Rhodium Atoms Research Articles

LaRhAl, La3Rh3Al4, and Ce5Rh5Al6 as a new family of ternary aluminides

The structures of three aluminides of similar composition are determined via X-ray diffraction. The structures consist of coordination polyhedra built of rhodium atoms that form alternating layers extending perpendicular to the short cell parameter. Compound Ce5Rh5Al6 is built of fragments of the structures of LaRhAl and La3Rh3Al4 in a ratio of 1: 1.

Pyrene-tagged carbohydrate-based mixed P/S ligand: spacer effect on the Rh(i)-catalyzed hydrogenation of methyl α-acetamidocinnamate.

The post-functionalization of a chiral catalyst offers the advantage of providing it with additional physical characteristics that, together with its enantioselective capacity, increase its overall synthetic value. Taking advantage of the modularity and polyfunctionality of carbohydrate-derived ligands, herein we report the synthesis of two mixed P/S catalysts functionalized with a pyrene group through the 6 position of the sugar by carbon chains of different lengths. Using the hydrogenation of methyl (Z)-α-acetamidocinnamate as the model reaction has shown that the proximity of the pyrenyl group to the catalytic center is detrimental to the activity and enantioselectivity of the hydrogenation process, the most efficient catalyst being the complex derived from pyrenebutyric acid 12. The study of the supramolecular π-π interaction of the most active complex 12 with SWCNTs by UV-Vis spectroscopy shows, that in ethyl acetate complex 12 is totally adsorbed onto the SWCNT surface, while in methylene chloride there is an equilibrium between the adsorbed and the free form of the complex, allowing the use of complex 12 and SWCNTs in a catch and release process. Interestingly, it has been determined that the nanocatalyst 12/SWCNT is more enantioselective than complex 12 alone, affording (S)-N-acetylphenyl alanine 16 in quantitative yield and 96% ee.

Stacking of dicarbonylacetylacetonatorhodium(I) molecules

Density functional theory calculations was used to understand the stabilisation of intermolecular rhodium-rhodium interactions as observed in [Rh(acac)(CO)2] crystals. Quantum theory of atoms in molecules calculations confirm that bonding paths, consistent with weak bonds stronger than hydrogen bonds, between the rhodium centres in different [Rh(acac)(CO)2] molecules, exist. The linear array of [Rh(acac)(CO)2] molecules were further stabilised by intermolecular hydrogen bonds. The weak interaction between the rhodium centres were further confirmed by natural bond orbital (NBO) calculations that showed donor-acceptor interaction between the a filled and an empty NBO on the rhodium atoms in adjacent [Rh(acac)(CO)2] molecules. A visualisation of the molecular orbitals of linear units of [Rh(acac)(CO)2] molecules showed σ bonding and anti-bonding interactions between the dz2, as well as between the pz orbitals, on rhodium of different [Rh(acac)(CO)2] molecules.

(η2,η2-Cycloocta-1,5-diene)[2-(diphenylphosphanylmethyl)pyridine-κ2N,P]rhodium(I) tetrafluoridoborate 1,2-dichloroethane monosolvate

The title compound, [Rh(C8H12)(C18H16NP)]BF4has been prepared as a precatalyst for applications in rhodium-catalysed additions of carbocyclic acids to terminal alkynes leading toanti-MarkovnikovZ-enol esters. Here the triclinic pseudopolymorph of the title compound is presented. In contrast to the earlier reported pseudopolymorph (orthorhombic space group) [Weiet al.(2013).Chem. Eur. J.19, 12067–12076], the triclinic polymorph contains half a molecule of dichloromethane as solvent in the asymmetric unit. The rhodium(I) atom exhibits a square-planar coordination. The estimated diffraction contribution of the disordered solvent (a half molecule of dichloroethane per asymmetric unit) was subtracted from the observed diffraction data using the SQUEEZE [Spek (2015).Acta Cryst.C71, 9–16] routine inPLATON. The given chemical formula and other crystal data do not take the solvent into account.

