At present there are at least two challenges that must be addressed for larger and more complex systems to be realized. First, single crystals of large molecules are difficult to obtain, thus precluding their full structural characterization; second, design of rigid entities that maintain their structure in the absence of guests in order to allow for reversible access to the voids as well as chemical functionalization of their voids and outside surface remains largely unexplored. Given these challenges and considering our recent work on metal -organic frameworks (MOFs), where we have demonstrated the use of secondary building units (SBUs) as means to the construction of rigid networks with permanent porosity, we sought to use the paddle-wheel cluster adopted by copper(II) acetate, Cu2(CO2)4, as a rigid SBU for addressing these challenges. Significantly, the assembly of such SBUs with polytopic carboxylate linkers generated rigid porous frameworks with open metal sites where it is possible to functionalize the pores with different ligands. 10 The possible structures in which square SBUs (such as the paddle-wheel) are linked by identical links are derived from fourconnected nets in which there is a planar (or near planar) vertex arrangement where all links are equivalent (quasiregular). 11