Reaction of CuX2 (X ) NO3, Cl, (SO4)1/2) salts and Hpymo‚HX (Hpymo ) 2-hydroxypyrimidine; X ) Cl, NO3) in aqueous amine solutions (amine ) NH3, CH3NH2) leads to formation of the poorly crystalline three-dimensional open-framework coordination polymer [Cu(pymo-N1,N3)2]∞ (1). Under the same reaction conditions, but in the presence of amine and group 1 metal salts of single charged voluminous anions, highly crystalline clathrates of the type {[Cu(pymo-N1,N3)2]‚(AX)1/3}∞ (1‚AX with A ) NH4, CH3NH3, Li, K, Rb; X ) ClO4, BF4, PF6) are obtained. The X-ray crystal structure of 1‚NH4ClO4 reveals that the combination of square-planar Cu2+ ion with 120° bond angles provided by Hpymo generates a three-dimensional porous [Cu- (pymo-N1,N3)2]∞ framework with ammonium and perchlorate ions and water molecules included in the pores. 1 possesses a rich host-guest chemistry. Indeed, N2 sorption at 77 K by empty host 1 reveals its microporous nature with a BET surface area of ca. 200 m2 g-1. Hydrated 1 loses water upon heating and when exposed to moist air regenerates the original hydrated material. Likewise, 1‚NH4ClO4 loses ammonia upon heating, giving the corresponding activated acidic material 1‚HClO4 which upon exposure to gaseous ammonia regenerates 1‚NH4ClO4. In addition, 1 reversibly and selectively sorbs AClO4 salts (A ) NH4, Li, Na, K, Rb) when exposed to AClO4 aqueous solutions giving highly crystalline 1‚AClO4 clathrates. Cl-, NO3-, SiF62-, and SO42- salts are, however, not taken up. Salt sorption curves are of type V and possess hysteresis loops. This behavior agrees with the observed guest-induced structure phase change taking place in the 1 framework after guest inclusion.