O2 adsorption on AunRh n = 1–5 neutral and charged clusters

Theoretical evidence is presented for the molecular and dissociative adsorption of O2 on free Au n Rh neutral, anionic and cationic clusters with 1 to 5 gold atoms, indicating that the stabilization of the activated di-oxygen species is a key factor for the unusual catalytic activities of Au-based catalysts. The structure, stability, for both molecular and dissociative O2 adsorption on Au n Rh n = 1–5 clusters has been investigated using density-functional theory. To find the transition states, the minimum energy paths have been explored for a few clusters. In general, lower values for the activation energy have been found when compared with the barriers that occur on pure Au n based clusters. The higher binding energies in the AuRh mix favor oxygen dissociation among any other possible reaction paths. The anionic clusters being the most reactive of all. The molecular bonding mechanism to these complexes involves charge transfer to the oxygen molecule with a concomitant activation of the O-O bond to a superoxo-like state. The characteristic planar structures of both pure gold and AuRh clusters prevail for most of the cases here studied. The odd-even characteristic catalytic activation of pure gold clusters is not observed once even a single rhodium atom has been added to the cluster.

Structural, Energetic, and Magnetic Properties of Ag $$_{n-m}$$ n - m Rh $$_{m}$$ m and Ag $$_{m}$$ m Rh $$_{n-m}$$ n - m Clusters with $$n \le 20$$ n ≤ 20 and $$m=0,1$$ m = 0 , 1

At first, a genetic algorithm in combination with either the parametrized density-functional tight-binding method or a Gupta-potential is used to determine the putative global minimum energy structures of mixed Ag $$_{n-m}$$ Rh $$_{m}$$ and Ag $$_{m}$$ Rh $$_{n-m}$$ clusters with $$n\le 20$$ and $$m=0,1$$ . Subsequently, the resulting structures are re-optimized with a first-principles method. The results demonstrate that the exchange of a single silver atom by rhodium leads to compact core-shell-like structures with structural motifs well known from the Lennard-Jones system. For the systems of the present study, AgRh $$_{n-1}$$ clusters retain their cube-based structural motif and the silver atoms typically avoid the corner positions within a cube if possible. Population analysis of both cluster systems shows that the total magnetic moment is mainly due to unpaired electrons on the rhodium atoms with a small ferro-magnetic contribution of the silver host in Ag $$_{n-1}$$ Rh and virtually no contribution to the total magnetic moment from the single silver atom in AgRh $$_{n-1}$$ clusters.

Synthesis of Phenanthridines via a Rhodium-Catalyzed C–C Bond Cleavage Reaction of Biphenylene with Nitriles

The reaction of biphenylene with various nitriles in the presence of catalytic amount of [Rh(cod)2BF4]/dppe under microwave irradiation afforded 9-substituted phenanthridines. The reaction with alkyl and aromatic nitriles provided the corresponding 9-substituted phenanthridines in 26–79% isolated yields. The reaction was also carried out with cyanopyridines and it provided heterocyclic compounds with the bipyridine and terpyridine scaffold. The synthesized bipyridine and terpyridine were complexed with [Rh(cod)Cl]2. The former provided a Rh(III) complex in which the cyclooctadiene moiety was oxidized to the tetrahydrofuran ring, whereas the latter gave a structurally fluxional complex (in solution) with only one pyridine ring coordinated to the rhodium atom.

A comparative study of AumRhn (4 ≤ m + n ≤ 6) clusters in the gas phase versus those deposited on (100) MgO.

A comparative theoretical study has been performed of the gas phase and deposited AumRhn (4 ≤ m + n ≤ 6) clusters. The combined use of a genetic algorithm and Density Functional Theory (DFT) calculations allows us to explore the potential energy surface and, therefore, find efficiently and automatically the global minimum configuration for each composition. Our results show interesting effects on the geometries of the clusters on deposition. This occurs because the rhodium atoms (electronically) prefer to be in contact with the MgO surface, sometimes promoting planar clusters to become three-dimensional when deposited, and three-dimensional clusters in the gas phase to become two-dimensional. Together with the change in geometries, the magnetic moment is reduced from the gas phase, as the electrons rearrange themselves when the cluster interacts with the substrate.

X-ray photoelectron spectroscopy as a probe of rhodium-ligand interaction in ionic liquids

We use X-ray photoelectron spectroscopy (XPS) to identify the interaction between the rhodium atom and phosphine ligands in six 1-octyl-3-methylimidazolium-based ionic liquids ([C8C1Im][X]). The formation of a mono-phosphine rhodium complex based upon addition of triphenylphosphine (PPh3) is confirmed by XPS in all ionic liquids studied herein. Due to the electron donation effect of the ligand, the rhodium atom becomes more negatively charged and thus exhibits a lower measured binding energy. The influence of the anion basicity on the formation of different types of rhodium complexes is also investigated. By introducing a biphosphine ligand, a chelated diphosphine rhodium complex is formed in ionic liquids with more basic anions and verified by both XPS and Infrared Spectroscopy (IR). The measured Rh 3d binding energies are correlated to the reaction selectivity of a hydroformylation reaction which inspires a method to design a metal catalyst to control the chemical reaction towards desired products in the future.

