Homoleptic dirhodium tetraoctanoate and its pyridine adduct: synthesis and crystal structures

Dirhodium tetraoctanoate (1), without any exogenous axial ligands, has been obtained in crystalline form by slow evaporation from ethanol. The molecules are linked by axially ligating each other to form infinite chains, which is similar to the only other two structurally characterized rhodium(II) carboxylates, Rh₂(O₂CCF₃)₄ and Rh₂(O₂CC₃H₇)₄, which also lack exogenous axial ligands. The infinite chains of 1 are cleaved by strong donor solvents such as pyridine to produce the corresponding pyridine adduct (2). Both the complexes were characterized by elemental analysis, MS, IR and NMR spectra.     

New rhodium(II)-rhodium(III) dinuclear complexes with 2-aminopyridine bridges

Summary The syntheses of the complexes [Rh₂(ap)₄X] (ap = the heterocyclic anion of 2-aminopyridine; X = Cl or Br) are described. The complexes have been characterized on the basis of elemental analysis, i.r., e.s.r. and electronic absorption spectra, and magnetic susceptibility measurements. The 2-aminopyridine anion behaves as bridging ligand, coordinatingvia the pyridine and amine nitrogen atoms in a way analogous to that in the dinuclear rhodium(II) carboxylates.     

The thermal behaviour of some rhodium complexes

The thermal behaviour of several complexes of rhodium has been investigated. Complexes containing nitrogen ligands readily decompose to the oxide, Rh₂O_3^? Complexes with phosphorus and arsenic ligands decompose to the same oxide of rhodium, although difficulty is encountered in removing all the phosphorus and arsenic. The suitability of the decomposition to Rh₂O₃ as an analytical technique for rhodium is discussed.     

The chemistry of hetero-allene and -allylic derivatives with rhodium : I. The reactions of rhodium(I)-hetero-allylic compounds with hetero-allenes; B. Carbon Disulfide

The rhodium(I) complexes Rh[X-C(Z)-Y] (PPh₃)₂, in which [X-C(Z)-Y] represents an uninegative unsaturated heteroallylic bidentate ligand, coordinating via two of the three hetero atoms (X, Y, Z  P, S or N), react at elevated temperature with an excess of the hetero-allene SCS to give the rhodium(I)-thiocarbonyl complexes Rh[X-C(Z)-Z](CS)(PPh₃). In the initial step a first CS₂ molecule is coordinated side-on by one of the CS double bands. Subsequent reactions can be blocked at this stage by addition of pyridine, resulting in RhCl(η²-CS₂)(PPh₃)(py)₂. The formation of the CS complexes occurs in two ways. Either by direct sulfur abstraction from the Rh^I(η²-CS₂) complex by PPh₃ or by a dimerisation of two CS₂ molecules and elimination of a CS moiety, resulting in a Rh^III-thiocarbonyl-trithiocarbonato complex, immediately followed by demolition of the trithiocarbonato-CS^?2₃ fragment, by PPh₃ to SPPh₃ and CS₂.Complexes containing a CS^?2₃ fragment, but no CS moiety, can also be identified by IR measurements. These products may be formed in a sidereaction upon elimination of CS.     

Dirhodium Complexes as Panchromatic Sensitizers,Electrocatalysts, and Photocatalysts

Dinuclear rhodium complexes are attractive candidates as homogeneous panchromatic photosensitizers and photocatalysts. Modification of the coordination sphere of the Rh₂(II,II) compounds results in photophysical and redox properties that are highly desirable for electro- and photocatalysis. Specifically, Rh₂(II,II) complexes have shown promising catalytic activity towards proton reduction to generate H₂, a clean fuel, and for the selective reduction of CO₂ to HCOOH. In addition, paddlewheel Rh₂(II,II) complexes provide robust platforms for the design of efficient and stable single-component photocatalysts. Optimization of the Rh₂(II,II) catalysts is crucial to realize their future application in devices or systems designed for the production of fuels from sunlight.     

