From metal carbonyl clusters

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Supported nanoclusters made from metal carbonyl clusters are emphasized here, because there are numerous characterization data on which to base the discussion. The synthetic methods are illustrated by the following examples. 

Supported nanoclusters have been prepared by decarbonylation of neutral or anionic metal carbonyl clusters on supports. The decarbonylation chemistry is not fully understood. The chemistry accompanying removal of the CO ligands from metal carbonyl clusters on metal oxides evidently involves hydroxyl groups or water on the surface of the metal oxide. 

It must be assumed that the samples listed in Table 2 have distributions of cluster sizes, although the available methods do not provide good evidence of the distributions. It seems likely that zeolite-supported metal clusters made from metal carbonyl clusters (Table 1) incorporate more nearly uniform clusters than samples made by conventional methods from metal salts, but this suggestion is not yet tested. 

Finally, of interest to workers in catalytic processes using carbonyl clusters, some work on CO hydrogenation catalysts derived from metal carbonyl clusters (ie molybdenum promoted Rh/Si02 and Rh/Zr02) has been published . 

There are numerous reports of attempts to prepare metal clusters (as distinguished from metal carbonyl clusters) in zeolite cages, [105, 108, 123-125] most often by reduction of exchange ions in the cages or by decarbonylation of metal carbonyl clusters. One of the challenges has been to confine the resultant clusters within the cages, and often the literature reports have failed to provide suffident... 

There is now ample evidence to support the premise that heterogeneous catalysts derived from metal carbonyl clusters can display fundamentally different behaviour from that observed with conventially prepared... 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

Neutral metal carbonyl clusters exemplified by Ir4(CO)i2, IrelCOlie, and Rh6(CO)i6 are adsorbed intact from solution (e.g., n-pentane) onto more-or-less neutral supports such as y-Al203 or Ti02. The clusters on these supports can often be extracted intact into solutions such as tetrahydrofuran. 

In contrast, when neutral metal carbonyl clusters are adsorbed on basic supports such as MgO or La203, surface anions typically form (e.g., [HIr4(CO)ii] and [IrslCOlis] from Ir4(CO)i2 and Ir6(CO)i6, respectively). 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

The field of surface-mediated synthesis of metal carbonyl clusters has developed briskly in recent years [4-6], although many organometallic chemists still seem to be unfamiliar with the methods or consider themselves ill-equipped to carry them out. In a typical synthesis, a metal salt or an organometallic precursor is brought from solution or the gas phase onto a high-area porous metal oxide, and then gas-phase reactants are brought in contact with the sample to cause conversion of the surface species into the desired products. In these syntheses, characteristics such as the acid-base properties of the support influence fhe chemisfry, much as a solvenf or coreactant influences fhe chemisfry in a convenfional synfhesis. An advanfage of... 

Good images indicating nearly uniform clusters of other metals are lacking, but evidence from EXAFS spectroscopy, combined with IR spectroscopy and extraction of clusters into solution, has provided a basis for structure determination of a number of small metal carbonyl clusters and clusters formed by their decarbonylation. Compilations of these are reported elsewhere [6,12,26]. 

These types of clusters represent some of the more modest sizes and geometries detected in homo- and hetero-metal carbonyl clusters. From dimetallic up to pentadecametallic clusters have been defined by crystal structures, and assembly of the metal centers in these clusters adopt a number of well-defined arrangements.83 Redox activity in these polymetallic clusters is anticipated and has been observed. Routes to large carbonyl polymetal clusters have been reviewed 83,84... 

Nakabayashi, M., M. Yamashita, and Y. Saito, Preparation of size-controlled ruthenium metal particles on carbon from hydro-carbonyl cluster complex. Chem. Lett., 1275-1278 (1994). 

General Methods. Methanol used in kinetic runs was distilled from sodium methoxide or calcium hydride in a nitrogen atmosphere before use. Freshly distilled cyclohexanol was added to the methanol in the ratio 6.0 ml cyclohexanol/200 ml MeOH and was used as an internal standard for gas chromatographic (GC) analysis. Benzaldehyde was distilled under vacuum and stored under nitrogen at 5°. Other aldehydes (purchased from Aldrich) were also distilled before use. The corresponding alcohols (purchased from Aldrich) were distilled and used to prepare GC standards. All metal carbonyl cluster complexes were purchased from Strem Chemical Company and used as received. Tetrahydrofuran (THF) was distilled from sodium benzophenone under nitrogen before use. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Mingos rules. This hypoelectronicity or electron poverty (fewer than the Wade-Mingos 2n + 2 skeletal electrons) in the bare metal cluster anions Enz (z < n + 2) leads to deltahedra not only different from those in the deltahedral boranes but also different from those in hypoelectronic metal carbonyl clusters of metals such as osmium. 

A second feature of metal halide cluster chemistry is that the early transition metals are more prone to form metal -metal bonds than are the later noble metals and coinage metals. Again the polynuclear metal carbonyls differ in this facet of metal-metal bond behavior, and, in fact, metal carbonyl clusters become more common on going from the left to the right of the Periodic Table. 

Finally, Basset and co-workers (88j) report that impregnation of alumina with metal carbonyl clusters leads to CO reduction through initial H2 formation from CO + OH (or adsorbed water) followed by catalyzed reaction on the surface. Above 250°C, the selectivity of the reaction toward methane formation increases greatly, but so does decomposition of the surface bound clusters. In all cases about half of the carbon monoxide was converted to C02 as would be expected for the production of H2 reducing equivalents. 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

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