Rhodium Carbonyl Clusters Research Articles

Alternative synthetic route for the heterometallic CO-releasing [Sb@Rh12(CO)27]3− icosahedral carbonyl cluster and synthesis of its new unsaturated [Sb@Rh12(CO)24]4− and dimeric [{Sb@Rh12Sb(CO)25}2Rh(CO)2PPh3]7− derivatives

The hetero-metallic [Sb@Rh12(CO)27]3− cluster has been known as for over three decades thanks to Vidal and co-workers, and represents the first example of an E-centered (E=heteroatom) icosahedral rhodium carbonyl cluster. However, its synthesis required high temperature (140–160°C) and elevated CO pressure (400atm). Applying the redox condensation method for cluster preparation, we herein report a new synthetic, high-yield route for preparing [Sb@Rh12(CO)27]3− under much milder conditions of temperature and pressure. Notably, when the same synthesis was carried out under N2 instead of CO atmosphere, the new isostructural but unsaturated derivative [Sb@Rh12(CO)24]4− was obtained, for which we report the full X-ray structural characterization. This species represents one of the few examples of an icosahedral cluster disobeying the electron-counting Wade-Mingos rules, possessing less than the expected 170 cluster valence electrons (CVEs). Judging from IR monitoring, the two species can be obtained one from the other by switching between N2 and CO atmosphere, making [Sb@Rh12(CO)27]3− a spontaneous CO-releasing molecule. Finally, the study of the chemical reactivity of [Sb@Rh12(CO)27]3− with PPh3 allowed us to obtain the new [{Sb@Rh12Sb(CO)25}2Rh(CO)2PPh3]7− dimeric compound, for which we herein report the full X-ray structural and 31P NMR analyses.

Performance of novel bimetallic carbonyl clusters as PEM fuel cell anodes, a comparative study

This work describes the performance of novel bimetallic catalysts, prepared from ruthenium, rhodium and iridium carbonyl clusters by a thermolysis procedure in o-dichlorobenzene. The electrochemical characterization by the rotating disk electrode technique in 0.5 mol L−1 H2SO4 showed that the RuxIry(CO)n, RuxRhy(CO)n and RhxIry(CO)n clusters are able to perform both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR), even in the presence of fuel cell contaminants such as CO and methanol, respectively. These promising results led us to evaluate the new materials as electrodes in a single fuel cell, using a Fuel Cell Test System designed and built in our group. The performance results of the three bimetallic clusters as anodes in a hydrogen PEM fuel cell are presented in this work. In the tests different H2 and O2 gas flows were fed into the cell to determine the most adequate ratio for maximum power. In the absence of CO, the results showed that although the three bimetallic materials had a similar performance to that of platinum with low flows of both reactants, RuxIry(CO)n showed the best electrocatalytic parameters. When the hydrogen fuel feed was contaminated with 100 ppm and 0.5% CO, the commercial platinum activity decreased considerably or was completely lost. However, while the current density of the novel materials also decreased in the presence of CO, it was significantly above that of platinum nanoparticles, the RhxIry(CO)n and RuxIry(CO)n catalysts showing the best performance in the presence of 100 ppm CO and 0.5% CO respectively. These results are promising in the context of PEM fuel cells using reforming hydrogen.

ChemInform Abstract: Rhodium-Catalyzed Carbonylation of Acetylenes Under Water-Gas Shift Reaction Conditions. Selective Synthesis of Furan-2(5H)-ones.

Rhodium-catalyzed carbonylation of acetylenes was studied. Under water-gas shift reaction conditions, internal acetylenes are selectively carbonylated to 3,4-disubstituted furan-2(5H)-ones (2). Use of D 2 O gave 5,5-dideuteriofuran-2(5H)-one (3), which indicates that the hydrogen comes from water. Use of molecular hydrogen in place of water gave hydroxymethylated product 4 and stilbene 5, and no furanone was obtained, indicating that water-gas shift reaction conditions are indespensable for the formation of furanones. As catalysts, rhodium carbonyl clusters such as Rh 4 (CO) 12 and Rh 6 (CO) 16 are the best among the transition-metal complexes tested. The presence of amines is essential for the selective synthesis of furanones. Effects of additives, solvents, and the pressure of carbon monoxide were examined

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.

