Rhodium Atoms Research Articles

In Situ Observation of Elusive Dirhodium Carbenes and Studies on the Innate Role of Carboxamidate Ligands in Dirhodium Paddlewheel Complexes: A Combined Experimental and Computational Approach.

Carboxamidates as equatorial ligands in dirhodium paddlewheel catalysts are widely believed to increase selectivity at the expense of reactivity. The results of the combined experimental and computational approach described in this paper show that one has to beware of such generalizations. First, 103Rh NMR revealed how strongly primary carboxamidates impact the electronic nature of the rhodium center they are bound to; at the same time, such ligands stabilize donor/acceptor carbenes by engaging their ester carbonyl group into peripheral interligand hydrogen bonding. This array benefits selectivity as well as reactivity if maintained along the entire reaction coordinate of a catalytic cyclopropanation. In settings where the hydrogen bond needs to be distorted for the reaction to proceed, however, it constitutes a significant enthalpic handicap. Representative examples for each scenario were analyzed by DFT; in both cases, the cyclopropanation step rather than carbene formation was found to be turnover-limiting. While this conclusion somehow contradicts the literature, it implied that the direct observation of highly reactive dirhodium carbenes in truly catalytic settings might be possible, even though the intermediates carry olefinic sites amenable to intramolecular cyclopropanation. Such in situ monitoring by NMR is without precedent, yet it was successful with the homoleptic catalyst [Rh2(OPiv)4] as well as with its heteroleptic sibling [Rh2(OPiv)3(acam)] comprising an acetamidate (acam); in the latter case, the carbene bound to the rhodium atom at the [O3N]-face was observed, which concurs with the computational data that this species is stabilized by the forecited interligand hydrogen bonding.

Doping position estimation for FeRh-based alloys

FeRh-based alloys have attracted significant attention due to their magnetic phase transition and significant magnetocaloric effects. These properties position them as promising candidates for fundamental research and practical applications, including magnetic cooling and targeted drug delivery. The study of FeRh alloys, particularly those where Rhodium or Iron atoms are substituted with other transition metals, is crucial as certain substitutions preserve the alloy’s magnetocaloric properties. However, even within a specific structural type and without considering competing phases, determining which atom (Fe or Rh) is replaced upon introducing a third element remains unclear. This paper addresses this ambiguity through ab initio calculations. We propose an approach to predict whether a dopant will replace Fe or Rh, offering insights into the electronic and structural factors influencing the substitution. Additionally, we present a dataset of ab initio calculations on doped FeRh alloys, which will support future data-driven modeling efforts. Our findings not only advance the understanding of FeRh-based alloys but also contribute to the design of novel materials for experimental and industrial applications.

Electrocatalytic CO2 conversion on boron nitride nanotubes by metal single-atom engineering

Atomically dispersed single metal on defective boron nitride nanotubes (BNNTs) show promising potential for CO2 electrocatalytic reduction reaction (CO2ERR). Using density functional theory calculations, we explore the CO2ERR mechanism on single-atom catalysts (SACs) supported on BNNTs as well as assess their electrocatalytic performance of these catalysts. Boron or nitrogen vacancies in BNNTs are doped with palladium, platinum, or rhodium atoms to form three-coordinated metal centers, denoted as M@BNNT-V (M = Pt, Rh, Pd; V = B vacancy or N vacancy). Pt@BNNT with boron vacancies (Pt@BNNT-Bvac) demonstrates a tendency to facilitate multi-electron reduction processes leading to the formation of complex molecules such as CH2O, CH3OH, and CH4 with a limiting potential (UL) of −0.36 V. Conversely, Rh@BNNT-Bvac and Pd@BNWNT-Bvac show significantly lower limiting potentials of −0.06 V and −0.12 V, respectively, for the reduction to formic acid (HCOOH), with Rh@BNNT-Bvac exhibiting enhanced activity and selectivity. Additionally, the deep reduction to other hydrocarbons or oxygenates, which typically requires potentials of −0.72 V and −0.60 V, appears less likely on both Rh and Pd doped BNNT-Bvac, further delineating the nuanced performance differences induced by metal type and coordination environment within the BNNT structure.

