Mixed-metal clusters carbonylation

More recent developments in cluster chemistry have included the anchoring of clusters to silica or alumina surfaces via sol-gel processing, the products finding applications as catalyst precursors. Very recently, mixed-metal clusters have been incorporated onto the inner walls of mesoporous silica with a pore diameter of about 30 A, and have been subsequently converted into discrete nanoparticles by thermolysis, which have been shown to act as hydrogenation catalysts. This is a particularly important new use for mixed-metal cluster systems, since there are many well characterized mixed-metal cluster carbonyls vide infra) that are readily available to act as precursors for such reactions. 

Table XI summarizes reports of ligand fluxionality at "very mixed -metal clusters. A number of studies (c.g.. Refs. 28, 8I, 116, 213, 214) have reported that both metal-localized and global carbonyl fluxionality occur but give little mechanistic detail. The following discussion focuses on the examples for which detailed studies have been undertaken and/or those for which mechanistic speculation is available.

The limited magnetic measurements of very mixed -metal clusters are summarized in Table XIII. The magnetic behavior of some anti-ferromagnetic very mixed -metal carbonyl clusters (Fig. 82) has been studied by Pasynskii and eo-workers. Temperature dependences of the magnetic susceptibilities of Cr2Co(/t3-S)3(/i-SBu )(CO)2() -C3H4R)2l (R = H. Me) have been determined us-... 

The reaction with [Mn(CO)5]2 or [Co(CO)4]2 also led to mixed-metal clusters.962,963 Osmium-palladium mixed-metal carbonyl clusters were made using the unsaturated cluster [Os3(CO)io(/r-H)2] and the carbido cluster [Os5(/U5-C)(CO)15].964 970 Treatment of [Os3(CO)10Gu-H)2] with [Pd2(/u-dppm)2Cl2] afforded the novel high-nuclearity osmium-palladium mixed-metal carbonyl clusters [Os5Pd6(CO)13( -CO)5(/u-H)2(/u-dppm)2], [Os5Pd6(CO)13(//-CO)6(/u-dppm)2], and... 

Anionic clusters are good nucleophiles (see Section III,A) and are often easy to make. On the other hand, the electrophilic nature of most monometallic complexes is obvious from ligand substitutions. The combination of these properties makes a strategy for cluster expansion. This strategy was used for the first time by Hieber (130) in making Fe4(CO)fc from Fe3(CO),7 and Fe(CO)s. It is probably active in many syntheses of large metal carbonyl clusters because the Re, Os, Rh, Ir, Ni, and Pt clusters involved are almost always anionic. However, simple stoichiometries can rarely be written for such reactions (122). This route makes mixed metal clusters accessible, e.g.,... 

Metal carbonyl dimers have proved to be useful reagents for the synthesis of mixed-metal clusters. This is particularly true for Fe and Co clusters since Fe2(CO) and Co2(CO)8 are readily available starting materials. The prediction of the reaction products is usually fruitless. However, examination of the available data indicates that the initial dimeric unit is preserved in approximately half of the reactions. Typical examples are illustrated in Eqs. (4-6) (102), (7) (28), and (8-10) (121). 

The reaction of a carbonylmetalate with a neutral metal carbonyl has been labeled a redox condensation by Chini et al. (40, 41) and has been as widely used as a pyrolysis reaction for synthesizing mixed-metal clusters. Carbonylmetalates usually react rapidly with most neutral carbonyls, even under very mild conditions. A large number of mixed-metal hydride clusters have been formed via this type of reaction, primarily because the initial products are anionic clusters that in many cases may be protonated to yield the neutral hydride derivative. 

The reaction of carbonylmetalates with monomeric and dimeric carbonyls has yielded many mixed-metal clusters, as illustrated by the reactions shown in Eqs. (28) (15), (29) (87), (30 (88), and (31) (88). 

The reaction of [Fe(CO)J2- and [CoICOJJ- with metal carbonyl trimers has also been shown to be useful for the preparation of mixed-metal clusters, Eqs. (33) (78) and (34) (147). 

A common feature of metal clusters is their stereochemical nonrigidity, in which carbonyl and hydride ligands exchange their coordination sites. Mixed-metal clusters are ideally suited for studies of the fluxional processes in clusters because of the low symmetry inherent in their metal framework. In such clusters, the majority of the ligands are in chemically nonequivalent positions and should thus be distinguishable by NMR... 

Finally, 1-electron oxidation of [Fe2Rc(CO)i23 with tropilium bromide has been reported to give a neutral mixed-metal cluster formulated as [Fe2Re(CO)i2]2 on the basis of elemental analyses (67) however, its IR spectrum, which shows carbonyl absorptions quite similar to those of the starting material, is inconsistent with such a formulation. 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

For the past few years metal carbonyl clusters have been under study In this laboratory from several viewpoints. First, we have a continuing interest in developing better methods for the directed synthesis of mixed-metal clusters and considerable progress has been made in this area (1-6). We have more recently been evaluating the reactivity features of mixed-metal clusters with a variety of substrates. We have chosen to concentrate our efforts on one particular cluster, l FeRuj O), 1, and to examine its reactivity in as much detail as possible. 

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

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