Highly selective silylformylation of internal and functionalised alkynes with a cationic dirhodium(II) complex catalyst

The tetracationic complex [Rh 2(MeCN) 2(Naft) 4](BF 4) 4 (Naft = μ-1,8-naphthyridine) was found to be an efficient catalyst for the silylformylation of internal and functionalised alkynes to yield useful synthetic intermediates. The complex exhibits an unprecedented chemoselectivity towards alkyne silylformylation instead of simple hydrosilylation, as well as a good stereoselectivity. The catalytic efficiency of the complex is markedly superior compared to that of previously reported catalysts such as [ Rh + C 7 H 8 BPh 4 - ] or Rh 4(CO) 12; incidentally, the performance of the latter catalyst was found to vary dramatically with its shelf-life, which indicates that the catalyst evolves with ageing towards other species, most notably higher nuclearity rhodium carbonyl clusters, which are more chemoselective towards silylformylation. Preliminary results on the determination of the catalytically active species in the case of complex [Rh 2(MeCN) 2(Naft) 4](BF 4) 4 indicate that the complex is reduced in situ to a dirhodium(I) species which maintains the dimeric, lantern-shaped structure.

High-yield syntheses of [Rh 7(CO) 16] 3− and [Rh 14(CO) 25] 4− working in ethylene glycol solution under 1 atm of CO

The anionic rhodium carbonyl clusters [Rh 7(CO) 16] 3− and [Rh 14(CO) 25] 4− can be easily prepared by a new simple and high yield one-pot synthesis starting from RhCl 3· nH 2O dissolved in ethylene glycol and involving two steps: (i) treatment of RhCl 3· nH 2O under 1 atm of CO at 50 °C to give [Rh(CO) 2Cl 2] −; (ii) addition of a base (CH 3CO 2Na or Na 2CO 3) followed by reductive carbonylation under 1 atm of CO at an adequate temperature (50 °C for [Rh 7(CO) 16] 3−; 150 °C for [Rh 14(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 [Rh 14(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 [Rh 7(CO) 14] − and [Rh 15(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.

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

AbstractHexanuclear 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.

The Effect of Charge on CO Binding in Rhodium Carbonyls: From Bridging to Terminal CO

The structures of cationic rhodium carbonyl cluster compounds containing one to six Rh atoms are established by infrared multiple photon dissociation spectroscopy. Comparison with their well-known neutral analogues reveals that ionization of the neutral compounds destabilizes bridge bound CO ligands in Rh2(CO)8 and Rh4(CO)12, leading to cationic complexes with only terminally bound CO. The destabilization is associated with removal of charge from a highest occupied molecular orbital that is bonding with respect to bridge-bound CO. Density functional theory calculations support this conclusion. The results provide a possible insight into electronic promoter effects in catalysis.

Synthesis and Electrochemistry of New Rh-Centered and Conjuncto Rhodium Carbonyl Clusters. X-ray Structure of [NEt4]3[Rh15(CO)27], [NEt4]3[Rh15(CO)25(MeCN)2]·2MeCN, and [NEt4]3[Rh17(CO)37

A reinvestigation of the redox chemistry of [Rh7(CO)16]3- resulted in the finding of new alternative syntheses for a series of previously reported Rh-centered carbonyl clusters, i.e., [H4-nRh14(CO)25]n- (n = 3 and 4) and [Rh17(CO)30]3-, as well as new species such as a different isomer of [Rh15(CO)27]3-, the carbonyl-substituted [Rh15(CO)25(MeCN)2]3-, and the conjuncto [Rh17(CO)37]3- clusters. All of the above clusters are suggested to derive from oxidation of [Rh7(CO)16]3- with H+, arising from dissociation either of [M(H2O)n]2+ aquo complexes or nonoxidizing acids. The nature of the previously reported species has been confirmed by IR, electrospray ionization mass spectrometry, and complete X-ray diffraction studies. Only the molecular structures of the new clusters are reported in some details. The ready conversion of [Rh7(CO)16]3- in [HRh14(CO)25]3- upon oxidation has been confirmed by electrochemical techniques. In addition, electrochemical studies point out that the close-packed [H3Rh13(CO)24]2- dianion undergoes a reversible monoelectronic reduction followed by an irreversible reduction. The irreversibility of the second reduction is probably a consequence of H2 elimination from a purported [H3Rh13(CO)24]4- species. Conversely, the body-centered-cubic [HRh14(CO)25]3- and [Rh15(CO)27]3- trianions display several well-defined redox changes with features of electrochemical reversibility, even at low scan rate. The major conclusion of this work is that mild experimental conditions and a tailored oxidizing reagent may enable more selective conversion of [Rh7(CO)16]3- into a higher-nuclearity rhodium carbonyl cluster. It is also shown that isonuclear Rh clusters may display isomeric metal frameworks [i.e., [Rh15(CO)27]3-], as well as almost identical metal frames stabilized by a different number of carbonyl groups [i.e., [Rh15(CO)27]3- and [Rh15(CO)30]3-]. Other isonuclear Rh clusters stabilized by a different number of CO ligands more expectedly exhibit completely different metal geometries [i.e., [Rh17(CO)30]3- and [Rh17(CO)37]3-]. The first pair of isonuclear and isoskeletal clusters is particularly astonishing in that [Rh15(CO)30]3- features six valence electrons more than [Rh15(CO)27]3-. Finally, the electrochemical studies seem to suggest that interstitial Rh atoms are less effective than Ni and Pt interstitial atoms in promoting redox properties and inducing molecular capacitor behavior in carbonyl clusters.

