Surface supported metal cluster carbonyls. Chemisorption decomposition and reactivity of Rh4(CO)12 supported on silica and alumina

Chemisorption of Rh₄(CO)₁₂ on to a highly divided silica (Aerosil “0” from Degussa), Leads to the transformation: 3 Rh₄(CO)₁₂ → 2 Rh₆(CO)₁₆ + 4 CO. Such an easy rearrangement of the cluster cage implies mobility of zerovalent rhodium carbonyl fragments on the surface. Carbon monoxide is a very efficient inhibitor of this reaction, and Rh₄(CO)₁₂ is stable as such on silica under a CO atmosphere. Both Rh₄(CO)₁₂ and Rh₆(CO)₁₆ are easily decomposed to small metal particles of higher nuclearity under a water atmosphere and to rhodium(I) dicarbonyl species under oxygen. From the Rh^I(CO)₂ species it is possible to regenate first Rh₄(CO)₁₂ and then Rh₆(CO)₁₆ by treatment with CO (P_co ? 200 mm Hg) and H₂O (P_H2O ? 18 mm Hg). The reduction of Rh^I(CO)₂ surface species by water requires a nucleophilic attack to produce an hypothetical [Rh(CO)_n]_m species which can polymerize to small Rh₄ or Rh₆ clusters in the presence of CO but which in the absence of CO lead to metal particles of higher nuclearity. Similar results are obtained on alumina.     

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.     

Radiolytic synthesis of Rh2(CO)4Cl2 and Rh6(CO)16 from RhCl3 in ethanol under CO atmosphere

Ethanolic solutions of Rh^III chloride exposed to γ-radiation under CO atmosphere are shown to be totally reduced into Rh^I complexes (Rh₂(CO)₄Cl₂ and Rh(CO)₂Cl^-₂) within a few hours with a radiolytic reduction yield of about 6.0 elementary reductions/100 eV (6.2·10^-7 mol·J^-1). The chloride ions freed in the medium inhibit further reduction through Rh(CO)₂Cl^-₂ formation. On addition of copper metal under the same conditions, Rh^III is transformed into Rh₆(CO)₁₆ with a conversion yield 50%. This cluster is formed via Rh₂(CO)₄Cl₂ although Rh(CO)₂Cl^-₂ is also present under these conditions. Rh₆(CO)₁₆ cluster is also formed under radiolysis by direct reduction of Rh₂(CO)₄Cl₂, but metallic rhodium and other reduced products are obtained at the same time.     

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.     

High Nuclearity Bimetallic Rhodium–Palladium Carbonyl Cluster Complexes. Synthesis and Characterization of Rh6(CO)16[Pd(PBu t 3)]3 and Rh6(CO)16[Pd(PBu t 3)]4

The reaction of Rh₄(CO)₁₂ with Pd(PBu ^t ₃)₂ yielded the high nuclearity bimetallic hexarhodium-tripalladium cluster complex Rh₆(CO)₁₆[Pd(PBu ^t ₃)]₃, 10, in 11% yield. Compound 10 was converted to the hexarhodium-tetrapalladium cluster Rh₆(CO)₁₆[Pd(PBu ^t ₃)]₄, 11, in 62% yield by reaction with an additional quantity of Pd(PBu ^t ₃)₂. Both compounds were characterized crystallographically. Structurally, both compounds consist of an octahedral cluster of six rhodium atoms with sixteen carbonyl ligands analogous to that of the known compound Rh₆(CO)₁₆. Compound 10 also contains three Pd(PBu ^t ₃) groups that bridge three Rh–Rh bonds along edges of the Rh₆ octahedron to give an overall D₃ symmetry to the Rh₆Pd₃ cluster. Compound 11 contains four edge bridging Pd(PBu ^t ₃) groups distributed across the Rh₆ octahedron to give an overall D_2d symmetry to the Rh₆Pd₄ cluster. Each Rh–Pd connection in both compounds contains a bridging carbonyl ligand that helps to stabilize the bond between the Pd(PBu ^t ₃) groups and the Rh atoms. Both compounds can be regarded as Pd(PBu ^t ₃) adducts of Rh₆(CO)₁₆.     

