Rhodium clusters hydrogenation

In addition to these homometallic (rhodium) clusters, several hetero-metallic clusters of the type [M M CO o]2, where M and M1 are each different metals selected from the Co, Rh, Ir triad (jc = 1-11), have been described and claimed to be useful catalysts in the reaction between carbon monoxide and hydrogen to produce oxygenated products (68, 69). These complexes can be prepared from the heterometallic dodecacar-bonyl complexes, [MuM (CO)12] (M, M1 = Co, Rh, or Ir y = 1-3), by simply mixing the appropriate dodecacarbonyl species in THF under nitrogen and then adding water (70). They can be isolated by adding a suitable cation e.g., Al3+, Mg2+, Ca2+, etc. 

Unfortunately, because of the exceptional number of interrelated equilibria between various rhodium clusters and Rh(CO) it seems unlikely that it will be possible to identify which rhodium species is responsible for the hydrogenation reactions. 

Mononuclear ruthenium complexes were found to be superior to carbonyl clusters during a comprehensive comparison of a variety of catalysts in the reduction of acetone [49]. Without solvent, most catalysts were highly selective, although the activity was quite low. The addition of water to the system vastly increased yields, in agreement with Schrock and Osborrfs observations into rhodium-catalyzed hydrogenations (Table 15.9) [41],... 

The rhodium cluster anions and cations reacted with benzene in a similar manner with a few minor variations. The small clusters reacted by loss of one hydrogen molecule. The loss of two molecules of hydrogen started at Rh6 for anions and Rh7 for cations. The loss of three hydrogen molecules started at n = 9 for cations and n = 12 for anions. At Rhi4, the coordination of benzene became the dominant process for both anions and cations. 

Brayshaw, S. K. et al.. Holding onto lots of hydrogen A 12-hydride rhodium cluster that reversibly adds two molecules of H-2. Angew. Chem. Ini. Ed., 44, 6875,2005. 

Niobium and cobalt clusters exhibit size-sensitive reactions with nitrogen with a reactivity pattern similar to that observed for hydrogen. The reactivity of rhodium clusters (n = 1-12) toward N2 has also been studied. In this case the atoms through the tetramer appear to be inert, with reactivity turning on at Rhj. Maximum reactivity occurs at Rh7, and subsequently drops off by roughly a factor of 2 in going from Rh, to Rh,. Iron clusters appear to be nearly unreactive toward N2. Attempts to induce low-pressure ammonia synthesis on gas-phase iron clusters indicate that hydrogenated iron clusters Fe H are also unreactive toward N2. ... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

Rhodium clusters modified with chiral diphosphines have been reported to catalyze the enantioselective hydrogenation of prochiral dehydroamino acids (Scheme 4). In ethanol, 23 is hydrogenated by Rh6(CO)I0(i ,R-DIOP)3 (80°C, 1 bar, 24 hr) giving the R-enantiomer of 24 with 47% yield... 

With rhodium clusters the nuclearity of the species involved is doubtful. Chini et al 231) showed in a clean stoichiometric reaction that propene is converted into a 1 1 mixture of butanal and 2-methylpropanal by Rh4(CO)i2 and hydrogen the hexanuclear cluster Rh6(CO)J6 is formed quantitatively ... 

A few reactions in which four components are reacted together in the presence of a transition metal cluster as the catalyst are known. These reactions involve an acetylene, an olefin, carbon monoxide and hydrogen, or a hydrogen donor and are catalyzed by the rhodium cluster Rh4(CO)i2. In a typical experiment, an acetone solution of diphenylacetylene is pressurized with ethylene (25 bar), CO (30 bar), and H2 (5 bar) at 150°C for 6 hr the reaction... 

The tetranuclear rhodium cluster Rh4(CO),2 has also been reported to catalyze the hydrocarbonylation of acrylic acid derivatives with isopropanol as hydrogen donor. As a typical example, ethyl acrylate reacts with CO and isopropanol... 

Polymer-bound rhodium clusters were used for catalytic hydrogenations of a,/3-unsaturated aldehydes to allylic alcohols. Amination of chloromethylated polystyrene with 2-(2-(dimethylamino)ethoxy)ethanol gave an amine-functionalized polymer. Using the aminated polystyrene and Rh6(CO)i6 in the presence of H2 and CO or CO and water, various a,/ -unsaturated aldehydes were chemoselectively hydrogenated to give allylic alcohols in high yields, generally >95% conversion and 80-100% selectivity, at 303 K. Under the reaction conditions, a number of anionic clusters form, which can be recovered as ions paired to the ammonium cations of the polymer. Clusters identified by... 

Polymers of poly(styrene-divinyIbenzene) functionalized with diphenyl phosphine groups containing rhodium clusters are stable active catalysts for the hydrogenation of ethylene and cyclohexene. ... 

The preparation and chemistry of rhodium clusters in Y zeolite has been extensively studied at IRC-CNRS(Lyon) [143,191-199] and by others [20,200-209]. Rhodium-exchange procedures in different zeolites (faujasites, mordenite, erionite,ZK-5,ZSM-34,ZSM-ll) were extensively studied by Shaimon et al. [197]. Rhodium clusters were prepared from [Rh(NH3)5Cl] +-exchanged faujasites with a calcination step before reduction, or from aqueous solutions of Rh(H20)g+ obtained by dissolving RhClj or Rh(N03)2 in water. In the latter case, highly dispersed rhodium was obtained by direct hydrogen reduction at 200°C omitting the preliminary calcination required to decompose [RhfNHjjjCl] complexes. 

As the most convenient reaction for this study we may take benzene hydrogenation, which occurs at low temperatures. In this reaction, the catalytic activity of rhodium is two orders of magnitude higher than that of palladium (Table III). It seems, therefore, of interest to compare the properties of (Rh+Pd)/Si02 catalysts. It can be expected that at low reduction temperatures (100 C) a metallic framework of an adsorbed rhodium cluster may be preserved, as in the case of Ru3(CO)] 2 on palladium (Fig. 2, curve (2)). 

Table III. Catalytic properties of (Rh+Pd)/Si02 catalysts, obtained using rhodium clusters of different nuclearity and reduced with H2 at 100°C, for benzene hydrogenation.

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