Active Rh Catalysts with Strong π-Acceptor Phosphine-Containing Porous Organic Polymers for Alkene Hydroformylation.

Phosphine-containing porous organic polymers (phosphine-POPs) are a kind of potential catalyst support for alkene hydroformylation. However, the synthesis of phosphine-POPs with strong π-acceptor is still a challenge. Herein, we report the synthesis of phosphine-POPs with different π-acceptor properties [POL-P(Pyr)3, CPOL-BPa&PPh3-15, and CPOL-BP&PPh3-15] and evaluated their performances as ligands to coordinate with Rh(acac)(CO)2 for hydroformylation of alkenes. We found that the Rh center with stronger π-acceptor phosphine-POPs showed better catalytic performance. Rh/CPOL-BPa&PPh3-15 with strong π-acceptor bidentate phosphoramidites showed obviously higher activity and regioselectivity (TON = 7.5 × 103, l/b = 26.1) than Rh/CPOL-BP&PPh3-15 (TON = 5.3 × 103, l/b = 5.0) with weaker π-acceptor bidentate phosphonites. Particularly, the TON of the hydroformylation reached 27.7 × 103 upon using Rh/POL-P(Pyr)3 which possessed tris(1-pyrrolyl)phosphane coordination sites. Overall, our study provides an orientation to design phosphine-POPs for hydroformylation reactions.

Triazolyl- vs Pyridyl-Functionalized N-Heterocyclic Carbene Complexes: Impact of the Pendant N-Donor Ligand on Intramolecular C-C Bond Formation.

Organometallic Rh(Cp*) (Cp* = η5-pentamethylcyclopentadienyl) complexes with monodentate N-heterocyclic carbene (NHC) ligands bearing a pendant anthracenyl substituent have been shown to undergo intramolecular C-C coupling reactions. Herein, two bidentate NHC ligands substituted with pyridyl or triazolyl donor groups were prepared along with the corresponding MII/III (M = RuII, OsII, RhIII, IrIII) complexes. While the Rh(Cp*) complex featuring an NHC-triazole bidentate ligand underwent the equivalent reaction as the monodentate Rh(NHC) complex, i.e., it formed a polydentate ligand, the pyridyl-pendant derivative was unequivocally shown to be unreactive. This contrasting behavior was further investigated by density functional theory (DFT) calculations that highlighted significant differences between the two types of Rh(III) complexes with pendant pyridyl or triazolyl N-coordinating groups. Modeling of the reaction pathways suggests that the initial formation of a dicationic Rh(III) species is unfavorable and that the internal ligand transformation proceeds first by dissociation of the coordinated N atom of the pendant group from the Rh center. After the formation of a neutral η4-fulvene ligand via combined proton/single electron transfer, a cycloaddition occurs between the exo-ene bond of fulvene and the 9' and 10' positions on the pendant anthracenyl group. The resulting experimental UV-visible spectrum recorded in methanol of the polydentate triazolyl-based Rh species revealed the loss of the vibronic coupling typically associated with an anthracenyl functional group. Moreover, TD-DFT modeling indicates the presence of an equilibrium process whereby the N-coordination of the pendant triazolyl group to the RhIII center appears to be highly labile. Charge decomposition analysis (CDA) of the DFT-modeled species with the dissociated triazolyl group revealed a pseudo-η3-allylic interaction between the π-type MOs of the transformed anthracenyl group and the RhIII center; thus, the singly attached chelating ligand is classified as having rare nonadenticity.

Organo-rhodium and -iridium complexes containing the thiosulfate (S2O32−) ligand

Reaction of the complexes [M2(μ-Cl)2(cod)2] (M = Rh, Ir) with sodium thiosulfate pentahydrate gives the dinuclear thiosulfate complexes [M2(μ-S2O3)2(cod)2]2−, which were isolated as their Ph3PCH2Ph+ salts. An X-ray structure determination on the Rh complex confirms the dimeric nature of the complex, with the two Rh(I) centres bridged by the terminal sulfurs of the thiosulfate ligands. Using ESI mass spectrometry, the dimer [M2(μ-S2O3)2(cod)2]2− and monomer [M(S2O3)(cod)]− are both observed, depending on the metal and capillary exit voltage. Reaction of sodium thiosulfate pentahydrate with the complexes trans-[RhCl(CX)(PPh3)2] (X = O or S) gave the anionic complexes Na[Rh(S2O3)(CX)(PPh3)], which were characterised by 31P{1H} NMR and IR spectroscopies, ESI MS, and in the case of the thiocarbonyl complex, by elemental analysis. Pentamethylcyclopentadienyl (Cp*) rhodium(III) and iridium(III) thiosulfate complexes [(Cp*M)2(μ -S2O3)3]2− were also synthesised by reaction of the chloride-bridged dimers [Cp*MCl2]2 with sodium thiosulfate, and isolated as their Ph3PCH2Ph+ salts. The complexes gave the expected parent dianions in ESI mass spectra, which underwent fragmentation of a thiosulfate ligand at higher capillary exit voltages to give ions [(Cp*M)2(S2O3)2S + H]−.

