Syntheses of p-phenylene- and p-biphenylylene-bridged methylated binuclear ferrocenes

p-Phenylene and p-biphenylene-bridged methylated cyclopentadienyl rings were prepared. Reaction of the dilithio salts of the ligands with (η5-C5Me5)Fe(acac) provides cleanly ferrocenes bridged by p-phenylene and p-biphenylene groups. The electrochemistry of these compounds shows that a mixed-valence state is stabilized only in the p-C6H4-bridged species; if the bridge is extended to two aromatic rings, the individual ferrocenes do not interact. Methyl substitution on the ring increases the interaction between the metals centers as shown for (η5-C5H5)Fe[(η5-C5Me4)C6H4(η5-C5Me4)]Fe(η5-C5H5), in which the mixed-valence state is less stable as compared with the analogous, fully methylated compound.