Carbonylative Rh4

High nuclearity carbonyls Rh4(CO)i2 and Rhs(CO)i6 have been extensively used as precursors for the preparation of supported rhodium catalysts. Early studies reported the use of a great variety of supports that includes metal oxides [159-166], zeolites [101, 167], polymers [168] and modified-silica surface [169]. 

The metal in powder form absorbs hydrogen when heated. When heated with carbon monoxide under pressure rhodium forms carbonyl, Rh4(CO)i2-... 

Interaction of the dibenzylideneacetone complex of palladium with a high surface area carbon gave a supported complex which on heating produced a Pd/C catalyst having a uniform distribution of palladium metal particle sizes. The rhodium carbonyls, Rh4(CO) 2 and Rh6(CO) 6, were adsorbed on silica to give... 

Rhodium carbonyl, Rh4(CO)I2, catalyzes the cyclocarbonylation of acetylenes with benzene to give indenones. For example 2,3-diphenyl -indenone is obtained from diphenyl acetylene and benzene in 10% yield, corresponding to a catalytic turnover of 11 (220°C, 25 bar, 7 hr)... 

Rhodium carbonyl clusters in Y-type zeolite were generated from Rh(III)-Y and CO, and characterized by IR in the contribution of LeFebvre et al. [629], who identified monovalent rhodium dicarbonyl, Rh(I)(CO)2 as well as polynuclear carbonyls, Rh4(CO)i2 and the most stable RhsfCOlig. The latter exhibited a pronounced activity for and selectivity to formation of alcohols and C2-C5 hydrocarbons. 

Rhodium carbonyl, Rh4(CO)ig, catalyses the cross-hydrocarbonylation of acetylenes and ethylene under pressure to give ap-unsaturated ethyl ketones [equation (28)]. The reaction shows high regioselectivity for terminal alkynes, giving the transAsomex,... 

Fachinetti and coworkers—acid cocatalyzed water-gas shift over Rh and Ru carbonyl catalysts. Fachinetti and coworkers133 145 published research on the acid cocatalyzed homogeneous water-gas shift reaction by Rh4(CO)12 and Ru3(CO)12 precursors in aqueous pyridine solution. For Rh carbonyl, experiments were carried out at 80 °C, PCo = 1 atm, and [Rh] = 0.02 mol/L in a solution of pyridine containing 3% H20. They observed the following reaction in anhydrous pyridine ... 

Substitution of four carbonyl groups on the four different metal atoms is exemplified by the structure of Rh4(CO)8(DPM)243) (Fig. 15) while the particular structure of Co4(CO)g(ffars)2 88 (Fig. 15) is probably connected with the rigidity of the bidentate ligand, ffars. 

As already shown in Eqs. (3), (4) and (5), fragmentation of carbonyl cluster anions, on reaction with carbon monoxide becomes easier with increasing negative charge. It is therefore not surprising that reduction of tetranuclear clusters with alkali metals in excess, gives rise to simple mononuclear derivatives [M(CO)4]-, as found for Co4(CO)12 in liquid NH3 13, e Rh4(CO)12 in THF under C056) and Ir4(CO)12 in THF under CO 17S ... 

With tertiary phosphines, phosphites and iso-nitriles, the substitution of the carbonyl groups in Co4(CO)i2 and Rh4(CO)i2 occurs very easily up to the tri-substituted complex. It is sufficient to add slowly at room temperature a solution of the ligand to a solution of the dodecacarbonyl ... 

The equilibrium concentrations of Rh2(CO)s and Rh4(CO)i2 were determined by infrared spectroscopy by monitoring the absorbance of the band at 1886.8 cm-1, which corresponds to the stretching of the bridging carbonyls of the tetrarhodium complex. More details of the experimental procedure can be found in the original papers. For our purpose, it is enough to say that the equilibrium concentrations of the rhodium complexes were quite low (< 10-3 mol dm-3), but the same was not true for the CO concentration ( 2 mol dm-3 see... 

Previous studies on various aliphatic and aromatic alkynes have evidenced that this type of furan-2-ones could also be produced in the presence of the rhodium carbonyl cluster [Rh4(CO)i2] [39-41]. Labeling experiments using D2O allowed the authors to assign the origin of the hydrogen atoms (Scheme 6). Noteworthy, dicobalt octacarbonyl follows a different catalytic... 

For instance let us consider the case of the tetrahedral carbonyl cluster [Rh4(CO)i2], together with the theoretical analysis of its molecular orbitals, Figure 1. 

Rhodium carbonyl cluster catalysts [Rh4(CO)i2] and [Rh6(CO)i6] were effective to produce lactones in carbonylation of alkynes (Scheme 5.10) [32,33]. In these systems, however, water is rather a reagent than a solvent and its amount can be as low as 216 pL in 45 mL CHCI3 [33]. 

Carbonyl compounds of metals of group 9 have been extensively used in the preparation of supported catalysts, mainly for two reasons the metals are active in several reactions of industrial interest and there are stable carbonyls of easy decarbonylation of the three metals of this group. Besides the stable neutral homo-nuclear carbonyl compounds CojjCOjg, Co4(CO)i2, Rh4(CO)i2, Rh,s(CO)iis and Ir4(CO)i2, other heteronuclear carbonyl species, mainly containing Co-Rh, Co-Ru and Rh-Fe, have been used in the preparation of catalysts. Both metals, Co and... 

The simple metal carbonyl hydride species HRh(CO)4 has been sought by many researchers including Hieber [97], Chini [98], Vidal [99] and Whyman [100]. Vidal was able to obtain evidence for new MIR bands when Rh4(CO)i2 was subjected to ca. 1400 bar CO/H2 pressure, and Whyman was able to corroborate this finding at ca. 400 bar CO/H2 pressure. 

In many instances, the formation of inactive dimers from active, monomeric catalytic species is observed during catalysis. When weak or unstable ligands are used, even larger rhodium carbonyl clusters like Rh4(CO)i2 and Rh5(CO)i5 can be observed [42-44]. The formation of dimers is often a reversible equilibrium (Scheme 6.2). This only leads to a reduction in the amount of catalyst available and does not kill the catalyst. One of the first examples was the formation of the so-called orange dimer from HRh(PPh3)3CO, already reported by Wilkinson [45]... 

Rhodium carbonyl complexes catalyze the silane-initiated cascade cyclization of 1,6,11-triynes to form fused aromatic tricyclic compounds. For example, reaction of 83 [X = G(G02Et)2] with methyldiphenylsilane catalyzed by the tetrarhodium carbonyl cluster Rh4(GO)i2 in toluene at room temperature gave an 88 12 mixture of the silylated and unsilylated fused tricycles 84a and 84b [X = G(G02Et)2] in 85% combined yield (Equation (55)). The ratio of silylated to unsilylated tricyclic product formed in the reaction of 1,6,11-triynes was dependent on the nature of the substrate (Equation (55)). For example, Rh4(GO)i2-catalyzed reaction of diaminotriyne 83 (X = NBn) with methyldiphenylsilane gave unsilylated tricycle 84b (X = NBn) in 92% yield as the exclusive product (Equation (55)). 

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