Further Polynuclear Systems

Dietrich-Buchecker, B. Frommberger, I. Luer, J.-P. Sauvage and F.Vogtle, Angew. Chem., Int. Ed. Engl, 1993, 32, 1434. 

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A further, drastic step towards increasing structural complexity can be made by going from mononuclear to polynuclear complexes. These are systems in which two (or more) metal complex subunits are connected by one (or more) bridging ligand(s) (Fig. 1). Because of this additional complexity, the photophysics of polynuclear complexes is expected to be different from, and more interesting than, that of simple mononuclear species. This is true, in principle, also from the photocatalytic point of view, where the additional possibilities of polynuclear systems, in particular with respect to multielectron redox processes and inter-component energy or electron transfer, could find applications. 

ECL investigations of dinuclear or polynuclear Ru(II) complexes have been recently performed with hope for developing more efficient electrochemiluminescent materials. Centrally or peripherally functionalized dendrimers with active RuL32 + chelate units can produce higher (up to four to five times) ECL intensities as compared to their monomeric RuL32 + precursors alone. It was also found that the ECL intensities of metallodendrimers become larger as the multiplicity of the involved Ru(II) units increases. Similar observations have been reported for binuclear Ru(II) complexes with weak interaction between both metallic centers.84-88 These results indicate that further studies in such direction may result in design of still more efficient ECL systems based on Ru(II) luminophores. 

A number of the above complexes may be alternatively considered as macrocyclic ligands or compartmental ligands, but as the emphasis has been primarily in terms of the local copper(I) stereochemistry and the polynuclear nature of the complexes, they have been included above. As there is no crystallographic data on biological copper(I) systems, this section will have to await the further refinement of the structure of Panutirus Interruptus hemocyanin.353... 

A ternary catalyst system based on Nd alcoholate provides one of the first examples in which the active species could be isolated and analytically characterized [620]. The preformation of Nd(0 Pr)3/TEA/DEAC (molar ratio = 1/10/1.5) results in a crystalline precipitate. Further characterization of single crystals gives evidence for a polynuclear Nd - A1 bimetallic complex in which Nd atoms are coordinated by Cl, alcoholate and alkyl ligands. 

The RhCl2(DMF)(py)2(BH4) system reported by Jardine and McQuillin [94] and described in the previous section, is also capable of hydrogenating quinoline under mild conditions. Surprisingly, this catalyst, discovered 30 years ago has not been further exploited or studied since then. Metal carbonyls were also found to display some hydrogenation activity for a variety of polynuclear N-heterocycles under Hj, H2/CO or CO/HjO [49, 50, 61] the heterocyclic ring was regioselectively hydrogenated in all cases were activity was observed, but conditions were rather harsh and turnover frequencies relatively low. This makes this type of catalyst unattractive for either practical uses or mechanistic studies. 

The authors conclude that [2.2.3]cyclazine (219) is intermediate in aromaticity between the polynuclear aromatic hydrocarbons and peripheral conjugated aromatic compounds involving delocalized 10 tt or 14 tc electrons. They also state that the resulting [2.2.3]cyclazines and benzo[2.2.3]-cyclazines are strongly dlatropic and claim that further theoretical study is required before the details of these novel conjugated systems can be fully understood. 

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