Encapsulation of Water Cluster, meso-Helical Chain and Tapes in Metal−Organic Frameworks Based on Double-Stranded Cd(II) Helicates and Carboxylates

Four new complexes [Cd₄(bpa)₂(MeCO₂)₈]·9H₂O (1), {[Cd₄(bpa)₄(suc)₂(H₂O)₄](suc)₂·17H₂O}_n (2), {[Cd₂(bpa)₂(glu)₂]·9H₂O}_n (3), and {[Cd₂(bpa)₂(adi)₂]·11H₂O}_n (4) (where bpa = N,N‘-bis(picolinamide)azine, H₂suc = succinic acid, H₂glu = glutaric acid, and H₂adi = adipic acid) have been synthesized by the reactions of the bpa ligand and different Cd(II) carboxylates in similar conditions. Their structures consist of double-stranded helical [Cd₂(bpa)₂] structural units and various carboxylates. Each of the [Cd₂(bpa)₂] structural units in 1 is capped at its both ends by acetate-chelated Cd(II) fragments through acetate bridges into discrete tetranuclear species, which are self-assembled into two-dimensional (2D) hydrogen-bonded sheets with cavities. The 2D hydrogen-bonded sheets in 1 are threaded by one-dimensional (1D) water tapes featuring (H₂O)₄ clusters to generate an unprecedented three-dimensional (3D) hydrogen-bonded polypseudo-rotaxane architecture. The [Cd₂(bpa)₂] structural units as secondary building blocks (SBUs) are extended into metal−organic frameworks (MOFs) in 2 and 3 as 1D chains, while in 4 as 2D gridlike sheets by bridging dicarboxylate. These MOFs host meso-helical water chains, (H₂O)₃₂ water clusters, and T4(1)4(2)6(2) water tapes, respectively. All of these water morphologies have not been documented so far, and the observations indicate that an SBU and various carboxylate ligands including acetate and alkyl dicarboxylate with different lengths and flexibilities can self-assemble into diverse host frameworks from tetranuclear clusters to 1D chains and ladders, finally to 2D sheets, in which guest water molecules with different morphologies can be tuned to fill the spaces of the host MOFs.