Bipyridine oligomers

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

FIG. 38. a-D-GalNAc 2,2 -bipyridine oligomers self-assembled around a copper(II) salt.277... 

The conventional Sonogashira coupling has been used to construct bipyridine oligomers in which a thiophene ring is attached to the bipyridine through an acetylene link, for example, 96 <200581169>. 

Figure 143. Formation of double-stranded helicates from bipyridine oligomers.

Layer-by-layer Ru3 cluster-based multilayers were fabricated onto preorganized self-assembled monolayer gold electrode surfaces by Abe et al. [15], in which [Ru3(q3-0)( i-0Ac)6(4,4/-bpy)2(C0)] was utilized as the synthetic precursor. The stepwise connection of oxo-centered triruthenium cluster units onto the gold electrode surface is a feasible approach for construction of Ru3 cluster-based oligomers on a solid surface, in which the bridging ligand 4,4 -bipyridine appears to mediate weak cluster-cluster electronic interaction between the Ru3 cluster centers. 

Pyridine-pyridine connections via a Stille reaction have been well precedented for the syntheses of bipyridine, terpyridine, tetrapyridine and other oligomers of pyridine [73-75]. These reactions are exemplified by the synthesis of tetrapyridine 84 [75] from terpyridyl chloride 83 and 2-tributylstannylpyridine. 

During the 1980s several laboratories prepared and investigated double-stranded helical complexes, systems containing either pyirolic ligands [75, 76] and derivatives [77-79] (with Zn2+, Ag+, Cu+) or oligomers of 2,2 -bipyridine [80, 81]. Helicates [80-84] may consist of up to five copper centers and these systems are reminiscent of the DNA double helix. 

Organometallic macro cycles and cyclic polymers were prepared by the photolytic ring opening of a silicon-bridged ferrocenophane with a bipyridine initiator. The relative amounts of cyclic oligomers and cyclic polymer, as well as the molecular weight of the cyclic polymer, can be controlled by the reaction temperature [228]. 

Recently, Angelescu et a/.[92] have studied the activity and selectivity for dimerization of ethylene of various catalysts based on Ni(4,4-bipyridine)Cl2 complex coactivated with A1C1(C2H5)2 and supported on different molecular sieves such as zeolites (Y, L, Mordenite), mesoporous MCM-41 and on amorphous silica alumina. They found that this type of catalyst is active and selective for ethylene dimerization to n-butenes under mild reaction conditions (298 K and 12 atm). The complex supported on zeolites and MCM-41 favours the formation of higher amounts of n-butenes than the complex supported on silica alumina, which is more favourable for the formation of oligomers. It was also found that the concentration in 1-butene and cw-2-butene in the n-butene fraction obtained with the complex supported on zeolites and MCM-41, is higher compared with the corresponding values at thermodynamic equilibrium. 

While the formation of the palladium species was shown to be fast (and in better than 90% yield), the kinetically less labile platinum(II) ion initially gave a mixture of oligomers however, these kinetic products slowly rearranged (over a month) to the required thermodynamic product 9. Models predict that free rotation of the 4,4 -bipyridine units is restricted such that they will prefer to be orientated perpendicular to the plane of the square such an arrangement has been confirmed for the above tetra-palladium structure. ... 

The bis(ethylthio) species (83a,b) may be mono-oxidized with MMPP or m-CPBA to afford the thiones (82a,b). If these species are treated with methylmagnesium bromide, a ring-coupling reaction occurs to provide (83b,c) in yields of 56 and 3%, respectively (Scheme 15). Alternatively (82a,b) may be treated with 6-lithio-2,2 -bipyridine to provide the oligomers (84a,b).88 A similar reaction has been used to prepare a chiral quaterpyridine derivative employing the mono-sulfone of (84a).89... 

Figure 53. Cisoid— transoidequilibrium of 2,2 -bipyridine and the structure of oligomer series 11 (n = 2, 5, 8, 9, 12).

As shown in Scheme 11.Id, these polymers consist of the main backbone of (i) a nonconductive polymer (25-27), or a polymeric ionomer (29) or (ii) a backbone of an ECP (28) to which pendant, localized redox-centers, such as ferrocene (Fc), bipyridine-complexes of Ru, Os, and so forth, or even low-molecular-weight thiophene oligomers, are covalently attached (25, 27, and 26, respectively). Covalent attachment is characteristic of the structure 28, whereas 29 contains a typical electrostatic bond between the electroactive bipyridine-complex of Ru and the polymeric ionomer s backbone. 

Treatment of 6,6 -dibromo-2,2 -dipyridine with phenylphosphine via Pd-promoted crosscoupling conditions gave a series of linear oligomers with terminal bromides as well as the cyclic dimer 24, in which the bipyridine moieties are held in the cis configuration by means of the two phenylphosphine A-oxide bridges <05JOC9835>. 

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