Clusters rhodium

Cox A J, Louderback J G and Bloomfield L A 1993 Experimental observation of magnetism in rhodium clusters Phys. Rev. Lett. 71 923... 

The interaction of small, well defined, rhodium clusters, Rh and Rhs, with O2 has been investigated (220) by matrix infrared, and UV-visible, spectroscopy, coupled with metal/02 concentration studies, warm-up experiments, and isotopic oxygen studies. A number of binuclear O2 complexes were identified, with stoichiometries Rh2(02)n, n = 1-4. In addition, a trinuclear species Rhs(02)m, m = 2 or 6, was identified. The infrared data for these complexes, as well as for the mononuclear complexes Rh(02)x, = 1-2 (229), are summarized in Table XI. Metal-concentration plots that led to the determination of... 

There appears to be concentration of rhodium in the surface of the iridium-rhodium clusters, on the basis that the total number of nearest neighbor atoms about rhodium atoms was found to be smaller than the nunber about iridium atoms in both catalysts investigated. This conclusion agrees with that of other workers (35) based on different types of measurements. The results on the average compositions of the first coordination shells of atoms about iridium and rhodium atoms in either catalyst Indicate that rhodium atoms are also incorporated extensively in the interiors of the clusters. In this respect the iridium-rhodium system differs markedly from a system such as ruthenium-copper (8), in which the copper appears to be present exclusively at the surface. 

The EXAFS results suggested that the iridium-rhodium clusters dispersed on alumina differed in size and/or shape from those dispersed on silica, based on the result that the total coordination nunbers of the iridium and rhodium atoms in the clusters were very different (7 and 5 in the alumina supported clusters vs. 11 and 10 in the silica supported clusters). These coordination numbers suggested that the clusters dispersed on alumina were smaller or that they were present in the form of thin rafts or patches on the support. The possibility of a "raft-like" structure in the case of the alumina supported clusters suggests an interaction between the metal clusters and the support which is much more pronounced for alumina than for silica. If the clusters on the alumina were present as rafts with a thickness of one atomic layer, one could have a situation in which the rhodium concentration at the perimeter of the raft was greater... 

Related substitution patterns are observed in tetranuclear cobalt and rhodium clusters. Thus, the small ligand, P(OMe) (L), occupies axial sites in [Co,(CO)1 L ] (x = 1,2) (17) whereas steric effects become important witfi (OPh) and the isomers shown in Fig. 2 are obtained with tetrarhodium derivatives. 

A consequence of the value of the ligand is that one of the simplest ways to restore catalyst activity is simply to add fresh catalyst precursor. Unfortunately, there are practical limits as the rhodium concentration increases. First one must consider metal complex solubility, particularly in the recycle catalyst solution in a liquid recycle system. Secondly, higher rhodium concentrations favor formation of various types of rhodium clusters.[11] As rhodium increments are added to a partially deactivated cata-... 

Wiped-Film Evaporator/02 Reactivation of Catalyst. In this technology [38], spent or-ganophosphine-modified rhodium catalyst is first concentrated in a wiped-film evaporator where most of the organophosphine is removed. The rhodium concentrate is contacted with air to break down phosphido bridges in rhodium clusters. This air treated concentrate may then be used as a catalyst precursor. This procedure is suitable in circumstances where most of the aldehyde dimers, trimers and tetramers are sufficiently volatile to be removed in a wiped-film evaporator. 

Such compounds may be contacted with partially deactivated hydroformylation catalyst under non-hydroformylation conditions to effect disruption of the phosphido bridges of a rhodium cluster (Equation 2.12). After the treatment period, the catalyst solution is again suitable for hydroformylation. 

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. 

The mechanism for ether formation is quite simple given the recent work of Chini (19) that shows that rhodium clusters can react to give strong acids ... 

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. 

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

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. 

GaCp can serve as a ligand in rhodium cluster compounds, effectively replacing up to four CO s in the parent Rh6(CO)16 cluster.90 Direct reaction of Rh6(CO)16 and an excess of GaCp led to the tri- and tetrasubstituted... 

The incorporation of bridging germanium ligands into high-nuclearity transition metal clusters has been accom-plished. Thermal reaction of Ph3GeH with rhodium carbonyl yields a mixture of germanium/rhodium cluster... 

Size and Structural Dependence of the Magnetic Properties of Rhodium Clusters. 

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. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

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