Preparation, structure, and dynamic and electrochemical behaviors of dinuclear rhodium(I) complexes with bridging formamidinato ligands

Dinuclear rhodium(I) complexes, [Rh(4-Me-pf) (cod)]2 (1), [Rh(3,5-Me2-pf) (cod)]2 (2), [Rh(4-Me-pf) (nbd)]2 (3), [Rh(3,5-Me2-pf) (nbd)]2 (4), and [Rh(2,6-F2-pf) (nbd)]2 (5), have been synthesized and characterized by X-ray structure analysis, 1H, 13C, and 19F NMR, UV–vis, ESI-TOF-MS, and elemental analysis. In these complexes, two rhodium atoms are bridged by two formamidinato ligands and each rhodium atom is coordinated by one chelating cod or nbd ligand to form an approximately square planar coordination structure with two nitrogen atoms and two double bonds. The Rh··Rh distances are in the range of 3.2668 to 2.9726 Å, suggesting a direct bonding interaction between two rhodium atoms. Variable temperature NMR studies in CD2Cl2 solution have revealed that 1–5 exhibit a novel dynamic behavior, that is, an interconversion between two enantiomers. The activation parameters for racemization have been determined by the line shape analyses of the 1H and 19F NMR spectra taken at various temperatures. Variable temperature NMR studies have also revealed that the rotation rates of the four aryl groups around N–C(aryl) bonds are extremely different in each complex. The cyclic voltammetry study has shown that the oxidation potentials corresponding to Rh23+/Rh2+ are 0.41 V in the cod complexes (1 and 2) while those in the nbd complexes (3–5) have shown negative shift by ca. 0.2 V. The reasons for the difference in dynamic behaviors and redox properties among these complexes have been discussed.

Crystal structure of [Rh(H2O)5Cl](C7H7O3S)2 and Cs[Rh(H2O)5NO3](C7H7O3S)3•H2O

The crystal structures of the compounds with the composition [Rh(H2O)5Cl](C7H7O3S)2 (I) and Cs[Rh(H2O)5NO3](C7H7O3S)3•H2O (II) are determined. Crystallographic data for I: a = 10.063(10) A, b = 7.771(8) A, c = 13.524(15) A; β = 100.44(3)°, space group P21, Z = 2, dcalc = 1.823 g/cm3; for II: a = 16.221(3) A, b = 9.646(19) A, c = 21.307(4) A, β = 92.06(3)°, space group P21, Z = 8, dcalc = 1.833 g/cm3. The compounds are characterized by IR spectroscopy and powder X-ray diffraction. The rhodium atoms have a distorted octahedral environment consisting of five oxygen atoms of coordinated water molecules and one chloride ion (I)/oxygen atom of the nitrate ion (II) with the following distances: Rh–OH2O of 1.993-2.075 A and Rh–Cl of 2.270 A for I; Rh–\({O_{{H_2}O}}\) of 1.996-2.031 A and Rh–\({O_{N{O_3}}}\) of 1.990 A for II.

Rhodium(III) Azuliporphyrins

The organometallic chemistry of azuliporphyrins has been extended to the first syntheses of rhodium(III) derivatives. Reaction of [Rh(CO)2Cl]2 with an azuliporphyrin in refluxing xylenes gave rhodium(III) azuliporphyrins in 62–67% yield. These derivatives incorporate solvent molecules as axial ligands and thereby introduce two carbon–rhodium bonds in a three-component reaction. The methylbenzyl ligands that are derived from xylenes overlie the π system of the porphyrinoid macrocycle, and proton NMR spectra show that these units are strongly shielded. The o-, m-, and p-xylene-derived complexes were characterized by X-ray crystallography, and this confirmed the presence of the central rhodium atom and the axial benzylic ligands. Additionally, when the azuliporphyrin was reacted with [Rh(CO)2Cl]2 in refluxing acetonitrile and then treated with acetone and basic alumina, a rhodium(III) azuliporphyrin with an acetone-derived axial ligand was generated. These results demonstrate that azuliporphyrins are superio...

