[Cu(4-oxopyrimidinate)2*nH2O]: a robust sodalite type metal-organic framework exhibiting a rich host-guest chemistry

Reaction of Cu2+ salts with 4-hydroxypyrimidine (4-Hpymo) in water:ammonia (9:1) solutions at room temperature leads to formation of either [Cu(4-pymo)2(NH3)2(H2O)2] (1) or [Cu(4-pymo)2·nH2O]∞ (2), depending on which crystal nucleation process occurs. Selective formation of 2 is possible by using non-coordinating Et3N as a base. X-ray diffraction analyses have been performed in both cases showing that 1 is a mononuclear compound in which the heterocyclic ligands monodentately coordinate the copper ions through the N1 nitrogen atom. 2 is a 3D sodalite type open framework, in which each 4-pymo ligand bridges two copper ions through both nitrogen atoms in the N,N′-exobidentate mode. Heating 1 at 110 °C in air generates an amorphous phase (2a), which shows the same chemical analysis and spectroscopic properties as dehydrated 2. Crystalline [Cu(4-pymo)2·nH2O]∞ (2), possesses interesting physico-chemical properties related to its porous nature. Indeed, this material reversibly absorbs N2 and water vapour with minimal structural changes.