Chlorido-[(1,2,5,6-η)-cyclo-octa-1,5-diene](1-ethyl-4-isobutyl-1,2,4-triazol-5-yl-idene)rhodium(I).

A new neutral triazole-based N-heterocyclic carbene rhodium(I) complex [RhCl(C8H12)(C8H15N3)], has been synthesized and structurally characterized. The complex crystallizes with two mol-ecules in the asymmetric unit. The central rhodium(I) atom has a distorted square-planar coordination environment, formed by a cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene (NHC) ligand, and a chlorido ligand. The bond lengths are unexceptional. A weak inter-molecular non-standard hydrogen-bonding inter-action exists between the chlorido and NHC ligands.

Response to the rebuttal of the article Pathological crystal structures.

We stand fully behind our earlier suggestion [Raymond & Girolami (2023). Acta Cryst. C79, 445-455] that the claim by Fish and co-workers [Chen et al. (1995). J. Am. Chem. Soc. 117, 9097-9098; Smith et al. (2014). Organometallics, 33, 2389-2404] of a linear two-coordinate rhodium(I) species is incorrect, and that the putative rhodium atom is in fact silver.

Binding ability of chloride ion with platinum and rhodium dinuclear complex containing ethylenediamine as co-ligand

In this study, the binding capability for Cl− ion was investigated with amidate-bridged platinum–rhodium dinuclear complex axially coordinated PPh3 to the rhodium atoms containing ethylenediamine (=en) as co-ligand. Mixing [PtRhCl3(piam)2(en)]·3H2O (2Cl) with PPh3 and NaPF6 provided [PtRhCl2(piam)2(en)(PPh3)]PF6 (2(PPh3)·PF6) which can bind Cl− ion at the axial position of platinum atom to be [PtRhCl2(piam)2(en)(PPh3)]Cl (2(PPh3)·Cl), where both crystal structures were determined by single-crystal X-ray analyses. UV–vis titration demonstrated that the formation constant of 2(PPh3)·PF6 with Cl− is 1.6 × 103 M−1, significantly smaller than that with NH3 as a co-ligand, which suggests that multinucleation is necessary for effective hydrogen bonds as well as the affinity of metals.

Stable anchoring of single rhodium atoms by indium in zeolite alkane dehydrogenation catalysts.

Maintaining the stability of single-atom catalysts in high-temperature reactions remains extremely challenging because of the migration of metal atoms under these conditions. We present a strategy for designing stable single-atom catalysts by harnessing a second metal to anchor the noble metal atom inside zeolite channels. A single-atom rhodium-indium cluster catalyst is formed inside zeolite silicalite-1 through in situ migration of indium during alkane dehydrogenation. This catalyst demonstrates exceptional stability against coke formation for 5500 hours in continuous pure propane dehydrogenation with 99% propylene selectivity and propane conversions close to the thermodynamic equilibrium value at 550°C. Our catalyst also operated stably at 600°C, offering propane conversions of >60% and propylene selectivity of >95%.

Single rhodium atom embedded two dimensional MoSi2N4: A promising electrocatalyst for oxygen reduction reaction

It is essential to develop electrocatalysts that exhibit excellent performance in the oxygen reduction reaction (ORR) for electrochemical energy applications. Recently, MoSi2N4 monolayer has been synthesized and attracted widespread interest. Herein, we constructed a series of structures by embedding transition metal (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt and Au) atom into MoSi2N4 with N vacancy (VN-MSN), denoted as TM@VN-MSN, and investigated their ORR activity systematically by first-principles calculations. It is found that TM@VN-MSN systems are stable and there is charge transfer between TM atom and MoSi2N4. After screening, Rh@VN-MSN has the best ORR catalytic activity with an overpotential of 0.35 V, even lower than that of classical Pt (111). Furthermore, an implicit solvation model is used to evaluate the solvent effect. The volcano plots and scaling relations are established based on free energies of intermediates, and verified ΔG*OH can use as a descriptor to explore the ORR activity on TM@VN-MSN. Moreover, d-band center theory and crystal orbital Hamilton populations (COHP) are used to reveal the ORR activity origin. This work not only provide a promising method for designing a new type of MoSi2N4-based ORR catalysts, but also expands its application in the field of electrocatalysis.

