A novel three-dimensional coordination polymer with a mixed-valence localized copper(I, II) dimeric unit, [Cu-2(4-pya) (3)](n) (4-pya = 4-pyridinecarboxylate), was hydrothermally synthesized via a simultaneous in-situa redox and hydrolysis reaction of Cu(II) and 4-cyanopyridine and crystallographically characterized to be a twofold interpenetrated three-dimensional coordination network with a cubic [Cu-16(4-pya)(12)] building unit.