Polymetallic assemblies

In addition to the more commonly observed oxo, hydroxo, and halo ligands, there are polymetallic assemblies for which rather exotic ligands are present in the cluster core. For example, a tetranuclear yttrium-aluminum-methine cluster was obtained according to the scheme shown in Figure 37 (left) (Dietrich et al., 2006a). 

Other approaches can be used to form polymetallic assemblies under thermodynamic control. One of the simplest is to exploit micellar structures that encapsulate lanthanide ions. For instance, during our early studies on lanthanide emission in the NIR, we used reverse micelles derived from sodium bis(2-ethylhexyl) sulpho-succinate to control the solvation of lanthanide ions [37]. While this approach gives great flexibility, micellar assembhes tend not to be well defined, meaning that a range of species exist in solution at any time. 

Metal-metal bonds involving platinum(ll) or palladium(ll) as a Lewis base have been known for several decades and are involved in a large variety of polymetallic assemblies, ranging from simple bimetallic compounds to high-nuclearity clusters... 

Building polymetallic assemblies with chiral components... 

The use of the unusual spacer, -hex-3-ene-l,5-diyn-diyl, has been reported recently for the construction of polymetallic assemblies (105 R= H, OMe X = H, Ru2(X-ap)4>. Voltammetry experiments were used to confirm the conjugation through the unsaturated spacer and the materials were found to show promising magnetic and (molecular wire) charge-transfer properties. 

An exciting development in dithiocarbamate chemistry is the synthesis by Beer and co-workers (325,1489) of a range of supramolecular complexes containing nickel(II), zinc(II), and copper(II) bis(dithiocarbamate) centers. These include the preparation of nano-sized resorcarene-based polymetallic assemblies, copper(II) dithiocarbamate macrocycles (62,326,1469,1470), crown-ether derivatives (1471), cryptands (492), and catenanes (544). Details of this work are given in this section and also within the copper and zinc sections (Section IV.H.l and IV.I.l). 

Mono- and polymetallic lanthanide-containing functional assemblies with heterocyclic ligands 99CSR347. 

To resolve this problem, Rickborn made an ingenious proposal that implicated the intermediacy of a bimetallic transition structure assembly v involving a bridging Znl2 molecule (Fig. 3.5). This would accommodate the needed spatial requirements of the methylene transfer process. The importance of the polymetallic re-... 

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... 

Biinzli, J.-C. G. Piguet, C. Lanthanide-containing molecular and supramolecular polymetallic functional assemblies. Chem. Rev. 2002,102,1897-1928. 

Fox OD, Cookson J, Wilkinson EJS, Drew MGB, MacLean EJ, Teat SJ, Beer PD (2006) Nanosized polymetallic resorcinarene-based host assemblies that strongly bind fullerenes. J Am Chem Soc 128 6990... 

Fig. 4.43. Self-assembly of a polymetallic contrast agent. Redrawn from V. Jacques, J.-F. Desreux, Top. Cur.

In addition to the amperometric response, a photoaction response can also be observed when electrodes coated with polymetalated porphyrin Aims are irradiated with Vis light in the presence of O2, or another suitable as an electron receptor agent. This characteristic is usually exhibited by pure M(TRPyP) Alms, as well as by the electrostatic assembled M(TRPyP)/M (TPPS) ones (125, 172, 185, 251-253,285, 383) and the electropolymerized ETRPyP modifled electrodes(53,172). 

The classical thermochemical approach has also already been applied to special template-assisted chemical reactions like the template-directed synthesis of oligonucleotides [260], for which a detailed system of kinetic elementary steps has been derived and solved. Other examples are the thermochemical model for the assessment of cooperativity in self-assembly processes proposed in Ref. [261, 262] and for the quantitative description of multicomponent self-assembly processes of polymetallic helicates [263, 264]. More thermochemical approaches applied within particular fields of supramolecular chemistry will be mentioned in the following. 

There exist a vast number of discrete, polymetallic coordination clusters both of the metal-metal bonded type and linked by a tremendous variety of bridging ligands, notably carboxylates. Such compounds are not coordination polymers but oligomers and hence can be more soluble, more well-defined and easier to characterise than coordination polymers, for which they can serve as useful model systems. We will discuss discrete, self-assembled complexes of semi-protected metal ions that act as hosts in solution or as 3D capsules in the next chapter and we will not cover these systems in detail here except to note in passing a couple of examples that are of particular interest. One particularly prominent cluster is Muij-acetate, the mixed-valence compound... 

Piguet, C., Borkovec, M., Hamacek, J., Zeckert, K., Strict self-assembly of polymetallic helicates the concepts behind the semantics , Coord. Chem. Rev. 2005, 249, 705-726. 

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