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 [Pd3(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.