High-yield syntheses of [Rh7(CO)16] and [Rh14(CO)25] working in ethylene glycol solution under 1 atm of CO

The anionic rhodium carbonyl clusters [Rh₇(CO)₁₆]³⁻ and [Rh₁₄(CO)₂₅]⁴⁻ can be easily prepared by a new simple and high yield one-pot synthesis starting from RhCl₃·nH₂O dissolved in ethylene glycol and involving two steps: (i) treatment of RhCl₃·nH₂O under 1 atm of CO at 50 °C to give [Rh(CO)₂Cl₂]⁻; (ii) addition of a base (CH₃CO₂Na or Na₂CO₃) followed by reductive carbonylation under 1 atm of CO at an adequate temperature (50 °C for [Rh₇(CO)₁₆]³⁻; 150 °C for [Rh₁₄(CO)₂₅]⁴⁻). These new syntheses are more convenient than those previously reported, especially since such clusters are not accessible via silica surface-mediated reactions. This different behavior is due to the particular stabilization on the silica surface and under 1 atm of CO of an anionic carbonyl cluster, called A, which does not allow the formation of a higher nuclearity carbonyl cluster, called B, which was shown to be the key-intermediate in the synthesis of [Rh₁₄(CO)₂₅]⁴⁻ working in ethylene glycol solution. Although it was not possible to isolate crystals of A and B suitable for X-ray structural determination, a combination of cyclovoltammetry, one of the few examples so far available of the use of this technique for anionic rhodium carbonyl clusters, infrared spectroscopy and elemental analyses suggest that A and B are probably the never reported [Rh₇(CO)₁₄]⁻ and [Rh₁₅(CO)₂₈]³⁻ clusters, respectively. In particular the tentative formulation of the two clusters was carried out by a non-conventional method based on the existence of a linear correlation between carbonyl frequencies of the main band and the [(charge/Rh atoms)/CO number] ratio.     

Oxidation of the [Rh(CO)2Cl2] anion in aqueous solution

The reaction of rhodium(I) carbonyl chloride, [Rh(CO)₂Cl]₂, with dichromate, cerium(IV) sulfate, hexachloroplatinic acid or p-benzoquinone in aqueous hydrochloric acid proceeds by consumption of 4 equivalents of oxidizing agent per mole or rhodium(I) in accordance with the equation Rh^I(CO)₂  4e + H₂O → Rh^III(CO) + 2H⁺ + CO₂A “cyclic” oxidation mechanism is suggested.     

Synthesis and molecular structure of [Rh3(μ-PPh2)3(CO)6(PPh2-H)], an unusual six-membered inorganic ring complex

Reaction of [{Rh(μ-Cl)(CO)₂}₂] with PPh₂H in CO-saturated ethanol yields [Rh₃(μ-PPh₂)₃ (CO)₆ (PPh₂H)], a red trinuclear cluster of rhodium containing a near-planar six-membered Rh₃P₃ ring; this compound reversibly undergoes elimination of CO and PPh₂H to afford [Rh₃(μ-PPh₂)₃(CO)₅].     

Reactions of metal carbonyl cluster complexes with multidentate phosphine ligands; reactions with bis(diphenylphosphino)methane

Reactions of Rh₆(CO)₁₆ with bis(diphenylphosphino)methane (dppm) gave Rh₆(CO)₁₄(dppm), Rh₆(CO)₁₂(dppm)₂, or Rh₆(CO)₁₀(dppm)₃, depending upon the reaction conditions. Rh₄(CO)₁₀(dppm) may be obtained from the reaction of Rh₄(CO)₁₂ with dppm, but this derivative rapidly decomposes in solution to give Rh₄(CO)₈(dppm)₂, Rh₆(CO)₁₄(dppm), and Rh₆(CO)₁₂(dppm)₂. Ir₄(CO)₁₀(dppm) and Ir₄(CO)₈(dppm)₂ have also been prepared, and their structures are discussed on the basis of infrared and ³¹P NMR spectroscopic data.     

Catalytic hydrogenation of 1,3-trans-pentadiene over Rh4(CO)12 supported on γ-Al2O3

Rh₄(CO)₁₂ anchored on γ-Al₂O₃ (Rh₄(CO)₁₂/Al₂O₃) has been studied as a catalyst for the hydrogenation of 1,3-trans-pentadiene. Under mild conditions (1 atm H₂ and temperatures between 60°C and 80°C) hydrogenation occurs at only one of the double bonds of the diene, and analysis of the products shows that the terminal double bond is preferentially hydrogenated. Hydrogenation of the second double bond of the conjugated diene occurring only after all the 1,3-trans-pentadiene has been consumed. In this respect Rh₄(CO)₁₂/Al₂O₃ behaves like toluene solutions of Rh₄(CO)₁₂. Anchoring of Rh₄(CO)₁₂ on the solid support gives a catalyst which is less active but more stable than toluene solutions of Rh₄(CO)₁₂. The effects of CO and of triphenylphosphine on catalytic activity and on specificity of Rh₄(CO)₁₂/Al₂O₃ have also been investigated and both shown to cause a reduction of the rate of hydrogenation of 1,3-trans-pentadiene.     

