Carbon rhodium carbonyl clusters

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 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. . ... 

The other neutral species we have studied with respect to Its potential participation In the fragmentation-aggregation reactions of rhodium carbonyl cluster Is Rh2(C0>3. This species has not been previously Implicated In these reactions In the case of anionic rhodium carbonyl clusters, although Its Involvement in such reactions for neutral clusters (equation 4) has already been shown. An Indication of the presence of this species In these types of reactions could be the formation of Rh6(CO)16 n The reaction of CRhs(C0)i5I- wtth carbon monoxide, as observed by Chini, et al., 1 9. and by us ii L under ambient conditions... 

These studies have Indicated that simple rhodium carbonyl complexes, e.g., mono- and binuclear species are Involved in the fragmentation and aggregation reactions of rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen. They indicate that it is possible to write formal equations for such reactions in the case of rhodium carbonyl anionic hydrido clusters (equation 25) and for the more particular situation when there are not hydrides present (equation 26)... 

An impressive example of functionalizing aromatics was presented by Moore and co-workers. Thus, pyridine can be regioselectively acylated with carbon monoxide and olefins (eq. (14)). In the presence of a rhodium carbonyl cluster, first-order rate dependence on the Ru3(CO)i2 concentration led to the belief that the cluster framework remains intact during the course of the acylation reaction. 

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] >... 

The application of zeolite-entrapped rhodium carbonyl clusters [prepared by exchanging Rh(NH3)6Cl3 into NaY zeolite followed by reduction in (CO H2) mixtures] as catalysts for the liquid-phase hydroformylation of alkenes has been discussed. More recently, infrared spectra of Rh6(CO)i6, supported on NaY zeolite by sublimation and treated with carbon monoxide at 100 C, have been found to be virtually identical to those obtained in the hydroformylation experiments. ... 

We noted in our discussion of rhenium and rhodium carbonyl clusters the importance of systems containing core atoms, particularly core carbon atoms that often feature in metal carbonyl clusters prepared by thermal decomposition of other clusters, when they result from reactions between pairs of carbonyl ligands that involve transfer of one oxygen atom from one carbon atom to the other, leading to... 

The reactions catalyzed by rhodium carbonyl clusters, Rli4(CO)i2 and Rh2Co2(CO)i2, proceeded smoothly at room temperature with high selectivity for the formation of 548 when X is a carbon tether. Heteroatom-tethered triynes are highly selective for the formation of 549. The SiCaT reaction was also applied to 1,7,12- and 1,7,13-triynes, which gave 6-6-5 and 6-6-6 fused tricyclic benzene derivatives, respectively, in high yields. ... 

Vidal JL, Walker WE (1981) Rhodium carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 3. Synthesis, characterization, and reactivity of HRh(CO>4. Inorg Chem 20(l) 249-254... 

Scheme 3-3. Some selected rhodium carbonyl clusters obtained from the reductive thermolysis of [Rh4(CO)i2] under N2 or H2 at atmospheric pressure, and [Rh(CO)2(acac)[ under superatmospheric pressure of carbon monoxide (acac = acetylacetonate). [95]...

It is not clear why the ruthenium catalyst is not able to induce the heterocyclisation from the presumably intermediate nitrene complex (Scheme 6). The benzamides 32 could be formed via the insertion of an intermediate isocyanate in the aromatic C-H bond of the solvent. This reaction has some precedents, where benzanilides were obtained by reaction of PhNCO with benzene, or directly from nitrobenzene, carbon monoxide and benzene, catalysed by rhodium carbonyl clusters [56-58], However, the reluctance of Ru3(CO)i2 to catalyse the reduction of nitrobenzene to phenylisocyanate in solvents such as benzene [22, 23] does not support this hypothesis. 

It should be stressed that, in this treatment of metal-carbonyl clusters, the number of nonbonding electron pairs allocated to each metal atom [3 pairs for each ruthenium atom of H2Ru8(CO)i8 2 pairs for each rhodium atom of Rhe(CO)i6] is not arbitrary but is chosen with two objectives in mind (o) to reduce the number of electrons formally remaining for skeletal bonding to fewer than the number of orbitals remaining, because only then is it realistic to assume that all these electrons can be accommodated in bonding MO s and (6) to provide a suitable number of electron pairs on each metal atom for metal carbon... 

In the case of rhodium, however, it was demonstrated early that in the synthesis of [Rh6C(CO)l5]2 the encapsulated carbon atom originated as chloroform, which had reacted with the rhodium carbonyl anion [Rh7(CO)l6]3- (59). In the cobalt analog, [Co6C(CO)l5]2-, the carbon atom is derived indirectly from carbon tetrachloride [via Co3(CO)9CCl] (60) Both these syntheses are performed under mild conditions, and there are apparently no examples of carbidocarbonyl clusters of cobalt or rhodium prepared directly from the metal carbonyls under pyrolysis conditions. 

The compound Na2 [Rh12(CO)30] can be prepared by reaction of Rh2(CO)4-Cl2 with sodium acetate in methanol under an atmosphere of carbon monoxide.1 It contains one of the fust polynuclear anions to be formed when the rhodium carbonyls or carbonyl halides are reduced by the action of alkaline reagents in alcohols or by alkali metals in tetrahydrofuran (THF). It provides a unique example of a double octahedral cluster carbonyl anion in which the noble gas rule is not obeyed,1 2 and it is a starting material for the preparation of other polynuclear rhodium carbonyl anions.1 3"5 The synthesis reported here is a modification of the original method. The starting material is Rh4(CO)i2, now easily prepared at atmospheric pressure.6"8 The reaction is fast, and the overall procedure requires about 6-7 hours with 80-85% yields. 

It was of interest in this respect to determine which rhodium carbonyl complexes result from the former reaction and whether the parallel formation of any organic products derived from the hydrogenation of carbon monoxide Is also occurring. The latter possibility was considered because of the presence of hydrides In the cluster and the involvement of hydrido carbonyl complexes in the hydrogenation of carbon monoxide. Unfortunately, It was not possible to detect any organic products formed from this reaction even after the cyclic repetition of the transformations below (equation 14). 

In contrast to hydroformylation of olefin derivatives, the addition of carbon monoxide and trialkylsilane to alkynes gives carbon-centered silanes exclusively when catalyzed by, for example, rhodium and Rh-Co carbonyl clusters [153, 154] and Rh2(pfb)4 (pfb = perfluorobutyrate) (eq. (13) [153]). 

The data in Table 4.3 correspond to a radius for the octahedrally coordinated carbon atom that Ues in the range 0.59-0.69 A. We noted earlier that the radius of the core carbon in osmium, rhenium, and rhodium clusters lie in the range 0.59-0.62 A. It appears likely that the enthalpy change ZE(M-C), needed to cleave the six M-C bonds in these molecular carbonyl clusters, will lie in the same range (239-306 kcal moT i.e., 38-51 kcal mol per MC link) that we have now calculated for the similarly coordinated carbon atoms in these extended lattice binary carbides MC or M2C. 

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