Oxygenation of aromatic hydrocarbons with hydrogen peroxide catalyzed by rhodium carbonyl complexes

Hexanuclear rhodium carbonyl cluster, Rh₆(CO)₁₆, catalyzes benzene hydroxylation with hydrogen peroxide in acetonitrile solution. Phenol and (at lower concentration) quinone are formed with the maximum attained total yield and turnover number 17% and 683, respectively. Certain other rhodium carbonyl complexes, containing cyclopentadienyl ligands, Rh₂Cp₂(CO)₃ and Rh₃(CpMe)₃(CO)₃, are less efficient catalysts. Cyclopentadienyl derivatives of rhodium which do not contain the carbonyl ligands, Rh(CpMe₅)(CH₂?CH₂)₂, RhCp(cyclooctatetraene) and Rh₂Cp₂(cyclooctatetraene) turned out to be absolutely inactive in the benzene hydroxylation. Styrene is transformed into benzaldehyde and (at lower concentration) acetophenone and 1‐phenylethanol. Copyright © 2008 John Wiley & Sons, Ltd.     

Preparation of polymer-anchored dinuclear rhodium(I) catalysts attached by the bridging groups

We describe a method to anchor dinuclear rhodium(I) complexes by substitution of the chlorine bridge in Rh₂(μ-Cl)₂L₄ with LiSR where R represents a polymeric chain. The catalytic activity of such complexes compares well with that observed in homogeneous phase.     

Halide substitutions in the [Rh2(O f2 p− )(OH)2(H2O) n ]3+ complex

Summary Transition metal complexes containing activated dioxygen continue to attract considerable attention and are widely used as models, either as oxygen carriers or as catalysts in biological and industrial oxidations ^(1,2).Cobalt complexes containing the coordinated O₂ molecule are well known^(3,4), but the rhodium analogues are rare^(5–8). The known complexes with coordinated O₂ are rhodium(III) compounds containing strong nitrogen donor ligands.Our recent studies have focused on a synthesis of dioxygen rhodium derivatives in which a superoxide anion (O _f2 ^p– ) is bound to the [Rh^III—Rh^III] core surrounded by other donor molecules, e.g. H₂O, RCO _f2 ^p– (7,8), to investigate the effect of a coordination sphere of the metal centre on the redox properties of the O _f2 ^p– radical combined to it⁽⁹⁾.The [Rh₂(O _f2 ^p– )(OH)₂(H₂O) _n ]³⁺ cation, the product of oxidative addition of dioxygen to the [Rh₂(H₂O)₁₀]⁴⁺ dimer, was isolated by us and examined in aqueous HC1O₄ ^(7,9). So far we have been unable to isolate its solid but the water molecules coordinated to the [Rh₂(O _f2 ^p– )-(OH)₂] ³⁺ moiety were found to undergo substitution relatively easily⁽⁸⁾.     

Synthesis, spectral and structural characterization of dinuclear rhodium (II) complexes of the anticonvulsant drug valproate with theophylline and caffeine

The dinuclear complex tetra(μ-valproato) dirhodium(II), Rh₂(valp)₄ (1), and its bis-adducts with theophylline, Rh₂(valp)₄(ThH)₂ (4), or caffeine, Rh₂(valp)₄(Caf)₂ (5), have been synthesized and characterized by elemental analysis, IR, UV-Vis, magnetic moment, ¹H and ¹³C NMR spectroscopic techniques. Spectral data for the complexes are consistent with a dinuclear structure as found for rhodium (II) tetracarboxylate adducts. Theophylline and caffeine bases in complexes 4 and 5, respectively, are axially coordinated to rhodium (II) atoms through the sterically hindered N(9) site. This is confirmed by X-ray crystal structure analyses of complexes 4 and 5.     