A synthetic route to heteronuclear clusters containing iridium and rhodium: X-ray crystal structures of [IrOs3(μ-H)2(μ-Cl)(CO)12] and [Ir2Rh2(μ-CO)(μ3-CO)2(CO)4(η-C5Me5)2]

The iridium and rhodium complexes [MCl(CO)₂(NH₂C₆H₄Me-4)] (M = Ir or Rh) react with [Os₃(μ-H)₂(CO)₁₀] to give the tetranuclear clusters [MOs₃(μ-H)₂(μ-Cl)(CO)₁₂]; the iridium compound being structurally identified by X-ray diffraction. Similarly, [IrCl(CO)₂(NH₂C₆H₄Me-4)] and [Rh₂(μ-CO)₂(η-C₅Me₅)₂] afford the tetranuclear cluster [Ir₂Rh₂(μ-CO)(μ₃-CO)₂(CO)₄(η-C₅Me₅)₂], also characterised by single-crystal X-ray crystallog     

Rhodium carbonyl cluster chemistry. Synthesis and chemical characterization of the anion [Rh6(CO)14]4−

The anion [Rh₆(CO)₁₄]^4? has been isolated from the reaction of [Rh₆(CO)₁₆] with alkali hydroxides in aqueous solution. It shows high reactivity towards electrophiles and in redox condensations with other rhodium clusters; and is rapidly decomposed by carbon monoxide.     

Magnetic properties and EPR spectroscopy of molecular metal clusters

The magnetic properties of molecular metal cluster compounds resemble those of small metal particles in the metametallic size regime. Even-electron metal carbonyl clusters with 10 or more metal atoms are paramagnetic, because their frontier orbital separations of less than 1 eV lead to high-spin electronic configurations. The rhodium cluster [Rh₁₇S₂(CO)₃₂]^3? gives EPR below 200 K withg=2.04, the first example of this type of paramagnetism in an even-electron carbonyl cluster of this 4d metal. Its spectral parameters are compared with those of osmium carbonyl clusters and some significant differences highlighted. Attempts have also been made to generate radical cations from lower-nuclearity, diamagnetic molecular clusters such as Rh₆(CO)₁₆ by chemical oxidation in sulphuric acid. An EPR active species (g=2.09) believed to be [Rh₆(CO)₁₆]⁺ has been obtained.     

[Fec3(μ3-CR)(CO)10]− Cluster anions as building blocks for the synthesis of mixed-metal clusters: II. Synthesis of HFe3Rh(μ4-η2-CCHR)(CO)11 clusters (R = H or C6H5) and study of their catalytic activity (R = C6H5) under hydroformylation and hydrogenation conditions

The mixed-metal vinylidene clusters HFe₃Rh(CO)₁₁(CCHR) (R = H, C₆H₅) have been synthesized via the reaction of [HFe₃(CO)₃CCHR][P(C₆H₅)₄] with [RhCl(CO)₂]₂ in the presence of a thallium salt. The reaction initially gives the [Fe₃Rh(CO)₁₁]CCHR][P(C₆H₅)₄] cluster which leads to the final products by protonation. Spectroscopic data indicate a μ₄-η² mode of bonding for the vinylidene ligand. A structure with a Fe₃Rh core in a butterfly configuration and in which the rhodium atom occupy a wing-tip site is proposed. The catalytic activity of HFe₃Rh(CO)₁₁(CCH(C₆H₅)) (80% yield) has been checked in hydroformylation and hydrogenation. In hydroformylation the cluster shows the same activity as Rh₄(CO)₁₂, whereas in hydrogenation the mixed-metal system shows specific activity; isomerization of 1-heptene to cis and trans 2-heptene takes place with no more than 14% heptane formation. The cluster is broken down during the catalysis, and some H₃Fe₃CO)₉(μ₃-CCH₂(C₆H₅)) is formed. The latter cluster is not an active catalyst, and under the same conditions use of Rh₄(CO)₁₂ results mainly in hydrogenation of 1-heptene. These observations suggest that the active species is a mixed iron-rhodium system.     

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.     

