Synthesis, Structural Characterization, and Water Vapor Sorption Behavior of Two Ligand Ratio-Dependent Supramolecular Networks, [Cd(2,2′-bpym)1.5(BDC)]·0.5(2,2′-bpym)·5H2O and [Cd(2,2′-bpym)0.5(BDC)(H2O)3]

Two ligand ratio-dependent supramolecular networks, [Cd­(2,2′-bpym)1.5(BDC)]·0.5­(2,2′-bpym)·5H2O (1) and [Cd­(2,2′-bpym)0.5(BDC)­(H2O)3] (2), (BDC2– = dianion of terephthalic acid and 2,2′-bpym = 2,2′-bipyrimidine) have been synthesized and structurally characterized by the single-crystal X-ray diffraction method. Structural determination reveals that compound 1 is a two-dimensional (2D) layered metal–organic framework (MOF) constructed via the bridges of Cd­(II) ions with 2,2′-bpym and BDC2– ligands, and compound 2 is a zero-dimensional (0D) 2,2′-bpym-bridged di-Cd­(II) monomeric complex. When the thermally dehydrated powders of 1 (at 100 °C) were immersed into water solution, most of the dehydrated powders of 1 underwent structural transformation back to rehydrated 1, but very little amounts of the dehydrated powders of 1 were decomposed to light-brown crystals of 2 or colorless crystals of a new coordination polymer (CP), [Cd­(2,2′-bpym)­(BDC)­(H2O)]·3H2O (3), with its one-dimensional (1D) zigzag chain-like framework being constructed via the bridges of Cd­(II) ions with the BDC2– ligand. Structural analysis reveals that all 3D supramolecular networks of 1–3 are further constructed via strong intermolecular interactions, including hydrogen bonds and π–π stacking interactions. Compounds 1 and 2 both exhibit significant water vapor hysteresis isotherms, and their cyclic water de-/adsorption behavior accompanied with solid-state structural transformation has been verified by de-/rehydration TG analyses and powder X-ray diffraction (PXRD) measurements.