Ligand-Protecting Strategy for the Controlled Construction of Multinuclear Copper Cores within a Ring-Shaped Polyoxometalate.

The ring-shaped polyoxometalate (POM) [PWO] contains a large cavity and is an attractive inorganic multidentate ligand for accumulating metal cations. Until now, several multinuclear metal cores are constructed within the {PW} framework in aqueous solvents. However, it is still challenging to control the number and arrangement of introduced metal cations because of the numerous coordination sites inside the {PW} framework. In this study, we developed a novel approach for the selective synthesis of several multinuclear copper-containing ring-shaped POMs in organic solvents using methoxy groups as organic protecting ligands for the reactive coordination sites. Reacting a tetra--butylammonium (TBA) salt of [PWO] () with 4 and 8 equiv of copper(II) acetate in the presence of methanol (MeOH), tetra- and octacopper cores were incorporated into the cavity to form TBAH[Cu(HO)PWO(OCH)]·28HO·3CHNO () and TBAH[Cu(HO)PWO(OCH)]·24HO·CHCN (), respectively. For both structures, methoxy groups served as protecting ligands and temporarily inactivated vacant coordination sites. Without MeOH, dodeca- and hexadecacopper cores were constructed inside the cavity to form TBAH[Cu(HO)PWO]·4HO () and TBAH[Cu(OH)(HO)PWO]·12HO·CHO (), respectively. The arrangement of copper ions on the same {PW} units could be controlled by the input number of copper ions. Moreover, all four POMs could be synthesized from by the stepwise addition of 4 equiv of copper(II) acetate, clarifying the introduction process.