Dendrimers organometallic complexes

The alkynyl-metal (metal-acetyhde) complex is one of the best building blocks for organometallic dendrimers, since it has some advantages compared to other organometallic complexes [18]. Most of the metal-acetylide complexes are thermally robust and stable, even when exposed to air and moisture. Metal-acetylide complexes are fairly accessible in high yields by well-established synthetic methodology [19]. These features are essential to the construction of dendrimers. 

In periphery-functionalized dendritic catalysts, the functional groups at the surface determine the solubility and miscibility and thus the precipitation properties. Many dendrimers functionalized with organometallic complexes do not dissolve in apolar solvents, and the presence of multiple metal centers at the periphery facilitates precipitation upon addition of this type of solvent. It is emphasized that the use of dendrimer-immobilized catalysts with the goal of recovery through precipitation is worthwhile only if the tendency to precipitation of the dendritic system exceeds that of its non-dendritic equivalent. 

Dendritic polymers can be covalently functionalized with organometallic complexes to obtain a dendritic catalyst with molecularly defined catalytic sites [5-7]. Moreover, a considerable number of reports on the applicability of functionalized dendrimers in catalysis have led to the idea of a dendritic effect on the catalyst activity/selectivity, which can either be positive or... 

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. 

McDonagh, A.M., Humphrey, M.G., Samoc. M., Luther-Davies, B. Organometallic complexes for nonlinear optics. 17. Synthesis, third-order optical nonlinearities, and two-photon absorption cross section of an alkynylruthenium dendrimer. Organometallics 18, 5195-5197 (1999)... 

One of the first applications of dendrimers as organometallic hosts was their use as enantioselective catalysts. Indeed, dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes. Brunner introduced the term dendrizymes for core-functionalized transition metal catalysts which might be used in enantioselective catalysis. The dendrimeric organometallic complex shown in Figure 34 is an example of such a dendrizyme inside which the chiral dendritic branches create a chiral pocket around the transition metal. 

Dendrimers with Cores Containing Transition Metal or Organometallic Complexes... 

Samoc M, Corkery TC, McDonagh AM, Cifuentes MP, Humphrey MG (2011) Organometallic complexes for non-linear optics. 49. Third-order non-linear optical spectral dependence studies of arylalkynylruthenium dendrimers. Aust J Chem 64(9) 1269-1273... 

Koprowski M, Sebastian RM, Maraval V, Zablocka M, Cadierno V, Donnadieu B, Igau A, Caminade AM, Majoral JP (2002) Iminophosphine palladium complexes in catalytic Stille coupling reactions from monomers to dendrimers. Organometallics 21 4680 687... 

Metal-acetylide complexes have been used as a unit of organometallic polymers that have metallic species in the main chain [20]. Representative examples are metal-poly(yne) polymers (19) of group 10 metals depicted in Scheme 5. These polymers are easily prepared from M(PR3)2Cl2 (M=Pt, Pd) and dialkynyl compounds catalyzed by Cu(I) salts in amine. Recently, this synthetic method was successfully applied to the construction of metal-acetylide dendrimers. 

Metal ions within organometallic dendrimers can be incorporated at the core, in the branches, or at branch points. Examples of dendrimers having metal-ion-containing cores included the dendritic metalloporphyrins [66,67] and related materials reported by Aida and Enomoto [68],Diederich et al. [69], Moore et al. [70], and Erechet et al. [71], dendritic terpyridine-ruthenium complexes report-... 

By employing coordination complexes as branch points, dendrimers can be synthesized that contain metal ions throughout their structure. The repetitive unit of such dendrimers contains M-C, M-N, M-P, or M-S bonds [53,62]. The metal ions act as supramolecular glue [63], in which the complexation chemistry directs the assembly and structure of the dendrimer [53]. One of the synthetic procedures used to prepare organometallic dendrimers with coordination centers in every layer is based on a protection/deprotection procedure in which two complexes are used as dendritic building blocks wherein one acts as a metal and the other as a ligand [64,76]. 

As mentioned in the previous section, there are good reasons to search for new reaction conditions for Heck and related reactions, which permit catalyst recovery, the use of less toxic solvents, and simpler product recovery. The use of liquid or supercritical (SC) CO2 addresses all of these issues [171]. Until recently, however, the use of supercritical COj had been limited to organometallic Pd complexes functionalized with perfluorinated ligands [172-174], due to the limited solubility of metal colloids in CO2, and often required the use of water as a co-solvent [175]. The work described here shows that dendrimers can be used to solubilize Pd nanoclusters in liquid and SC CO2. This new finding opens the door to the combined benefits of a catalyst that promotes Heck couplings, but without the need for toxic ligands or solvents. 

---