Multimetallic Complex Composed of Redox-Active Bridging Quinonediimine Ligand

A (pi)-conjugated quinonediimine ligand possessing two terminal redox-active ferrocenyl groups, N,Nʹ-bis{(4ʹ-ferrocenylcarboxamide)phenyl}-1,4benzoquinonediimine (Lox), was synthesized. The quinonediimine ligand Lox was reduced to Lred composed of the phenylenediamine moiety on treatment with hydrazine hydrate. The reaction of Lox with the palladium(II) complex 1 bearing the N,Nʹ-bis(2-phenylethyl)-2,6-pyridinedicarboxamide ligand afforded the multimetallic complex 2. Variable temperature 1H NMR studies of 2 indicated the existence of syn and anti isomers, in which the anti isomer is enthalpically more favorable than the syn isomer in CD2Cl2 by 0.9 kcal mol^-1, but entropically less favorable by 2.6 cal mol^-1 K^-1 from the van’t Hoff plot. The redox function of the quinonediimine moiety is modulated by complexation with 1. The complex 2 showed three separate redox waves, which are assignable to the successive one-electron reduction of the quinonediimine moiety and oxidation of the two terminal ferrocenyl groups. Chemical reduction of 2 in CH2Cl2 with [Co(Cp)2] resulted in the appearance of ESR signals with weak 105Pd coupling centered around g = 2.004. The unpaired electron appears to locate mostly on the quinonediimine moiety with some delocalization onto the palladium center.