Molecular tricorns: self-assembly of trinuclear palladium(II) complexes

Cyclometalation of the ligand 1,3-bis(1-alkylbenzimidazol-2-yl)benzene (1) with palladium carboxylates leads to a trimeric complex [Pd(3)(ligand)(3)(carboxylate)(3)] (3). Studies in solution show that the trinuclear core is stable but that the carboxylates are labile, undergoing intra- and intermolecular exchange on an NMR time scale. The structural analogue of 1, 2,6-bis(1-alkylbenzimidazol-2-yl)pyridine (4), gives only the mononuclear species [Pd(4)(carboxylate)(2)], characterized by X-ray diffraction. This complex forms a trimer if one carboxylate is labilized by the addition of strong acid; the resulting trinuclear species is readily cleaved by nucleophiles but can include weakly basic anions within its cavity.