New bipolar and half-semiconductor materials formed by (6, 0) boron nitride nanotube decorated with different concentrations of rhodium atom

New bipolar and half-semiconductor materials formed by (6, 0) boron nitride nanotube decorated with different concentrations of rhodium atom

Modelling of the transition phase and energy for the adsorption of CO on Rhodium atoms interfaced with Niobium.

Modelling of the transition phase and energy for the adsorption of CO on Rhodium atoms interfaced with Niobium.

Crystalline Rhodium‐Tin Complexes with Radical Trianion and Tetraanion Phthalocyanine Ligands: Observation of Nimine(Pc)−Rh Coordination Bond

AbstractCrystalline {Cryptand(Na+)}[(COD)RhICl⋅SnII(Pc3−)]−⋅2C6H4Cl2 (1) and {Cryptand(Cs+)}[(COD)RhI⋅SnII(Pc4−)]−⋅C6H5CH3 (2) complexes were obtained via the interaction of [SnII(Pc3−)]− and [SnII(Pc4−)]2−, respectively, with organometallic {(COD)RhCl}2 dimer (COD is 1,5‐cyclooctadiene). Dissociation of {(COD)RhCl}2 followed by the Rh−Sn binding is observed at the formation of 1. Elimination of the chlorine atom at the rhodium atom is observed in 2, and rhodium is additionally coordinated to the imine nitrogen atom of Pc4−. The complexes contain mono‐ Pc⋅3− and doubly reduced Pc4− species, respectively, that is supported by the data of XRD analysis as well as optical and magnetic properties of 1 and 2. There is an alternation of C‐Nimine bonds in the macrocycles, which gradually increases with increasing negative charge on the macrocycle. The difference between shorter and longer bonds increases from 0.051 Å in Pc3− to 0.075 Å in Pc4−. The formation of 1 is accompanied by an essential blue shift of the Q‐band of starting SnPc and the appearance of a new intense band at 1031 nm. The even stronger shift of the Q‐band is observed in the spectrum of 2, but the band in the near‐IR range becomes weaker. The value of effective magnetic moment of 1 is 1.76 μB at 300 K corresponding the contribution of the Pc3− radical trianions (S=1/2). Only weak magnetic coupling with the Weise temperature of −3 K is observed in 1 due to weak π–π interaction between the macrocycles in the chains. Paramagnetic Pc3− species additionally monitored by EPR spectroscopy show a strong temperature dependence of g‐factor and linewidth of the EPR signal. Complex 2 is diamagnetic and EPR silent.

Carbonyl-bis-{6,6'-[(3,3'-di-tert-butyl-5,5'-dimeth-oxy-1,1'-biphenyl-2,2'-di-yl)bis(oxy)]bis-(dibenzo[d,f][1,3,2]-dioxaphosphepine)}hydridorhodium(I) toluene-d 8 2.25-solvate.

The crystal-structure determination of the title compound, [RhH(C46H44O8P2)2(CO)]·2.25C7D8, is reported. The bis-phosphite ligand, C46H44O8P2, is well known as Biphephos. One specific characteristic of this hydrido rhodium(I) monocarbonyl complex is that one bis-phosphite ligand is coordinated in the expected bidentate mode and the other is coordinated in a monodentate mode. Thus, one phosphite moiety remains non-coordinating. All three coordinating phospho-rus atoms occupy the equatorial positions in the trigonal-bipyramidal environment around the rhodium atom. The crystals of the hydrido rhodium(I) monocarbonyl complex contains deuterated solvent mol-ecules (toluene-d 8). Most of them are included in the model, but the contributions of about 0.84 toluene per unit cell were removed from the diffraction data, using the SQUEEZE procedure in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9-18].

