Synthesis of quasi-linear and segmented bis- to penta-2,2′-bipyridine polytopic ligands built via a convergent approach

Reliable and practical routes for the preparation of segmented oligomeric 2,2'-bipyridine-based ligands possessing rigid and conjugated spacers are presented. The first series of ligands bears a single alkyne function as a bridge and has been built by Pd(0)-catalyzed cross-coupling reactions between ethynylated and bromo-substituted derivatives of a,2'-bipyridine (bipy). These new ligands provide access to numerous hexameric, octameric, and decameric pyridine-based materials. Optimum conditions were found with [Pd(PPh3)(4)] (6 mol %) in benzene containing diisopropylamine at 80 degreesC. The second series of soluble ligands was synthesized around a 1,4-diethynyl-2, 5-didodecyloxybenzene bridging unit. The synthesis required a protection/deprotection methodology, as well as a chemioselective palladium-catalyzed Sonogashira-Hagihara cross-coupling protocol to obtain the target multitopic ligands. Within this strategy, the pivotal 15b, 17, and 19b intermediates bearing one or two bipy and phenyl units are required and such entities have been isolated in excellent yield. The products are highly soluble and photostable. In each case, the final step involves a double cross-coupling reaction between the appropriate constituents, with the best preparative conditions involving [Pd(PPh3)(4)] (6 mol %) in n-propylamine at 70 degreesC. The main advantage of this methodology lies in its synthetic versatility and adaptability for creating multitopic metal-binding scaffolds with a potentially large variety of bridging units and phenyl substituents. Spectroscopic data for the new oligomers show a steady decrease in optical energy with an increasing degree of oligomerization. The different results obtained with these ligands highlight the importance of the rigid 1,4-diethynylphenyl linker in directing the outcome of the nanosized molecules.