Binuclear and polynuclear rhodium complexes containing chiral dithiolate ligands derived from lactic acid

Two precursors of the chiral dithiolato ligands, di‐[(2R)‐acetylmercaptopropyl] phthalate and isophthalate, 1 and 2 respectively, were synthesized from (S)‐lactic acid. Reactions of 1 and 2 with [Rh₂(μ)‐OMe)₂(cod)₂] (cod = 1,5‐cyclo‐octtadiene) yielded rhodium thiolato complexes of different nuclearities. The mixtures of complexes were analyzed by gel‐permeation chromatography (GPC). The reaction with ligand 1 produced a mixture of oligomeric complexes, where the binuclear species was the main component. Higher‐nuclearity complexes were the main products of the reaction with ligand 2. The rhodium complexes, in the presence of PPh₃, were tested as catalyst precursors for the asymmetric hydroformylation of styrene. Moderate activity and regioselectivity were achieved in most cases, but no enantioselective discrimination was observed. Copyright © 2000 John Wiley & Sons, Ltd.     

Mechanism of the hydroxycarbonylation of 1-hexene catalyzed by immobilized rhodium complexes of pyridine ligands

Summary In this work, a mechanistic study of the hydroxycarbonylation of 1-hexene to heptanoic acid and the water gas shift reaction (WGSR) catalyzed by the rhodium(I) complexes, [Rh(COD)(amine)₂](PF₆) (COD = 1,5-cyclooctadiene, amine = 4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with water under CO is discussed. Catalytic cycles for these reactions bearing common Rh-H catalytic species are proposed.     

A new rhodium complex with carbon dioxide

The complex (PH₃P)₃Rh₂(CO)₂(CO₂)₂·C₆H₆ was prepared by action of carbon dioxide on complexes of zerovalent rhodium.     

State of rhodium(III) in sulfuric acid solutions

The formation of rhodium(III) sulfate complexes under moderately rigorous temperature conditions was studied by ¹⁰³Rh and ¹⁷O NMR spectroscopy. The complexes [Rh₂(μ-SO₄)₂(H₂O)₈]²⁺, [Rh₂(^*μ-SO₄)(H₂O)₈]⁴⁺, and [Rh₃(μ-SO₄)₃(μ-OH)(H₂O)₁₀]²⁺ were found to be the most stable species in aged solutions.     

Characterization of Au-Rh/TiO2 catalysts by CO adsorption; XPS, FTIR and TPD experiments

On X-ray photoelectron spectra of the Au-Rh/TiO₂ catalysts the position of Au4f peak was practically unaffected by the presence of rhodium, the peak position of Rh3d, however, shifted to lower binding energy with the increase of gold content of the catalysts. Rh enrichment in the outer layers of the bimetallic crystallites was experienced. The bands due to Au⁰-CO, Rh⁰-CO and (Rh⁰)₂-CO were observed on the IR spectra of bimetallic samples, no signs for Rh⁺-(CO)₂ were detected on these catalysts. The results were interpreted by electron donation from titania through gold to rhodium and by the higher particle size of bimetallic crystallites.     

Electronic structure and spectra of rhodium(II) tetracarboxylate complexes

The DFT B3LYP geometry optimization was carried out and the IR spectra were calculated for rhodium(II) tetracarboxylate complexes Rh₂(O₂CR)₄ (R = H, CH₃, CF₃, C₆H₅) and for the compound Rh₂(O₂CH)₄(H₂O)₂ with two axially coordinated water molecules. A minor influence of the substituent R on the electronic structure and geometric and spectral characteristics of the cage was noted. From the calculation results, it was concluded that the Rh(II)-Rh(II) stretching vibrations should be attributed to about 300 cm^?1. The results obtained for rhodium(II) dimers were compared with analogous data for Mo₂(O₂CH)₄. Analysis of the electronic structure including consideration of the natural bond orbitals indicates the presence of a strong Rh(II)-Rh(II) single bond and a quadruple Mo(IV)-Mo(IV) bond. The electronic spectra of Rh₂(O₂CR)₄ (R = H, C₆H₅) and Rh₂(O₂CH)₄(H₂O)₂ were simulated by the TDDFT technique.     

