Synthesis and Structures of the Capped and Sandwiched Cobalt Heptamolybdate Polyoxometalate (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} • 6 H2O and Its Extended Structure (NH4)6{[Co(H2O)2]3[Mo7O24]2}