Proton-induced motion in a molecular cup-and-ball zinc funnel complex

A Zn(II)-funnel metal complex built around a calix[6]arene cavity can undergo a dramatic conformational change upon a protonic stress with the use of a primary amine intermediate. In the resting state, the strong amine ligand is bound to the Zn(II) cation and the hydroxyl arm is pending out of the cavity. Addition of an acid causes the protonation of the amine and the self-coordination of the hydroxyl arm into the calix cavity. This switch, which is fully reversible and operated with sequential additions of acid and base, is reminiscent at the molecular level, of a cup-and-ball toy.