Metal carbonyl clusters synthesis

Huttner, G. Knoll, K. RP-bridged metal carbonyl clusters Synthesis, properties, and reactions, Angew. Chem., Int. Ed. Engl 1987, 26, 743 Angew. Chem. 1987, 99, 765. 

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. 

Synthesis of Metal Carbonyl Clusters and Decarbonylated Clusters... 

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. 

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. 

Synthesis of metal carbonyl clusters on oxide surfaces (followed by extraction into a solvent and workup) is occasionally a more convenient and efficient method for preparation of a metal carbonyl cluster than conventional solution chemistry. This synthetic strategy offers the green chemistry advantage of minimizing solvent use, as the reaction often occurs in the absence of solvent. 

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... 

Synthesis methods such as those described earlier for monometallics have been applied with metal carbonyls incorporating two metals. The resultant supported species may be small supported metal clusters [41,42], and, as for monometallics, the usual products are supported species that are nonuniform in both composition and structure [42]. There are several examples of well-defined metal carbonyl clusters in this category but hardly any examples of well-defined decarbonylated bimetalhcs on supports. 

Figure 1. Ship-in-a-bottle synthesis of metal carbonyl clusters in NaY zeolite.

Fe(CO)s], [Fe2(CO)g], [Co2(CO)8] and [Os3(CO)i2]) have been reacted with dicyanobenzene to form intrazeolite [M(Pc)] complexes [140]. Another class of materials prepared by the intrazeolite template synthesis method has been mixed ligand metal carbonyls and metal carbonyl clusters, frequently by reductive car-bonylation of metal ions in zeolite cages [175]. However, because these are frequently decomposed in situ to form, for example, nanoparticles, they are outside the scope of this chapter, and will be considered here only when they are used as precursors for metal complexes. 

Recently, Wade has pointed out a formal analogy between the electronic structures of carboranes and polyboranes and those of metal carbonyl clusters based on the assumption that certain triangulated polyhedra require the same number of skeletal orbitals whether there are BH or CH units as well as transition metal atoms at their apices (120). This assumption is quite reasonable as the synthesis of a large number of polyboranes and carboranes in which a transition metal atom takes the place of a BH skeletal unit may be carried out (70). 

An alternative to this physical method of preparing structurally uniform metal clusters on supports involves chemistry by which molecular metal carbonyl clusters (e.g., [Rh6(CO)i6]) serve as precursors on the support. These precursors are decarbonylated with maintenance of the metal frame to give supported nanoclusters (e.g., Rh6). Advantages of this chemical preparation method are its applicability to many porous supports, such as zeolites (and not just planar surfaces) and the opportunities to use spectroscopic methods to follow the chemistry of synthesis of the precursor on the support and its subsequent decarbonylation. Zeolites, because their molecular-scale cages are part of a regular (crystalline) structure, offer the prospect of regular three-dimensional arrays of nanoclusters. 

Molecularly or ionically dispersed metal carbonyl clusters on metal oxides have been prepared in high yields by reaction of metal carbonyl clusters with support surfaces or by syntheses on support surfaces from mononuclear precursors (Gates and Lamb, 1989 Iwasawa, 1993 Ichikawa, 1992 Gates, 1994). Synthesis of supported metal carbonyl clusters has been reviewed recently (Gates, 1995,1998), and only a few examples are included here. 

Another class of synthesis reaction is deprotonation of a hydrido metal carbonyl cluster on a basic surface. For example, [H4Os4(CO)i2] reacts with MgO or with y-Al203 to give [H3Os4(CO)i2], which is part of a surface ion pair on the support (Budge, Scott, and Gates, 1983). 

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. 

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