Zinc– and palladium–porphyrin based turnstiles

The design and synthesis of two new Zn(II) and Pd(II) strapped-porphyrin based molecular turnstiles have been achieved. These molecules are based on a porphyrin backbone as a stator and a handle as a rotor. The junction between the two parts is ensured by covalent bonds using two opposite meso positions. Both compounds have been characterised in solution by NMR spectroscopy and in the solid state by X-ray diffraction on a single crystal. The dynamic of the two systems have been investigated in solution by 1- and 2-D NMR techniques such as ROESY. The switching between the open and closed states is described. Whereas for the Pd(II) complex, the handle freely rotates around the stator, for the Zn(II) analogue owing to the presence of a water molecule bound to the zinc atom and hydrogen bonded to the pyridyl moiety of the rotor, the rotation is blocked. The unlocking of the rotational movement was achieved using an external agent, mainly dimethylaminopyridine as a strong coordinating ligand.