Porous metal-organic polyhedra: 25 A cuboctahedron constructed from 12 Cu2(CO2)4 paddle-wheel building blocks.

Construction of large molecules that are amenable to design and functionalization is of primary current interest as it represents an important step in the achievement of molecular complexity.1-8 We believe that large porous molecules serve as intriguing initial objects in this direction since the openings to their voids may be useful in regulating the selective release and binding of smaller molecules.9 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 Replacing the vertices by (in the present report) a square of vertices is a process which we have called augmentation.10e In the case of polyhedral structures the augmentation process is usually called truncation, and there are three possibilities11 for square units: The truncated octahedron with 6 squares, the truncated cuboctahedron with 12 squares, and the truncated icosidodecahedron with 30 squares. We have previously used 1,4benzenedicarboxylate (BDC) with 180° (straight) links to produce an infinite periodic structure.10c The analogous 1,3-benzenedicarboxylate (m-BDC) with 120° between functional groups is ideal for building a finite truncated cuboctahedron structure with 12 linked paddle-wheels. Here we have used this design principle toward the synthesis of large discrete molecular units. We report the synthesis of a porous metal-organic polyhedron Cu24(m-BDC)24(DMF)14(H2O)10‚(H2O)50(DMF)6(C2H5OH)6, hereafter termed a-MOP-1 (a ) anorthic ) triclinic) which is constructed from 12 paddle-wheel units bridged by m-BDC to give a large metal-carboxylate polyhedron.