Structural studies of Rh4(CO)12 derivatives in solution and in the solid state

The crystal structures of Rh₄(CO)₁₀(PPh₃)₂ and Rh₄(CO)₉P(OPh)₃₃ are reported. ³¹P-¹H NMR studies on Rh₄(CO)₁₂-_x {P(OPh)₃}_x(X  1, 2 and 3) show that each derivative exists as only one isomer in solution whereas the analogous triphenylphosphine derivatives can exist as different isomers. A quantitative redistribution of triphenylphosphites occurs on mixing Rh_4-(CO)₁₂-_xL_x with Rh₄(CO)₁₂-_yL_y (L  P(OPh)₃; x  0, 1, 2, y  x + 2; x  0, y  x + 4) to give Rh₄(CO)₁₂-_zL_z[z $\text{1}\text{2}$(x + y)]; a related rapid intermolecular randomisation of carbonyls occurs on mixing Rh₄(¹²CO)₁₂ with Rh₄(¹³CO)₁₂.     

Reactions of rhodium carbonyl clusters with heterobidentate ligands. Synthesis and structural characterization of the Rh6(CO)15[(C6H5)2PC6H4N(CH3)2] and {Rh6(CO)15[(C6H5)2PC6H4NH(CH3)2]}[GaX4] cluster compounds

The reaction of heterobidentate 4-(dimethylamino)phenyldiphenylphosphine with labile cluster Rh₆(CO)₁₅(NCMe) yielding a new monosubstituted cluster Rh₆(CO)₁₅(Ph₂PC₆H₄NMe₂) was described. The reactions of the resulting cluster with GaX₃ compounds were studied. The structure of all obtained compounds was completely characterized both in a solid phase and in a solution by X-ray diffraction analysis, mass spectrometry, and IR and polynuclear NMR spectroscopy.     

Structural characterization of the anions [Rh6(CO)15X]− [X = COEt and CO(OMe)]

The anions [Rh₆(CO)₁₅X]^?, with X = COEt and CO(OMe), have been studied by single-crystal X-ray diffraction. They contain octahedral rhodium clusters, with mean metalmetal distances of 2.779 and 2.765 », respectively. The carbonyl stereochemistry in the two anions is similar to that of Rh₆(CO)₁₆, with one terminal CO group replaced by the X ligand. The RhC(carbomethoxy) bond distance (1.96(2) ») is significantly shorter than the RhC(acyl) distance (2.06(2) »).     

Organometallic chemistry and homogeneous catalysis of Rh and Rh-Co mixed metal carbonyl clusters

The reactions of hydrosilane and/or alkyne as well as isonitriles with rhodium and rhodium cobalt mixed metal carbonyl clusters, e.g., Rh₄(CO)₁₂ and Co₂Rh₂(CO)₁₂, are studied. Novel mixed metal complexes, e.g., CoRh(CO)₅ (HCCBu ⁿ ), (R₃Si)₂Rh(CO) _n Co(CO)₄, Rh(R–NC)₄Co(CO)₄, Co₂Rh₂(CO)₁₀(HCCR), and Co₂Rh₂(CO)₉(HCCBu ⁿ ), are synthesized and identified. The catalytic activities of these rhodium and rhodium-cobalt mixed metal complexes are examined in hydrosilyation, silylformylation, and novel silylcarbocyclization reactions. Possible mechanisms for these reactions are proposed and discussed.     

Hydroformylation of olefins catalysed by rhodium and cobalt clusters supported on organic (Dowex) resins

Co₂Rh₂(CO)₁₂ or a mixture of Co₄(CO)₁₂ and Rh₄(CO)₁₂ were impregnated onto organic Dowex® resins to study the hydroformylation of olefins.Pressure, reaction time and amount of catalyst effect the alcohol production and the ratio of straight chain to branched alcohols in the Co₂Rh₂(CO)₁₂—Dowex amine resin catalytic system. 50 bar of synthesis gas pressure, 17 h total reaction time and 0.050 g catalyst were the optimum conditions in the hydroformylation of 1-hexene at 100 °C, where the maximum yield of alcohols is 95%. Other hexene isomers and different olefins were studied as hydroformylation substrates.The presence of amine groups on the support or the addition of Et₃N to the hydroformylation solution is necessary for the single-step alcohol formation. This conclusion was deduced from studies of the supports with a similar matrix but different functional groups.The mixture of Co₄(CO)₁₂ and Rh₄(CO)₁₂ supported on organic amine Dowex resin is an excellent catalyst, giving 99% alcohol production. The ratio of Rh/Co has a marked effect on the product distribution. The best alcohol conversions are reached with 2.6 or 3.6 of Rh/Co weight ratio.Co₂Rh₂(CO)₁₂ decomposes to give a known, but not optimum, composition of Co and Rh on the support. Therefore a mixture of Co₄(CO)₁₂ and Rh₄(CO)₁₂ is a better catalyst precursor than Co₂Rh₂(CO)₁₂.