Structural analysis and water adsorption properties of chloranilate anion–terpyridine metal complexes forming hydrogen-bonded frameworks

Abstract

Emerging microporous hydrogen-bonded organic frameworks (HOFs) are expected to overcome water shortages owing to their potential in harvesting and releasing water at low energies. To investigate their water adsorption properties, two distinct types of HOFs are synthesized. The frameworks are composed of chloranilic acid (H₂CA), terpy complexes, and crystal water. The complexes are denoted as [M(terpy)₂](H₂CA)_0.5(HCA)(CA)_0.5·H₂O (M–A) and [M(terpy)₂](CA)·6H₂O (M–B), where M represents Fe²⁺, Co²⁺, or Ni²⁺. Structural characterization results reveal that M–A contains H₂CA, chloranilate monoanion (HCA⁻), and chloranilate dianion (CA²⁻). M–A complexes comprise one-dimensional chains of H₂CA–CA²⁻ and HCA⁻–water, forming a three-dimensional framework via hydrogen bonding with [M(terpy)₂]²⁺. M–B complexes contain [M(terpy)₂]²⁺, CA²⁻, and six water molecules. The CA²⁻ and water molecules form a two-dimensional layered arrangement via hydrogen bonding, and the water molecules form tetramers within the layers. N₂ adsorption measurements indicate that both the M–A and M–B complexes are non-porous. In water adsorption–desorption experiments, M–A adsorbs one water molecule per unit with minimal structural changes, whereas M–B adsorbs six water molecules per unit and undergoes a multi-step isothermal adsorption process, indicating significant structural changes. Furthermore, the adsorption properties were observed to vary with the central metal ion. The differing adsorption behaviors of M–B may be attributed to the hydrogen bonding distances within the crystalline water tetramers forming the hydrogen bonded network. The HOFs explored in this study may be utilized for selectively adsorbing water molecules in low-humidity environments, such as arid regions.