Effect of Alkoxy Substituents on the Regioselectivity of Catalytic C-H Activation in Benzoic Acids: Experimental and DFT Study

This work demonstrates the influence of the catalyst and alkyne nature on the regioselectivity of rhodium-catalyzed annulation of alkoxy-substituted benzoic acids (such as 3-methoxybenzoic, 3,4-dimethoxybenzoic, and piperonylic acids) with alkynes. Here, X-ray diffraction and DFT calculation data gave evidence that the observed regioselectivity is provided by both steric and coordination effects of methoxy groups. The latter is the result of weak non-covalent C–H⋯O interactions with the supporting ligand rather than with the rhodium atom. We believe that these results are also valid for other reactions of the C-H activation of methoxy-substituted arene compounds.

Structure and 119Sn Mössbauer-spectroscopic characterization of BaRhSn2

Abstract The stannide BaRhSn2 was synthesized by induction melting of an arc-melted RhSn2 precursor compound with barium in a sealed tantalum ampoule. The structure of BaRhSn2 was refined from single-crystal X-ray diffractometer data: MgCuAl2 type, Cmcm, a = 437.56(4), b = 1242.35(10), c = 767.30(6) pm, wR2 = 0.0845, 469 F 2 values and 16 variables. The rhodium and tin atoms form a two-dimensional [RhSn2] δ− polyanionic network with short Rh–Sn (273–274 pm) and Sn–Sn (303–312 pm) distances. The large barium atoms lead to a substantial orthorhombic distortion of the (lonsdaleite-related) tin substructure, forcing a break of the Sn–Sn bond in b direction. This change in the tin substructure is reflected in the 119Sn Mössbauer spectrum. The tin atoms exhibit a higher s electron density which is expressed in an increased isomer shift of δ = 2.08(1) mm s−1 as compared to the previously reported stannide CaRhSn2 with a three-dimensional [RhSn2] δ− polyanionic network and δ = 1.96(4) mm s−1.

Dry reforming of methane over single-atom Rh/Al2O3 catalysts prepared by exsolution.

Single-atom catalysts often show exceptionally high performance per metal loading. However, the isolated atom sites tend to agglomerate during preparation and/or high-temperature reaction. Here we show that in the case of Rh/Al2O3 this deactivation can be prevented by dissolution/exsolution of metal atoms into/from the support. We design and synthesise a series of single-atom catalysts, characterise them and study the impact of exsolution in the dry reforming of methane at 700-900 °C. The catalysts' performance increases with increasing reaction time, as the rhodium atoms migrate from the subsurface to the surface. Although the oxidation state of rhodium changes from Rh(iii) to Rh(ii) or Rh(0) during catalysis, atom migration is the main factor affecting catalyst performance. The implications of these results for preparing real-life catalysts are discussed.

Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z' = 2 Structure from Pairwise C-H···Metallochelate-π Contacts.

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of (E)-2-(((2-ethylphenyl)imino)methyl)phenol (HL1) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL2), which in turn react with the dinuclear complex of [Rh(η4-cod)(µ-O2CCH3)]2 (cod = cycloocta-1,5-diene) to afford the mononuclear (η4-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ2N,O}rhodium(I), [Rh(η4-cod)(L1)] (1) or (η4-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ2N,O}rhodium(I), [Rh(η4-cod)(L2)] (2) (L1 or L2 = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL2 exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H+···-O(phenol) form (ketoamine form). 1H NMR spectra for HL2 in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η4-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in 1 in a structure with Z' = 2. The supramolecular packing in HL2 was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2. Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1. This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1. Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.

How Do Positions of Phosphito Units on a Calix[4]Arene Platform Affect the Enantioselectivity of a Catalytic Reaction?

