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

The ring-shaped polyoxometalate (POM) [P 8 W 48 O 184 ] 40- 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 {P 8 W 48 } 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 {P 8 W 48 } 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- n -butylammonium (TBA) salt of [P 8 W 48 O 184 ] 40- ( P8W48 ) 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 TBA 11 H 13 [Cu 4 (H 2 O) 4 P 8 W 48 O 176 (OCH 3 ) 8 ]·28H 2 O·3CH 3 NO 2 ( Cu4 ) and TBA 14 H 2 [Cu 8 (H 2 O) 12 P 8 W 48 O 176 (OCH 3 ) 8 ]·24H 2 O·CH 3 CN ( Cu8 ), 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 TBA 14 H 2 [Cu 12 (H 2 O) 16 P 8 W 48 O 184 ]·4H 2 O ( Cu12 ) and TBA 16 H 8 [Cu 16 (OH) 16 (H 2 O) 4 P 8 W 48 O 184 ]·12H 2 O·C 3 H 6 O ( Cu16 ), respectively. The arrangement of copper ions on the same {P 2 W 12 } units could be controlled by the input number of copper ions. Moreover, all four POMs could be synthesized from P8W48 by the stepwise addition of 4 equiv of copper(II) acetate, clarifying the introduction process.