Rhodium carbonyl cluster anion

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 ... 

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. 

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). 

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)... 

The nitrido carbonyl cluster anion [Co-yNlCO) 5 is found to have a carbonyl stereochemistry quite different from that of its rhodium analog . The 2-D nutation Co-59 NMR of solid CO4iridium carbonyl clusters have been formed on MgO and zeolites and make stable CO hydrogenation catalysts . ... 

Indeed, there is a unity with the field of heterogeneous catalysis. As evidence of this, similar (or identical) Rh (C0)2 sites can be prepared either by CO chemisorption on preformed metal particles [69] or by decomposition of rhodium carbonyl clusters on the oxide surface [62-66]. Further evidence for this can be seen from the observation of metal carbonyl clusters under operating supported metal catalysts. For example, ruthenium catalysts for the conversion of synthesis gas to polymethylene [122] afford mixtures of cluster species at elevated temperatures (120°C) and pressures (1000 atm) [123]. One of these was Ru3(CO)i2 others appear to be ill-characterised. A similar observation has been recently reported for Ru/MgO and Os/MgO synthesis gas conversion catalysts [124]. On this basic support, two anionic clusters were isolated, viz. [Ru5C(C0)i5] and [OsiQC(CO)24] 7 which may be synthesised in solution by thermolysis in basic or reducing media. It is unclear whether these clusters are actually effecting the catalysis. They may instead, as highly stable species, be formed in a side reaction. 

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. 

Another simple rhodium carbonyl complex also known to be Involved In the fragmentation and aggregation reactions of clusters Is Rh2(C0)s This species has been shown to participate In the reactions of neutral rhodium carbonyl species In either matrixes or solutions (equation 4), but It has not yet been Implicated In the chemistry of large anionic 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). 

Octahedral cluster iridiiam carbonyl derivatives have also been described (275), but these have been investigated in less detail than their cobalt or rhodium analogs. They are obtained by the alkali-metal reduction of Irit(C0)i2 in a manner somewhat similar to the preparation of octahedral cluster cobalt carbonyl derivatives by the alkali-metal reduction of Coit(C0)i2 Reaction of Irij(C0)i2 with sodium metal in tetrahydrofuTcin first gives the tetrahedral cluster anion Hirij (CX7) i" aind then the dark brown, octahedral cluster anion Ir0(CO)i5 , which can be isolated as its tetraethylammonixam salt (275). ... 

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]"-,... 

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