ChemInform Abstract: Chiral Cyclopentadienyls: Enabling Ligands for Asymmetric Rh(III)‐Catalyzed C—H Functionalizations

AbstractReview: 50 refs.

First-principle study of silicon cluster doped with rhodium: Rh2Sin (n = 1–11) clusters

The geometries, stabilities and electronic properties of rhodium-doped silicon clusters Rh2Sin (n = 1–11) have been systematically studied by using density functional calculations at the B3LYP/GENECP level. The optimized results show that the lowest-energy isomers of Rh2Sin clusters favor three-dimensional structures for n = 2–11. Based on the averaged binding energy, fragmentation energy, second-order energy difference and HOMO-LUMO energy gap, the stabilities of Rh2Sin (n = 1–11) clusters have been analyzed. The calculated results suggest that the Rh2Si6 cluster has the strongest relative stability and the doping with rhodium atoms can reduce the chemical stabilities of Sin clusters. The natural population and natural electron configuration analysis indicate that there is charge transfer from the Si atoms and 5s orbital of the Rh atoms to the 4d and 5p orbitals of Rh atoms. The analysis of electron localization function reveal that the Si–Si bonds are mainly covalent bonds and the Si–Rh bonds are almost ionic bonds. Moreover, the vertical ionization potential, vertical electron affinity, chemical hardness, chemical potential, vibrational spectrum and polarizability are also discussed.

Chiral Cyclopentadienyls: Enabling Ligands for Asymmetric Rh(III)-Catalyzed C-H Functionalizations.

Transition-metal catalyzed C-H functionalizations became a complementary and efficient bond-forming strategy over the past decade. In this respect, Cp*Rh(III) complexes have emerged as powerful catalysts for a broad spectrum of reactions giving access to synthetically versatile building blocks. Despite their high potential, the corresponding catalytic enantioselective transformations largely lag behind. The targeted transformations require all the remaining three coordination sites of the central rhodium atom of the catalyst. In consequence, the chiral information on a competent catalyst can only by stored in the cyclopentadienyl unit. The lack of suitable enabling chiral cyclopentadienyl (Cp(x)) ligands is the key hurdle preventing the development of such asymmetric versions. In this respect, an efficient set of chiral Cp(x) ligands useable with a broad variety of different transition-metals can unlock substantial application potential. This Account provides a description of our developments of two complementary classes of C2-symmetric Cp(x) derivatives. We have introduced a side- and back-wall concept to enforce chirality transfer onto the central metal atom. The first generation consists of a fused cyclohexane unit having pseudo axial methyl groups as chiral selectors and a rigidifying acetal moiety. The second ligand generation derives from an atrop-chiral biaryl-backbone and which possesses adjustable substituents at its 3,3'-positions. Both ligand families can be modulated in their respective steric bulk to adjust for the specific needs of the targeted application. The cyclopentadienes can be metalated under standard conditions. The corresponding chiral rhodium(I) ethylene complexes are relatively air and moisture and represent storable stable precatalysts for the targeted asymmetric Rh(III)-catalyzed C-H functionalizations. These complexes are then conveniently oxidized in situ by dibenzoyl peroxide to give the reactive Cp(x)Rh(III)(OBz)2 species. For instance, this catalyst is used for directed C-H activations of aryl hydroxamates and the subsequent enantioselective trapping with olefins, providing dihydroisoquinolones in very high enantioselectivities. In addition, we have established highly selective intramolecular trapping reactions with tethered higher substituted alkenes giving dihydrobenzofurans with quaternary stereogenic centers. Concerning intermolecular reactions, allene coupling partners allow for an enantioselective hydroarylation yielding substituted allylated compounds. A trapping process of the cyclometalated intermediate with diazo reactants enables the enantioselective construction of isoindolinones. Moreover, the catalysts can be used for the construction of atropchiral biaryl motives using a dehydrogenative Heck-type reaction. The development of flexibly adjustable chiral Cp(x) ligands is described in this Account showcasing their applicability for a variety of Rh(III) catalyzed C-H functionalization reactions. These Cp(x) derivatives hold promise as powerful steering ligands for further transition-metals used in asymmetric catalysis.