The combination of deconvolution and density functional theory for the mid-infrared vibrational spectra of stable and unstable rhodium carbonyl clusters

Geometrical optimization and frequency calculations on [HRh(CO) 4] were carried out using density functional theory methods with the B3LYP, PBE and B3PW91 functionals in conjunction with the LanL2DZ and DGDZVP basis sets. The accuracy of each calculation was verified by comparing the predicted and the experimentally obtained deconvoluted mid-infrared experimental C O stretching frequencies. The use of the density functional PBE with DGDZVP as the basis set was found to be the most accurate. The same method was then applied to [(C 2H 5)CORh(CO) 4], [Rh 2(CO) 6(μ-CO) 2], [Rh 4(CO) 9(μ-CO) 3] and [Rh 6(CO) 12(μ 3-CO) 4]. Again, vibrational spectral patterns and relative band intensities were in very good agreement with those experimentally observed after BTEM deconvolution. The only inconsistency was a constant shift in the predicted wavenumber assignments of ca. 1.5% for the terminal carbonyl stretching modes. In addition, the optimized geometries were also in good agreement with available X-ray structures of isolatable [Rh 4(CO) 9(μ-CO) 3] and [Rh 6(CO) 12(μ 3-CO) 4]. DFT not only proved to be a valuable tool in validating and confirming the structure of the reactive and unstable species but it also allowed better assignment of the observed spectra especially when vibrational modes were overlapping. The combination of advanced multi-component deconvolution, like band-target entropy minimization (BTEM), with DFT spectral prediction appears to have considerable potential for exploratory in situ studies of reactive rhodium carbonyl systems.

Novel rhodium and ruthenium carbonyl cluster complexes with face- and edge-bridging GaCp* ligands. Synthesis and structural characterization of the Rh6(CO)12(µ3-GaCp*)4 and Ru6(η6-C)(µ2-CO)(CO)13(µ3-GaCp*)2(µ2-GaCp*) clusters

Reactions of hexanuclear carbonyl clusters of rhodium Rh(6)(CO)(16) and ruthenium Ru(6)(eta(6)-C)(micro(2)-CO)(CO)(16) with GaCp*(Cp*= C(5)Me(5)) in the mild conditions result in substitution of CO ligands and formation of the Rh(6)(CO)(12)(micro(3)-GaCp*)(4) and the Ru(6)(eta(6)-C)(micro(2)-CO)(CO)(13)(micro(3)-GaCp*)(2)(micro(2)-GaCp*) cluster derivatives.

Superloading of Tin Ligands into Rhodium and Iridium Carbonyl Cluster Complexes

The reactions of Rh4(CO)12 and Ir4(CO)12 with Ph3SnH have yielded the new Rh-Sn and Ir-Sn cluster complexes M3(CO)6(mu-SnPh2)3(SnPh3)3, 1 (M=Rh) and 2 (M=Ir). Both compounds contain triangular M3 clusters with three bridging SnPh2 and three terminal SnPh3 ligands. The M-M bonds are unusually long. Molecular orbital calculations indicate that this is due to the importance of M-Sn bonding and weak direct M-M interactions. Reaction of 1 with Ph3SnH at reflux in 1,2-dichlorobenzene solvent yielded the complex Rh3(CO)3(SnPh3)3(mu-SnPh2)3(mu3-SnPh)2, 3, which contains eight tin ligands: three terminal SnPh3, three edge-bridging SnPh2, and two triply bridging SnPh ligands.

