Cluster rhodium-carbonyl

There are many related compounds, including rhodium carbonyl cluster anions, which are present in the solutions cataly2ing ethylene glycol formation and which may be the catalyticaHy active species or in equiUbrium with them (38). 

The catalyst precursor generally used for the reaction is rhodium dicarbonyl acetylacetonate. However, detailed infrared studies under the reaction conditions (ca. 1000 bar CO/H2 and 200°C) have shown both the [Rh(CO)4] and the [Rh12(CO)34 36]2 anions to be present in various concentrations at different stages of the reaction (62, 63). It is suggested that rhodium carbonyl clusters, characterized as having three intense infrared absorptions at 1868 10, 1838 10, and 1785 10 cm-1, are responsible for the catalysis (62), and it is believed that the reaction is dependent upon the existence of the following equilibria ... 

The rhodium carbonyl cluster [Rh6(CO)i6], in combination with the diamine N,N,N, N -tetramethyl-l,3-propanediamine is an effective catalytic system for the... 

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... 

The simple procedure for the carbonylation of allyl halides has been extended in the high yielding solid-liquid two-phase conversion of allyl phosphates into amides (60-80%) under the influence of a rhodium carbonyl cluster in the presence of primary or secondary amines (Scheme 8.8). A secondary product of the reaction is the allylamine, the concentration of which increases as the pressure of the carbon monoxide is reduced, such that it is the sole product (ca. 80%) in the absence of carbon monoxide [28],... 

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]. 

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]... 

Recently, an ESR spectrum from Aujj at 4.4 K, with g= 1.9204 and linewidth 67 mT, has been reported [54]. The intensity of the signal followed the Curie-Weiss law, and the g-value and linewidth were independent of temperature. If the linewidth were determined by the electronic spin-spin relaxation time Tj, it would be expected to increase with temperature, so its temperature-independence suggests inhomogeneous broadening by unresolved hyperfine coupling to P and Au nuclei. Similar behaviour has been found in rhodium carbonyl clusters [18]. 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

Homogeneous Catalytic Oxidation with Phosphine-Substituted Complexes of Rhodium Carbonyl Clusters... 

The catalyst was examined (226) before and after the pretreatment with CO/H2 by means of infrared (IR) spectroscopy. IR bands characteristic of terminal and bridging carbonyl groups associated with rhodium carbonyl clusters were observed following this pretreatment of the catalyst. However, these spectra were apparently not identical to those of the known rhodium clusters, Rh4(CO)12 or Rh6(CO)16. The differences were associated with the positions of the bands due to bridging carbonyl groups. It is conceivable that... 

For hydroformylation over cobalt and rhodium zeolites the active species have not been defined. However, in the case of RhNaY the in situ formation of a rhodium carbonyl cluster has been identified (226) by infrared spectroscopy. Interestingly, this cluster appears to be different from known compounds such as Rh4(CO)12 and Rh6(CO)16. This does suggest that alternative carbonyl clusters may possibly be formed in zeolites due to the spatial restrictions of the intracrystalline cavities. The mechanism of hydroformylation in these zeolites is probably similar to that known for homogeneous catalysis. 

S. Martinengo, G. Ciani, A. Sironi, and P. Chini, Analogues of Metallic Lattices in Rhodium Carbonyl Cluster Chemistry. Synthesis and X-ray Structure of the [Rh15 ( i-CO)14(CO)13]3 and [Rh14((j,-CO)15(CO)9]4 Anions Showing a Stepwise Hexagonal Close-Packed/Body-Centered Cubic Interconversion, J. Am. Chem. Soc. 100,7096-7098 (1979). 

Tri-/i-carbonyl-nonacarbonyltetrarhodium is the starting material for the synthesis of a large number of rhodium carbonyl cluster compounds.10... 

Rhodium Carbonyl Cluster Chemistry Under High Pressures of Carbon Monoxide and Hydrogen... 

The potential participation of HRh(C0)4 In the reactions of high nuclearlty rhodium carbonyl clusters could be analogous to the formation of CNi2 (COgH]- upon fragmentation of CN112(C0)21H2 2— by carbon monoxide (equation 7)(12/... 

Our Interest In understanding the behavior of rhodium carbonyl clusters in systems which catalytlcally convert CO H2 Into alcohols 3. prompted us to test the potential presence of mononuclear and bI nuclear rhodium carbonyl complexes In these systems. A positive characterization of these species under these circumstances would show a previously unknown behavior of rhodium carbonyl clusters under high pressure of carbon monoxide. It could also show the existence of a parallel behavior between the chemistry of these species under ambient and high pressures of carbon monoxide, and it may shed some light on the catalytic reactions occurring in those systems. 3. ... 

Because of our previous success In applying Fourier-transform infrared spectroscopy to the study of the rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen 2. A, we have applied the same technique and equipment in this work. 3. The temperature has been kept In all these experiments below 200° with maximum pressures of 832.0 atm to maximize the trend towards fragmentation of clusters. The absence of bases, e.g., salts or amines, in the systems under evaluation in this work was desirable to eliminate the ambiguity that would result from the enhancement of the fragmentation of clusters by carbon monoxide In a basic medium. . ... 

Rhodium carbonyl clusters with encapsulated heteroatoms, e.g., CRh6(C0)i5C]2 , [RhgPtCO l12 and [Rhi7S2(C0)32]3 , have been observed to be more resistant to fragmentation under 500-1000 atm of C0 H2 than homometal lie rhodium clusters. Polynuclear species account for 95-98 and 80-85 percent, respectively, of the total rhodium content of those systems, while the greatest catalyst activity has been noted with the latter complexes. CRh(C0>4l]— Is the only fragmentation product detected In both Instances. . ... 

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