Metal complexes dendritic structure

Metal-acetylide complexes including metal-poly(yne) polymers often show unique properties [21-23]. Thus, metal-acetylide dendrimers are of interest because amplification of the functionality due to metal-acetylide units based on three-dimensional assembly with a regular dendritic structure is expected. 

Dendrimers containing metal complexes in the branches. In these compounds (Fig. lc), metal complexes may play the role of connectors along the branches of a dendritic structure as in the case of (tpy)Ru(tpy)2+ (tpy=2,2 6, 2"-ter-pyridine) [6], or may be attached to specific sites as in the case of cobalt carbonyls [7]. 

The properties induced by the dendritic framework depend on the location of the functional groups within the structure. Periphery-functionalized dendrimers offer high accessibility of the metal complex, which allows reaction rates that are... 

Pyridine-based ligands which have been used for dendrimers are 2,2-bipyridine (bpy) 17,2,3-bis(2-pyridyl)pyrazine (2,3-dpp) 18 and its monomethylated salt 19, and 2,2 6, 2"-terpyridine 20. Their transition metal complexes possessing dendritic structures were first reported in the collaborative work of Denti, Campagne, and Balzani whose divergent synthetic strategy has led to systems containing 22 ruthenium centers. - The core unit is [Ru(2,3-dpp)3] 21 which contains three... 

Synthesis of dendritic structures with a metal complex as core can proceed in two different ways ... 

A typical method for fabricating multiple complex layers is illustrated in Figure 2.11,12 First, an Au/mica or Au/ITO plate is immersed in a chloroform solution of tpy-AB-SS-AB-tpy (tpy=2,2 6, 2" -terpyridyl), providing Au-S-AB-tpy SAM on the plate. In the case of connecting the Fe(II) ion, the tpy-terminated plate is immersed in 0.1 M Fe(BF4)2 aq or (NH4)2Fe(S04)2 aq to form a metal complex. Subsequently, the metal-terminated surface is immersed in a chloroform solution of the ligand Lj or L2 to form a bis(tpy)iron structure (Fig. 2b). The latter two processes are repeated for the preparation of multilayered bis(tpy)iron (II) complex films with linear structures. When L3 is used instead of Lj or L2, the resulting molecular wires have a dendritic structure (Fig. 2c). 

Brunner s concept of attaching dendritic wedges to a catalytically active metal complex represented the first example of asymmetric catalysis with metal complex fragments located at the core of a dendritic structure [5,6]. Important early examples of catalysts in core positions were Seebach s TAD-DOL systems (TADDOL = 2,2-dimethyl-a,a,a/,a/-tetraphenyl-l,3-dioxolane-4,5-dimethanol) [38,39]. In general, the catalytic performance of such systems was either unchanged with respect to the simple mononuclear reference system or significantly lower. In no case has the potential analogy of this core fixation and the existence of efficient reactive pockets in enzymes been vindicated. This may be due to the absence of defined secondary structures in the dendrimers that have been employed to date. 

Welded microstructures can be extremely complex and often change drastically over a very short distance. The fusion zone or weld metal is a dendritic structure that has solidified from a molten state. Bordering the fusion zone are transition, unmixed and partially melted zones, and the heat-affected zone (HAZ). These zones can be reheated and altered by subsequent weld passes, in multipass welding. For alloys with structures that depend strongly on thermal history, such as steels, the final microstructure can be extremely complex. Since welded structures are often quite susceptible to corrosion, overalloyed filler metals are often used to enhance the weld corrosion resistance. For stainless steels with sufficiently high carbon content, sensitization in the HAZ is another major problem. (Frankel)5... 

A large number of covalently linked systems are currently being synthesized and investigated, differing in the nature of A, B, and L, as well as in the number of functional units in the supramolecular system (nuclearity). It is common to call simple two-component donor-acceptor systems such as that of Eq. 2 dyads , and progressively more complex systems triads , tetrads , pentads , etc.. Systems where all the A and B units are organic molecules are dealt with in Chapter 1 of this section. The present chapter deals with systems where at least one of the A/B functional units is a transition metal coordination compound. From this definition, however, are excluded (a) systems where A and/or B are porphyrins or related species (dealt with in Chapter 2) and (b) systems of high nuclearity with dendritic structures (dealt with in Chapter 9). 

As far as electron transfer properties directly involving dendrimers are concerned, it can be generally considered that these reactions may be observed whenever the macromolecular structure contains one or more units featuring redox levels at accessible potentials. The first dendrimers prepared were purely organic macromolecules, with no unit appropriate for electron transfer reactions. Later, however, the introduction of metal and organometallic complexes in the dendritic structure opened new possibilities to the chemistry of dendrimers. Indeed, the incorporated metal units exhibit important properties such as absorption and emission of visible light (relevant for the construction of antenna systems see Volume V, Part 1, Chapter 7) and redox levels at accessible potential, which are necessary for electron transfer reactions. Successively, purely organic electroactive units have also been used to functionalize the dendrimers. 

Electrochemical characterization has been performed in acetonitrile solution on a great variety of dendrimers built with metal complexes as branching centers in the whole structure [16]. An interesting example is that represented by the decametallic dendrimers 30-32, where the metal centers are linked by bridging ligands made of two, closely connected bis-chelating units such as 2,3-dpp (2,3-dpp = 2,3-bis(2-pyridyl)pyrazine) [76, 78]. In the dendritic species, each mononuclear component brings its own redox properties, more or less affected by intercomponent inter-... 

Positive influence of scandium has a complex character. Thus, the additions of Sc not only increase the yield stress and the ultimate tensile strength, but improve also the workability of alloy and its production ductility, decrease the grain size in as-cast condition, eliminate the dendrite structure and lead to the formation of a fine-grained equiaxial structure, sharply increase the recrystallization temperature (Tab.2), improve metal weldability and raise its corrosion resistance. 

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