Two metal–organic frameworks (MOFs) based on Zn(II)-squarate bridges associated with non-covalent and covalent bonds of dpe ligands, [Zn(dpe)(C₄O₄)(H₂O)₂]_n (1), and {[Zn(Hdpe)(C₄O₄)_0.5(H₂O)₃][Zn(C₄O₄)₂(H₂O)₂]}_n (2) (C₄O₄ ^{2 −} (squarate) = dianion of 3,4-dihydroxycyclobut-3-ene-1,2-dione (H₂C₄O₄); dpe = 1,2-bis(4-pyridyl)ethane), have been synthesized and characterized by spectroscopic analysis and single-crystal X-ray diffraction studies. Compound 1 is a two-dimensional (2D) metal–organic framework (MOF), being constructed via the crosslinkage of one-dimensional (1D) [Zn(μ_1,3-squarate)] chains and 1D sinusoidal-like [Zn(gauche-dpe)] chains. Compound 2 is a 2D tri-layered composite network composed of one cationic polythreading [Zn(Hdpe)(C₄O₄)_0.5(H₂O)₃]²⁺ MOF and two anionic [Zn(C₄O₄)₂(H₂O)₂]^{2 −} MOFs with the penetration of the monoprotonated anti-form Hdpe⁺ ligands dangling above and below the 2D Zn(II)-(μ_1,2,3,4-squarate) layer of cationic [Zn(Hdpe)(C₄O₄)_0.5(H₂O)₃]²⁺ MOF into the windows of two 2D Zn(II)-(μ_1,3-squarate) layers of anionic [Zn(C₄O₄)₂(H₂O)₂]^{2 −} MOFs. Both of their 2D MOFs in compounds 1 and 2 are then extended to 3D supramolecular architectures via the intermolecular hydrogen bonding interaction among the squarate, protonated dpe ligands and water molecules.

Two supramolecular isomers of [Ni(4-bpd)₂(NCS)₂] (4-bpd = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) with 2D 4⁴ square-grid and 3D 6⁵·8 frameworks are co-crystallized in a one-pot reaction, both of which exhibit interesting luminescence properties and reversible adsorption–desorption with respect to guest solvents.

In this highlight, we have focused our attention on several examples of coordination polymers (CPs) and metal–organic frameworks (MOFs) that exhibit water-induced reversible SCSC or solid-state structural transformations under thermal conditions. Dissociation or removal of coordinated or solvated water molecules and exposure to moisture in air may cause reversible SCSC or solid-state structural transformations, which are mainly demonstrated through single-crystal X-ray diffraction, powder X-ray diffraction and cyclic TG measurement and manifested by various physical properties, such as colour, magnetism, luminescence, etc., due to changes in coordination number and geometry, structural dimensionality, interpenetration, etc.