Study of the state of rhodium in the potassium rhodiumundecatungstosilicate/alumina system by diffuse-reflectance IR spectroscopy

Adsorption of CO on the catalytic system K₅[SiW₁₁RhO₃₉]/Al₂O₃ was studied by diffuse-reflectance IR spectroscopy. The electronic state of rhodium and thermal stability of the system in the redox cycles were studied. The oxidized heteropolycompound (HPC) contains the charged forms Rh⁺, Rh²⁺, and Rh³⁺. Their ratio strongly depends on the pre-treatment temperature of the sample. In the reduced HPC two main forms of rhodium exist: the ionic Rh⁺ and metallic Rh⁰. The supported HPC structure is stable in an oxidative medium at temperatures below ∼900 K. The properties of the K₅[SiW₁₁RhO₃₉]/Al₂O₃ system are retained in the redox cycles up to ∼670 K, and the highly dispersed state of rhodium is observed up to 900 K.     

Heteropolyacids enhanced the catalytic activity of Rh6(CO)16 in the hydroformylation of alkenes

Active homogeneous catalytic systems based on Rh₆(CO)₁₆‐heteropolyacids for the regioselective hydroformylation of styrene and 1‐octene and their derivatives have been developed. The effects of the amount and the type of the heteropolyacid have been studied and showed a significant improvement of the conversion of styrene and the selectivity towards branched aldehydes. Other rhodium complexes were also considered in the study and the results showed the advantages of the rhodium cluster Rh₆(CO)₁₆ associated with the heteropolyacid H₃PW₁₂O₄₀,25H₂O (HPA‐W₁₂). The effects of temperature, type of solvent and CO/H₂ pressure have also been considered in order to optimize the reaction conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Catalytic asymmetric hydrogenation with the cluster Rh6(CO)10[(-)DIOP]3

Rh₆(CO)₁₀[(-)DIOP]₃ has been isolated after ligand exchange between Rh₆(CO)₁₆ and DIOP. The molecular cluster is an efficient catalyst for asymmetric reduction of various prochiral olefins; optical yields up to 47% have been achieved; the results are compared with those obtained with mononuclear rhodium (I) complexes.     

Diminished electron density in the Vaska‐type rhodium(I) complex trans‐[Rh(NCBH3)(CO)(PPh3)2]

Vaska‐type complexes, i.e. trans‐[RhX(CO)(PPh₃)₂] (X is a halogen or pseudohalogen), undergo a range of reactions and exhibit considerable catalytic activity. The electron density on the Rh^I atom in these complexes plays an important role in their reactivity. Many cyanotrihydridoborate (BH₃CN⁻) complexes of Group 6–8 transition metals have been synthesized and structurally characterized, an exception being the rhodium(I) complex. Carbonyl(cyanotrihydridoborato‐κN)bis(triphenylphosphine‐κP)rhodium(I), [Rh(NCBH₃)(CO)(C₁₈H₁₅P)₂], was prepared by the metathesis reaction of sodium cyanotrihydridoborate with trans‐[RhCl(CO)(PPh₃)₂], and was characterized by single‐crystal X‐ray diffraction analysis and IR, ¹H, ¹³C and ¹¹B NMR spectroscopy. The X‐ray diffraction data indicate that the cyanotrihydridoborate ligand coordinates to the Rh^I atom through the N atom in a trans position with respect to the carbonyl ligand; this was also confirmed by the IR and NMR data. The carbonyl stretching frequency ν(CO) and the carbonyl carbon ¹J_C–Rh and ¹J_C–P coupling constants of the C_ipso atoms of the triphenylphosphine groups reflect the diminished electron density on the central Rh^I atom compared to the parent trans‐[RhCl(CO)(PPh₃)₂] complex.     

Binuclear rhodium(II) complexes with leucine and proline

The dimeric rhodium(II) complexes [Rh₂(leu)₄(H₂O)₂]- (ClO₄)₄ and [Rh₂(pro)₄(H₂O)₂](ClO₄)₄ have been prepared and characterized by elemental analyses, i.r., u.v.–vis. and ¹H-n.m.r. spectroscopy. The amino acid molecules are coordinated as bridging ligands via their carboxylato groups. Cyclic voltammetry in DMF has shown that the complexes undergo a quasi-reversible reduction to yield dimers containing a Rh ₂ ³⁺ core. Oxidation processes within the 0–1.5V range were not observed.