An anionic heptacopper(II) oxo-cluster {Cu-7} was obtained from a simple reaction in aqueous solution without heating. In a similar reaction, another cage compound {Sb-12} with an encapsulated {Cu2} core was produced, implying the structural versatility of copper(II)-oxide assemblies constructed from potassium antimonyl tartrate as the ligand in aqueous solution. The ferromagnetic S = 7/2 ground state of {Cu-7} is confirmed by magnetization measurements.