Hexarhodium Clusters on Lanthana: Synthesis, Characterization, and Catalysis of Ethene Hydrogenation

Rhodium carbonyl clusters were prepared on the surface of La2O3 powder (calcined at 673 K) by a surface-mediated synthesis from La2O3-supported Rh(CO)2(acac) in the presence of CO at 1 atm and 373 K. The cluster preparation and subsequent decarbonylation by treatment in He were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment in He at 573 K removed the carbonyl ligands, giving site-isolated La2O3-supported clusters that are well approximated as Rh6 octahedra, being characterized by a first-shell Rh−Rh coordination number of 3.9 ± 0.4 at a distance of 2.64 ± 0.02 Å. The supported clusters were characterized by IR and EXAFS spectroscopies in the presence of ethene and H2 reacting catalytically to give ethane. The EXAFS first-shell Rh−Rh coordination number was found to be about 4, consistent with the presence of Rh6 octahedra, which are inferred to be the catalytically active species. IR spectra show that both hydrocarbons and hydride ligands were present on the working cluster catalyst, including π-bonded ethene and others, inferred to be ethyl, ethylidyne, and di-σ-bonded ethene. The concentration of hydride on Rh6 increased during the initial induction period in a flow reactor as the catalytic activity increased almost proportionately; hydrides are inferred to be reactive intermediates. 1H NMR spectroscopy showed that hydride remained on the clusters following catalysis. The results suggest that the hydrogenation of ethene on Rh6/La2O3 proceeds by insertion of π-bonded ethene into a Rh−H bond to form ethyl, which is subsequently hydrogenated to give ethane. Rh6/La2O3 is about 50 times more active for ethene hydrogenation catalysis than Rh6/γ-Al2O3, and we suggest that the difference is related to the electron-donor properties of the supports.

Reaction Intermediates in the Solution Photochemistry of a Tetrarhodium Carbonyl Cluster: FTIR Structural Characterization and Transient Kinetics

The photochemistry of the tetranuclear rhodium carbonyl cluster, Rh4(CO)12, in heptane solution has been studied using pulsed laser photoexcitation coupled to time-resolved step-scan FTIR spectrometry. It was found that excitation of the cluster with 266 nm light results in CO dissociation to form a Rh 4(CO)11(Solv) species, transformations of which were observed in the nanosecond time domain. Monitoring the reaction course revealed the presence of two successive transients, I and II, which eventually reform the parent cluster. The FTIR spectrometric study showed that these transients represent two isomeric forms of the Rh4(CO)11(Solv) species. The second transient exhibits the structure typical of ground-state substituted Rh 4(CO)11(L) derivatives with the solvent molecule in an axial position of the basal plane of the metal tetrahedron. The first isomer is a “nonequilibrium” form of the same species where the solvent molecule occupies either a radial site at the basal rhodium triangle or a vacancy at the apical rhodium atom. The kinetics of the reaction sequence is consistent with a unimolecular isomerization of I into II and trapping of the latter by CO evolved at earlier stage of reaction. In the presence of excess CO in the reaction mixture the first transient is effectively scavenged to form Rh4(CO)12. The kinetic parameters of the isomerization process and the trapping of I by CO have been determined.

Rh4(CO)12-derived functionalized MCM-41-tethered rhodium complexes: preparation, characterization and catalysis for cyclohexene hydroformylation

Abstract The preparation of Rh 4 (CO) 12 -derived functionalized silicate MCM-41-tethered catalysts has been studied by infrared (IR) spectroscopy, X-ray diffraction (XRD) and N 2 adsorption–desorption. Silicate MCM-41 is first functionalized with phosphine, amine and thiol donor ligand groups. Then the functionalized MCM-41 is reacted with Rh 4 (CO) 12 by coordination of surface donor ligands to the rhodium to produce phosphinated and aminated MCM-41-tethered unidentified rhodium carbonyl clusters and MCM-41-tethered [Rh(μ-S(CH 2 ) 3 Si(O s ) 3 )(CO) 2 ] 2 (where O s represents surface oxygen). The functionalized MCM-41 and Rh/functionalized MCM-41 possess the structural ordering of mesoporous MCM-41, but exhibit reduced pore sizes, total pore volumes and BET surface areas. The tethered rhodium carbonyl catalysts behave differently with different donor ligands attached in cyclohexene hydroformylation under equimolar CO and H 2 at 2.7 MPa and 100 °C. Only the aminated MCM-41-tethered catalyst displays good activity, selectivity and recycling for the formation of cyclohexane carboxaldehyde. The influences of supported donor ligands on the activity and stability of tethered catalysts for hydroformylation are discussed. The mesoporous structure of MCM-41 is maintained stable during the catalytic reaction.

