Rh clusters

More recently N has been encapsulated in [Rhu(N)2(CO)25] and [Rh23(N)4(CO)38]3-. The latter is the largest Rh cluster so far characterized. It consists of an irregular polyhedron of 21 Rh atoms encapsulating a pair of particularly close (257.1 pm) Rh atoms as well as 4 N atoms each of which is located in a semi octahedral site. 

The other product (VI) is formed in traces. These reactions form Au-Ru Au-Os , Ag-Fe and Ag-Rh clusters and compounds with bridged Ag— and Au—bonds. Syntheses of cluster compounds containing Cu, Ag or Au and other transition metals are available . 

Calculations on the basis of density functional theory have been used to check the structure of Rh clusters supported on zeolite X the results indicate a slightly twisted prism, nearly matching the octahedron inferred on the basis of the EXAFS data [28,29]. (These clusters were inferred not to be bare but to have hydride ligands, as described later). 

Using functionalized polystyrenes and [Rh6(CO)16], polymer-bound Rh-cluster complexes were obtained in the presence of CO and H20, which exhibited high catalytic activities for hydrogenation of aldehydes to alcohols.407,408... 

Experiments on Rh clusters5,106 reveal an oscillatory pattern of the average magnetic moment, with large values for N = 15, 16, and 19, and local minima for N = 13-14, 17-18, and 20. DFT calculations have been performed for selected clusters in that size range, usually assuming symmetric structures except for the smallest clusters.108-112 The conclusion reached by the various researchers is that the Rh clusters are magnetic. However, different experiments for the same cluster size show a lot of dispersion. 

Two syntheses of Rh4(CO)i2 at ordinary pressure have been developed 81, 279 making this compound easily available, and CO scrambling in this molecule has been carefully examined 110,112,164. Strong nucleophiles including CO at high pressure destroy the Rh cluster 45, 396, but with phosphines of limited donor strength, with cyclooctatetraene, and with acetylenes up to four CO groups could be replaced with retention of the Rh4 unit 45, 247, 396. Rh4-like Rhg-clusters show catalytic activity (7 04, 364 which has been used for hydroformylation 88 and oxidation (297) reactions. 

Very few examples are known, apart from solid state structures, where the sulfide acts as a bridge to more than four metal atoms. Two Rh clusters [Rh17(S)2(CO)33]3 and [Rh,0S(CO)22]2 have been reported in the literature77,78 where the sulfide is found interstitially in the center of the metal cluster and has contacts to nine and eight Rh atoms, respectively. In the former example an almost linear S—Rh—S unit (with d(Rh—S) as short as 2.16 A) is encapsulated in an Rh16 cluster, with four Rh—S contacts at about 2.33 A and four more at about 2.8 A.77 Other related systems are, for example, the [M6S,7]4 ions (13 M = Nb, Ta) which have been recently prepared by Holm and co-workers.80 Here, among other types of sulfide coordination, a /vS has been found in the base of the bell-shaped ions. 

Temperature dependent 103Rh-NMR spectra of the Rh cluster show that the Rh skeleton is stereochemically non-rigid. At - 80 °C the spectrum is in agreement with the X-ray structure (3 doublets 1 4 4). At room temperature only one doublet is observed118. ... 

The reaction of [Rh7(CO)ig] with trifluorosulfonic acid also results in selective oxidative coupling of two Rh, clusters to give [Rh,4(CO)26] -Even though neither of the protonated species [Rh7(CO)i6H,3 (x = 1, 2) were detected by NMR spectroscopy over a range of temperatures during the course of the reaction (216), the reaction probably follows a similar pathway to that of the hexanuclear dianions. 

Two factors concerning the solubility of the Rh-clusters need to be taken into account ... 

Clusters can be isomerized or transformed in various ways under the action of CO . Complex [FejRhCfCOlig]", used in hydroformylation, is converted at 100°C and 30xl0 Nm" (23x10 Torr) CO/H2I/I into Fe4Rh2C(CO),e and [Fe4RhC(CO),4] . The latter further transforms into [Fe3RhjC(CO),5] ". Reversible cluster fragmentation was observed with Co-Rh clusters ... 

For our purposes, it is relevant to conclude that CO adsorption is predominantly molecular on Pt, Ir, Pd, and Rh and that desorption occurs at about 500 K. Controversy exists in the literature regarding CO dissociation on Rh surfaces 19). On supported Rh catalysts and Rh clusters, CO dissociation has been observed. Some authors also reported CO dissociation on certain single-crystal surfaces, whereas other studies indicate no or insignificant dissociation on clean Rh surfaces 19). 

In this context, it is noteworthy that the IR spectrum of bridge-bonded Rhjj(CO)g [generated from Rh(CO)4 matrix-dimerization reactions and Rh/CO matrix-concentration studies (753)] shows bands in the terminal region at 2060, 2043, and 2038 cm , and in the bridge region at 1852 and 1830 cm , that are very close to the proposed, terminal-bridged, chemisorbed, CO species on clean Rh films and supported Rh clusters (4,34, 50,162) (see Table X). Moreover, the matrix, Vqq frequency observed for RhCO (2013 cm ) (154) falls between the vco values for the proposed terminal and bridge sites of chemisorbed CO on... 

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