A Novel Redox-Active Conjugated Palladium Homobimetallic Complex

Abstract

The π-conjugated molecule N,N′-bis(4′-dimethylaminophenyl)-1,4-benzoquinone diimine (L2) was incorporated into the palladium(II) complex 1 bearing the N,N′-bis(2-phenylethyl)-2,6-pyridinedicarboxamide ligand, to afford the redox-active conjugated palladium(II) homobimetallic complex 2. An X-ray crystal structure determination of 2 reveals that two palladium complex units are bridged by the quinone diimine moiety to form the C2-symmetrical 2:1 complex with an anti configuration, and that the bridging π-conjugated spacer moieties are aligned along a straight line almost parallel to the a axis, to form the columns of the π-conjugated molecules in the molecular packing. Variable temperature 1H NMR studies of the conjugated complex 2 indicate that the syn configuration is enthalpically more favorable than the anti configuration in CD2Cl2 by 1.0 kcal/mol, but entropically less favorable by 4.5 cal/mol from the van’t Hoff plot. The redox function of the quinone diimine moiety is modulated by complexation with the palladium complex 1. The conjugated complex 2 shows three separate redox waves assignable to the successive one-electron reduction of the quinone diimine moiety and one-electron oxidation process of the two terminal dimethylamino groups. Chemical reduction of 2 in THF with CoCp2 resulted in the appearance of ESR signals with weak 105Pd coupling centered around g = 2.0041. The added electrons are considered to be delocalized over the PdII-quinone diimine d-π* system in the complex.