Mechanistic study of the hydrogen exchange and hydrogenation of propene over alumina supported rhodium and ruthenium carbonyl cluster complexes

The mechanism of propene–deuterium reaction over alumina-supported Rh and Ru carbonyl cluster complexes was studied with kinetic investigation as well as microwave spectroscopic analysis of monodeuteropropene. Irrespective of the nuclearity and the kind of metals, similar catalytic behavior was observed in both C 3H 6–C 3D 6 and C 3H 6–D 2 reactions over Rh 4(CO) 12/Al 2O 3, Rh 6(CO) 16/Al 2O 3 and Ru 3(CO) 12/Al 2O 3, suggesting the formation of common surface complexes as [M(CO) 2] n by impregnation. Different catalytic behavior of fixed carbonyl cluster complexes from that over freshly reduced metal surface (Rh/Al 2O 3) and CO preadsorbed metal surface(CO/Rh/Al 2O 3), suggests the existence of an unique catalytically active species such as metal–metal bridged hydrides for the hydrogen exchange process of propene molecules.

Transition-Metal Catalyzed Multi-Component Carbocyclization and Related Reactions of Bifunctional Molecules.

The selective [2 + 2 + 2] cycloaddition of 1, 6-diynes takes place to give polycyclic 1, 3-cyclohexadienes or arenes using ruthenium or palladium complex catalysts. The rhodium carbonyl cluster is an efficient catalyst for the three-component coupling between carbon monoxide, hydrosilanes, and 1, 6-diynes or 1, 6-enynes. Organotitanium (III) complexes are excellent reagents for the intramolecular carbon-carbon bond formation between the nitrile and carbonyl groups as well as the intramolecular pinacol coupling of dials.

Rhodium Supported on Faujasites: Effects of Cluster Size and CO Ligands on Catalytic Activity for Toluene Hydrogenation

NaY zeolite-supported rhodium carbonyls, with the predominant species being [Rh6(CO)16], were prepared by carbonylation of adsorbed [Rh(CO)2(acac)] at 125°C. The zeolite-supported rhodium carbonyl clusters were treated in He or in H2at 200, 250, or 300°C to remove the carbonyl ligands. When the decarbonylation took place in He, the resultant clusters had Rh–Rh coordination numbers (determined by extended X-ray absorption fine structure spectroscopy) between 3.5 and 3.9, with hardly any evidence of Rh–C contributions, indicating that the rhodium cluster frames were nearly intact and that decarbonylation was nearly complete. In contrast, when the treatment took place in H2, partially decarbonylated rhodium clusters and aggregates formed, having Rh–Rh coordination numbers between 3.8 and 6.7; the rhodium aggregation increased with increasing temperature. The clusters formed by treatment in He at 200, 250, or 300°C all had similar catalytic activities for toluene hydrogenation. In contrast, the catalytic activities of the clusters and aggregates (partially decarbonylated at temperatures <300°C) formed in the presence of H2showed a strong dependence on treatment temperature, which is explained by inhibition by the remaining CO ligands and by cluster and aggregate size effects, with the aggregates being more active than the smaller clusters. Comparison of the data with those reported separately for rhodium clusters in NaX zeolite shows that NaY and NaX zeolites were nearly equivalent as supports. Comparison of the data with separately reported data indicates that the rhodium clusters are approximately the same in activity as iridium clusters of nearly the same size in NaY zeolite.

Solid-state NMR studies of interstitial phosphorus atoms in rhodium carbonyl clusters

Pure samples and as-prepared mixtures of Rh 9 and Rh 10 carbonyl clusters with interstitial P atoms have been studied quantitatively by 31P MAS and 1H– 31P CP/MAS NMR. Information on the 31P chemical shift tensor of the Rh 9 and Rh 10 clusters has been derived from spinning sideband simulations. The chemical shift anisotropy is slightly larger in the Rh 10 clusters (340–400 ppm) than in the Rh 9 clusters (230–300 ppm), while the asymmetry parameters are similar ( η=0.1–0.4). The results contribute to the understanding of the relationship between the shielding anisotropy and the structure of the cluster cavity. © 1997 Elsevier Science B.V.

ChemInform Abstract: Catalysis by Polymer‐Bound Rhodium Carbonyl Clusters. Selective Hydrogenation of α,β‐Unsaturated Aldehydes to Allylic Alcohols in the Presence of H2 and CO.

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