Anionic rhodium clusters

A few homometallic HNCC have also been produced by the condensation reactions of anionic carbonyl clusters with cationic complexes. For instance, sequential buildup of rhodium HNCC via incorporation of Rh(CO)2 fragments has been achieved by reacting [Rh(CO)2(MeCN)2] in acetonitrile with a series of anionic rhodium clusters as illustrated by the reactions shown in Eqs. (11) and (12) (223). 

Rh4(CO)25]4, and [Rh,3(CO)27]3- are under discussion (142). Up to now the role of these anionic rhodium clusters remained uncertain because of the labile fragmentation and rearrangement processes, the identity of the active species is not apparent. 

Another system in which intact cluster catalysis seems likely is the synthesis gas (CO/H2) production of ethylene glycol catalyzed by an anionic rhodium cluster of uncertain or undisclosed identity. From high-temperature... 

Scheme 6.123 Role of halogenides in the formation of anionic rhodium clusters.

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. 

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). 

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. 

With both cobalt and rhodium, clusters of higher nuclearity are accessible from the hexanuclear anions. Condensation of [Co6C(CO)l5]2- with Co4(CO)l2 in isopropanol at 60°C results in the isolation of [Co8C(CO)lg]2-[Eq. (22)] (64). 

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)... 

Polymer-bound rhodium clusters were used for catalytic hydrogenations of a,/3-unsaturated aldehydes to allylic alcohols. Amination of chloromethylated polystyrene with 2-(2-(dimethylamino)ethoxy)ethanol gave an amine-functionalized polymer. Using the aminated polystyrene and Rh6(CO)i6 in the presence of H2 and CO or CO and water, various a,/ -unsaturated aldehydes were chemoselectively hydrogenated to give allylic alcohols in high yields, generally >95% conversion and 80-100% selectivity, at 303 K. Under the reaction conditions, a number of anionic clusters form, which can be recovered as ions paired to the ammonium cations of the polymer. Clusters identified by... 

Neutral and anionic organoimidoimido clusters of rhodium)I) with diolefin and carbonyl ligands are accessible and stable. The diolefin neutral compounds [Rh4(/i-N-/7-tolyl)2(diolefin)4] are a convenient entry into this chemistry for which the synthetic route is an apparently simple replacement reaction, but the reproducibility of the results requires a careful technique. These tetranuclear compounds contain a trimetallic triangular cluster face capped on both sides by two pnra-toly-limido ligands and a further rhodium fragment coordinated to a para-to y ring in a 7 -fashion, giving an overall valence electron count of 64e. 

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

The process is catalyzed by a number of cobalt and rhodium clusters 361). Anionic clusters such as [FeCo3(CO),2] and [Coj(CO),5] show greater catalytic activity than would be expected from neutral clusters of the same size 361). Other mixed-metal clusters, such as Zo2C04(CO),5 362) and Pt3Co2(CO)9(PPhj)j 363), have also been reported to be active for the stereospecific dimerization of norbomadiene to give 81. In the presence of BF3 Et20, Pt3Co2(CO)9(PPhj)3 converts norbomadiene into 81 with 100% yield and 100% selectivity (20 °C, 30 min, CT 31). 

Cluster Compounds of Co, Rh, and Ir. In addition to the above-mentioned neutral cluster compounds, there is a large number of anionic carbonyl clusters and metal carbonyl carbides. Carbonyl carbides are formed when the interstice inside the metal cluster is sufficiently large to accommodate the carbon atom. Carbonyl carbides possessing at least four metal atoms are known. The most thoroughly investigated carbides are those of rhodium because they are very stable and resist air oxidation. Carbonyl clusters of group 9 elements containing even more than 20 metal atoms are now known [M6(CO)i5] - (M = Co, Rh, Ir), lM CO)uT. [M6(CO)i5C]"-(M = Co,Rh), [Co8(CO),sC] -, [Rh,(CO)i,] -, [Rh8(CO)i,C], [Ir8(CO)22]"-,... 

The [Rh(CO)4] anion has been used as building block in the synthesis of various rhodium clusters, such as [Rh7(CX)),6] ", [RbsfCO),]", [Rhii(CO)23] , ° and [Rh7N(CO) 5] ". It has also been used in the synthesis of rhodium cluster carbides isotopically enriched on the interstitial carbon atom. ... 

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 anionic cluster [Ir6(CO)i5] is octahedral and an increasing number of Ir clusters have been reported recently though their preparations are more difficult and yields usually smaller than for rhodium. [Iri4(CO)27] has the highest nuclearity so far and is obtained as black crystals by oxidizing [Ir6(CO)i5] with ferricinium ion (Fig 26.9b). 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

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 ... 

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