Exploring the programmable assembly of a polyoxometalate-organic hybrid via metal ion coordination

The conformational flexibility and programmed assembly of a dumbbell-shaped polyoxometalate-organic hybrid molecule comprising two Dawson-type polyoxometalates linked by a 2,2'-bipyridine unit, which can be coordinate to metal ions, in this case of Zn(2+), are described. SAXS, UV/vis, and NMR spectroscopic techniques confirm that the hybrid molecules exist as the trans dumbbell in metal-ion-free solutions and can be reversibly transformed into the cis dumbbell through coordination upon the addition of ZnCl2 into a DMSO solution containing the hybrid. Subsequent addition of EDTA reverses the switching process by extracting the Zn(2+) cations from the hybrid. During the interchange process between trans and cis dumbbells, a further reorganization of the hybrid molecules occurs through bond rotation to minimize steric clashes between the polyoxometalate subunits, in order to stabilize the corresponding dumbbell conformation. The Zn(2+)-controlled conformational transformation of the hybrid can be further utilized to manipulate the hybrid's solvophobic interaction-driven self-assembly behavior in the metal-ion driven reversible formation of 140 nm sized vesicles, studied by laser light scattering techniques.