The rapid growth in the area of metal-organic frameworks (MOFs) continues to provide open structures with interesting compositions, architectures and properties.2,3 However, this progress, although significant, has not witnessed many discussions on the relationship between porosity and interpenetration of open frameworks; a topic addressed here. We have recently used a synthetic strategy utilizing secondary building units (SBUs) for achieving stable, highly porous, and functionalized open networks.4-6 Here, the extended 3-D framework of crystalline MOF9, Tb2(ADB)3[(CH3)2SO]4‚16[(CH3)2SO], illustrates another aspect of SBUs: namely, their ability to support the existence of large free volume in interpenetrating structures, which thus far have had the propensity to form assemblies containing very little or no free volume.2 In an effort to prepare open networks having Ln2C6O12 core units (already known in molecular clusters)7 as SBUs, we copolymerized the long dicarboxylate linker ADB with Tb (III) ions:1b A mixture of Tb(NO3)3‚5H2O (35 mg, 0.08 mmol) and H2ADB (32 mg, 0.12 mmol) was dissolved in dimethylsulfoxide (DMSO) (10 mL).8 Diffusion from a solution mixture containing.