LaRh3Ga2 – Structure Determination from a Trilling

AbstractThe gallide LaRh3Ga2 was obtained by arc‐melting of the elements and subsequent annealing of the product button in a high‐frequency furnace. The sample was studied by powder X‐ray diffraction and the structure was refined from single crystal data of a trilling: ErRh3Si2 type (CaCu5 superstructure), Imma, a = 760.1(2), b = 975.6(3), c = 564.0(2) pm, wR = 0.0358 for 717 F2 values and 23 variables; domain ratio 0.36(1) / 0.28(1) / 0.35(1). The rhodium atoms build up Kagomé networks (281.8 and 282.2 pm Rh–Rh), which are condensed through the gallium atoms (249.6–251.0 pm Rh–Ga). The lanthanum atoms fill large voids left by the three‐dimensional [Rh3Ga2] network. They have coordination number 18 (6 Ga + 12 Rh) in form of a hexa‐capped hexagonal prism. The crystal chemical relationship with the aristotype CaCu5 is briefly discussed on the basis of a group‐subgroup scheme.

Axial coordination dichotomy in dirhodium carbenoid catalysis: a curious case of cooperative asymmetric dual-catalytic approach toward amino esters.

One of the most recent developments in asymmetric catalysis is to employ two or more catalysts under one-pot reaction conditions. This article presents some interesting mechanistic insights on a cooperative dual-catalytic protocol relying on the catalytic ability of dirhodium carbenoid (derived from rhodium(II) tetracarboxylate and a diazo compound) and a chiral spirophosphoric acid ((R)-SPA) in an asymmetric N-H insertion reaction. We have employed DFT(M06 and B3LYP) computational methods to identify the stereocontrolling transition states wherein a chiral (R)-SPA protonates a dirhodium-bound enol intermediate. A true cooperative action elicited by both catalysts has been noted in the enantioselective protonation. More importantly, whether the second axial ligand on the remote rhodium atom could influence the energetic features of the reaction has been probed for the first time. In all steps (such as nitrogen extrusion, addition of amine to the dirhodium carbenoid, and the enol formation), except that in the stereocontrolling event, no major effect of axial ligation has been noticed. However, the presence of the axial ligand helps in stabilizing the protonation transition state and reduces the activation barrier for protonation, suggesting a vital role in stereoselectivity. The predicted sense of stereoselectivities is in good agreement with the experimental results.

Anchored Wilkinson Catalyst: Hydrogenation of β Pinene

The hydrogenation of β pinene over homogeneous and anchored Wilkinson’s catalysts was studied. Phosphotungstic acid was used to anchor the Wilkinson catalyst to an alumina support by an interaction between the heteropoly acid and the rhodium atom of the complex. The hydrogenation of β pinene over the anchored catalyst was accompanied by some isomerization to α-pinene which was subsequently hydrogenated along with the β pinene to the pinanes with selectivities to the cis (endo) pinane in the 85–88 % range at near 100 % conversion. With the homogeneous Wilkinson catalyst an 88 % selectivity to cis pinane was also observed but only at 82 % conversion. The use of a 1 % Rh/Al2O3 catalyst for this hydrogenation gave some unexpected results. There was a facile isomerization to α pinene which was not hydrogenated further. The reaction stopped at about 30 % conversion at which time the cis and trans pinanes were present in near equal amounts in about 16–18 % yields.

Atomically dispersed rhodium on a support: the influence of a metal precursor and a support.

The influence of the support type and the metal precursor on the dispersion of rhodium after calcination and reduction was determined. The combination of electron microscopy and X-ray absorption analysis allowed the quantification of the amount of atomically dispersed rhodium in the samples. Higher amounts of atomically dispersed rhodium atoms are obtained when metal impregnation is performed with a rhodium acetate precursor in comparison to a rhodium chloride precursor over supports of the same composition. The stability of rhodium is improved with the addition of promoters; the co-presence of samaria and ceria in the support and metal impregnation with a rhodium acetate precursor leads to the highest amount of atomically dispersed rhodium remaining after reductive treatment at 773 K.

A Base-Stabilized Silyliumylidene Cation as a Ligand for Rhodium and Tungsten Complexes

The reaction of the base-stabilized silyliumylidene triflate [LSi(DMAP)]OTf (1·OTf; L = PhC(NtBu)2, DMAP = 4-dimethylaminopyridine) with [Rh(cod)Cl]2 and W(CO)5THF afforded [{L(DMAP)Si}2{μ-Rh(μ-Cl)2Rh(cod)}](OTf)2 (2) and [L(DMAP)Si→W(CO)5]OTf (3), respectively. Their crystal structures determined by X-ray crystallography show that the silyliumylidene cation acts as a two-electron donor coordinating to the rhodium and tungsten atoms.