Dendritic polyallyl and polyferrocenyl bipyridine ligands: Synthesis, MALDI-TOF characterization and ruthenium(II) complexation studies

A series of 6- and 18-armed dendritic polyallyl- and polyferrocenyl-containing bipyridine ligands were synthesized through the coupling reaction of 4,4′-bis(bromomethyl)-2,2′-bipyridine with AB₃ and AB₉ dendrons. All these bipyridine ligands were successfully characterized using standard physico-chemical techniques as well as MALDI-TOF mass spectrometric analysis. The complexation studies of these ligands toward RuCl₂(bpy)₂ indicated that, in contrast to the bulky 18-ferrocenyl bipyridine ligand 7, the 6-allyl 4 and the 18-allyl 5 bipyridine ligands react with Ru(bpy)₂Cl₂ to give the corresponding ruthenium(II) complexes 9 and 10. In the case of ligand 7, the steric bulk of the two nonaferrocenyl wedges at the 4,4′-position of the bipyridine moiety prevents the conversion of the transoid structure of the ligand to the cisiod structure needed for chelation to the metal. Thus, the 18-ferrocenyl ruthenium(II) dendrimer was not obtained. Metallodendrimers 9 and 10 have been characterized by a combination of analytical methods, especially MALDI-TOF mass spectrometric techniques. The hydrogenation of the 6-allyl ruthenium(II) dendrimer 9 in the presence of Pd/C catalyst gave the expected n-propyl complex 11. This reaction constitutes a new way for the direct synthesis of alkyl bipyridine metallodendrimers. The coordination of the alkene dendritic bipyridine ligand to the metal before the catalytic hydrogenation is absolutely necessary, because of their poisoning effect for the catalyst.