Advancements in squaric acid-based metal‒organic frameworks: transitioning from two-dimensional layers to three-dimensional networks

Squaric acid (C₄H₄O₄) is an organodioic acid characterized by a conjugated structure that features two carbonyl groups, two hydroxyl groups and a four-membered ring. Compared with other ligands, squaric acid has a shorter molecular length and a more rigid conformation. Its strong coordination ability and distinctive coordination mode endow it with unique functionalities, making squaric acid-based metal‒organic frameworks (SA‒MOFs) the focus of considerable interest. In recent decades, the diverse structural properties of both 2D and 3D squaric acid-based MOFs have demonstrated their potential for precise structural design. Furthermore, SA-MOFs show significant promise for high-performance applications in various fields, such as materials science and gas adsorption. This paper provides a systematic review of 2D and 3D squaric acid-based MOFs, focusing on their structures and properties from the perspective of synthesis methods. Additionally, it offers insights into SA-MOF materials from foundational principles while discussing future prospects and challenges associated with these innovative frameworks.