Theoretical study on the mechanism of the carbonylation cyclization of 1,5-diynes with hydrosilanes

The carbonylation cyclization of unstaturated hydrocarbons with hydrosilanes is an appealing strategy for the construction of valuable silicon-containing carbonyl compounds. Despite significant advances, the detailed mechanistic investigations on these reactions are still scarce. Herein, density functional theory (DFT) calculations are carried out to provide mechanistic insights into Rh-catalyzed carbonylative cyclization of 1,5-diyne with triethylsilane. The calculated results indicate that (1) In the silylrhodation process, distinguished from the classical Chalk−Harrod mechanism and the modified Chalk−Harrod mechanism, H atom on the Rh center first transfers to CO ligand to yield a aldehyde group. Then the terminal alkyne of 1,5-diyne inserts into Rh−Si bond to form C−Si bond. With the subsequent H atom retransfer to Rh center, β-(E)-silyl-substituted enyne can be obtained. (2) In the CO insertion process, internal alkyne prior to CO ligand coordinates to the Rh center. (3) The yielded metallacyclohexanone intermediate undergoes H-transfer and C−C cyclization steps rather than direct C−C cyclization forming the η3-binding intermediate. Finally, after the reductive elimination, the product (1E,3Z)-1-pentylidene-3-[(triethylsilyl)methylidene]indan-2-one is formed. The computational results demonstrate that H-transfer is the rate-determining step.

Cyclometalated Rhodium and Iridium Complexes Containing Masked Catecholates: Synthesis, Structure, Electrochemistry, and Luminescence Properties.

Two neutral cyclometalated rhodium and iridium coordination assemblies [(F2ppy)2M(η-Cat)], M = Rh, (2) and M = Ir, (3) (F2ppy: 2,4-difluorophenylpyridine), displaying a masked catecholate (η-Cat = η-O∧O) are described. The catecholate ligand is π-bonded to an organometallic Cp*Ru(II) moiety. The latter brings stability to the whole system in solution and suppresses the formation of the related paramagnetic semiquinone complex. The determination of the molecular structure of the iridium complex [(F2ppy)2Ir(η-Cat)] (3) corroborates the formation of the target compound and reveals the generation of a rare two-dimensional (2D) honeycomb supramolecular architecture in the solid state, in which the Δ-enantiomer self-assembles with the Λ-enantiomer through encoded π-π interactions among individual units. The electrochemistry of complexes 2 and 3 was investigated and showed that reduction occurs at very negative potentials (∼-2.2 V versus saturated calomel electrode (SCE)), while oxidation of the cyclometalated Rh and Ir centers occurs at 0.8 and 0.86 V. In contrast to complexes with 1,2-dioxolene chelates, which are nonemissive, the heterodinuclear diamagnetic complexes 2 and 3 were found to be emissive at room temperature both in solution and in the solid state. Moreover, at 77 K in a solid state, both compounds display opposite emission behavior, for instance, complex 3 displays a blue-shifted emission, while rhodium compound 2 exhibits red-shifted emission to lower energy.

Inverse Isotope Effects in Single-Crystal to Single-Crystal Reactivity and the Isolation of a Rhodium Cyclooctane σ-Alkane Complex.

The sequential solid/gas single-crystal to single-crystal reaction of [Rh(Cy2P(CH2)3PCy2)(COD)][BArF4] (COD = cyclooctadiene) with H2 or D2 was followed in situ by solid-state 31P{1H} NMR spectroscopy (SSNMR) and ex situ by solution quenching and GC-MS. This was quantified using a two-step Johnson–Mehl–Avrami–Kologoromov (JMAK) model that revealed an inverse isotope effect for the second addition of H2, that forms a σ-alkane complex [Rh(Cy2P(CH2)3PCy2)(COA)][BArF4]. Using D2, a temporal window is determined in which a structural solution for this σ-alkane complex is possible, which reveals an η2,η2-binding mode to the Rh(I) center, as supported by periodic density functional theory (DFT) calculations. Extensive H/D exchange occurs during the addition of D2, as promoted by the solid-state microenvironment.