Polymetallic clusters

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... 

In a more general context, metal carbonyls on zeolites can be a unique way to prepare highly dispersed metal catalysts. In the present work, this is especially the case for iron as no other mild methods are operative. It is expected that the method could be applied to the preparation of bi- and polymetallic catalysts even though the starting material are not bi- or polymetallic clusters, but more conveniently homometallic clusters. 

Demonstration of the critical roles of the open conformations of polymetallic clusters highlights theoretical analysis in cuprate chemistry. Polymetallic clusters in various synthetic reactions are currently attracting the attention of synthetic and mechanistic chemists alike [40, 180-183]. 

A new field of coordination chemistry is that of polymetallic cage and cluster complexes [Mm(p-X)xLJz with molecular (i.e. discrete) structure. They contain at least three metal atoms, frequently with bridging ligands X and terminal ligands L. These compounds link the classical complexes (m = 1) and the non-molecular (m - oo) binary and ternary compounds of the metals.1 Molecular polymetallic clusters (with finite radius) also provide a link with the surfaces (infinite radius) of metals and their binary compounds.2"5 Polymetallic complexes are known for almost all metals except the actinides. 

Figure 10 The core structure of the polymetallic cluster [MoFe4S6(PEt3)4Cl] ethyl groups on phosphorus atoms omitted for clarity [43,44].

Transition metal (TM) chemistry stands in contrast to this. Many compounds involve metal centres with partially filled d shells, and/or with one or several unpaired electrons. Therefore, it is not always straightforward to predict the orbital occupation pattern of a given stable compound. For intermediates on a reactive pathway, this is an even greater problem. This is also true for organometallic chemistry, despite the fact that many compounds obey the 18-electron rule and have closed-shell singlet ground states. Thus, there are many 16- or even 14-electron intermediates, odd-electron species [1], and polymetallic clusters and complexes for which the spin state is not readily predicted. 

Insufficient information exists currently for complex selective reactions, limited to phenomenological results with few electrocatalysts and reactants. The design of polymetallic clusters and of catalysts with controlled crystallite size, the exploration of redox catalysts, the tailoring of the physical catalyst structure, and the selection of reactors and operating conditions to enhance or suppress multiple reaction paths await further study. The exploitation of unconventional reduction or oxidation potential regimes for specificity control, which has been only occasionally attempted or appreciated, appears to be especially attractive. 

Sinfelt, J.H., inventor Exxon Research and Engineering Company, assignee. "Polymetallic cluster compositions useful as hydrocarbon conversion catalysts." U.S. Patent 3,953,368. 16 pages. 1976. 

Transition metal supramolecular arrays can be constructed with the aid of n-bonding interactions by using various [2n]cyclophanes and polymetallic clusters. This area is only in its infancy and its potential is illustrated by the formation of a supramolecular hexagonal two-dimensional network by using metal centers linked through the [2.2.2]paracyclophane ligand [102],... 

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