Three chiral diphosphites, (S,S)-5,17-bis(1,1′-binaphthyl-2,2′-dioxyphosphanyloxy)-25,26,27,28-tetrapropyloxycalix[4]arene (1), (S,S)-5,11,17,23-tetra-tert-butyl-25,27-dipropoxy-26,28-bis(1,1′-binaphthyl-2,2′-dioxyphosphanyloxy)calix[4]arene (2) and (S,S)-5,11,17,23-tetra-tert-butyl-25,26-dipropoxy-27,28-bis(1,1′-binaphthyl-2,2′-dioxyphosphanyloxy)calix[4]arene (3), based on conical calix[4]arene were investigated in the rhodium-catalyzed asymmetric hydrogenation of α-dehydroamino esters. High conversions were observed after 24 h under 5 bar of hydrogen whatever the employed diphosphite, and the chiral induction increases in the order 1 < 3 < 2. This may be due to the presence of the calix[4]arene moiety, which by its presence modifies the second coordination sphere of the catalytic center. The larger steric hindrance around the rhodium atom leads to the higher enantiomeric excess.

Versatile Reactivity of Half-Sandwich Rhodium(III) Iminophosphonamide Complexes.

Novel 18e̅ and 16e̅ pentamethylcyclopentadienyl rhodium(III) complexes [(η5-C5Me5)RhX(NPN)] (1a,b, X = Cl; 2a-c, X = PF6, BAr4F) with chelating zwitterionic iminophosphonamide (NPN) ligands (Ph2P(NR)(NR'); a, R = R' = p-Tol; b, R = p-Tol, R' = Me; c, R = R' = Me) were synthesized and characterized by single-crystal X-ray diffraction. In the 16e̅ complexes 2, the rhodium (Rh) atom is efficiently stabilized by π-donation of unshared N electrons, thus hampering coordination of the external ligands and rendering the 18e̅ complexes labile. Due to low coordination enthalpy, the cationic 18e̅ monocarbonyl and pyridine adducts 2a·L are stable only at low temperatures. At room temperature, 2·CO adducts readily give stable carbonyl-carbamoyl complexes [(η5-C5Me5)Rh(CO){(CO(NR')Ph2P(NR)}]+ (4) formed as a result of CO insertion into the Rh-N bond, thus showing high nucleophilicity of the N atoms in 18e̅ complexes. High basicity of the Na+NPN- precursors caused side deprotonation of the η5-C5Me5 ligand during the synthesis of 1 that yields unstable fulvene Rh(I) complexes [(η4-C5Me4CH2)Rh{Ph2P(NR)(NR')2}] (3a,b). Complex 3a undergoes a facile reaction with isoprene to yield an unusual [(η5:η1-C5Me4(CH2)C(Me)═CHCH2)Rh(NPN)] complex─the first example of intermolecular 1,4-metallacycloaddition of diene to the Rh-fulvene complex.

The role of the spatial arrangement of single rhodium sites on ZSM-5 in the oxidative methane carbonylation to acetic acid

Single atom rhodium catalysts based on microporous ZSM-5 zeolite (SiO2/Al2O3 = 30) were studied in the reaction of oxidative methane carbonylation to acetic acid. For highly dispersed rhodium distribution on the zeolite surface, nitrogen-containing polymers, namely chitosan hydrochloride, polyethyleneimine, and polyvinylpyrrolidone, were used. Using XAS spectroscopy, all rhodium was found to be as single atom metal sites in the zeolite structure, which contributes to the acetic acid formation. The catalytic properties of rhodium zeolite catalysts in the oxidative methane carbonylation to acetic acid were revealed to depend not only on the single atom rhodium distribution and the catalyst acidity, but also on the spatial rhodium atom arrangement in the zeolite, which was established by the XPS. According to DFT calculations in combination with EXAFS modeling, rhodium anchored at the zeolite channel intersections was shown to be coordinated with four zeolite oxygen atoms and one hydroxyl group oxygen atom and be the dominant catalytic species in the acetic acid formation.

Two Rhodium(III) Ions Confined in a [18]Porphyrin Frame: 5,10,15,20-Tetraaryl-21,23-dirhodaporphyrin.

Invited for the cover of this issue is the group of Ewa Pacholska-Dudziak at the University of Wroclaw. The image depicts two rhodium atoms being fixed into the skeleton of 21,23-dirhodaporphyrin in place of two core nitrogen donors. Read the full text of the article at 10.1002/chem.202201513.