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 Zn2+, 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
Zn2+ 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
Zn2+-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.