Two New Adenine-Based Co(II) Coordination Polymers: Synthesis, Crystal Structure, Coordination Modes, and Reversible Hydrochromic Behavior

We report the synthesis of two new three-dimensional coordination polymers (CPs) based on Co(II), adenine, and aromatic tetracarboxylate linkers. Adenine exhibits bidentate binding modes in both CPs, coordinating through the N-3 and N-9 sites in a first compact CP and through the more rare N-3 and N-7 sites in a second open, flexible, and H2O-responsive CP. These differences together with an analysis of the extended coordination structures made of adenine reported in the Cambridge Structural Database illustrate the rich coordination versatility of adenine as a building block for CPs. Although the latter CP is nonporous to N-2 or CO2, it shows a reversible and detectable color change from pink to purple, and vice versa, upon hydration and dehydration, respectively.