Coordination networks constructed from a flexible ligand: single-crystal-to-single-crystal transformations and thermoresponsive and electrochemical performances

Coordination networks (CNs), which have attracted great interest as electrode materials for supercapacitors (SCs), are generally constructed from Co and Ni ions with excellent redox ability and large rigid ligands with good productivity. There are scarce examples of CNs composed of copper ions and flexible ligands. In this work, a new fresh yellow crystal 2D CN [Cu^I₂(L1)Cl₂]_∞ (1a) is successfully synthesized using CuCl and N,N′-bis(3-pyridyl)adipoamide (L1) with a high degree of flexibility. The fresh yellow [Cu^I₂(L1)Cl₂]_∞ crystals (1a) can be spontaneously oxidized to the green isomorphic CN [Cu^II₂(L1)Cl₂]_∞ (1c) in air, and this structural transformation between 1a and 1c is reversible by treatment with redox reagents, which can be also observed for the isomorphic CNs [Cu^I₂(L1)Br₂]_∞ (2a) and [Cu^II₂(L1)Br₂]_∞ (2c). Moreover, 1a, 1c, 2a and 2c show thermoresponsive properties with different degrees and disparity in electrochemical performance due to the different electronic structures of copper atoms and halogen species. Noticeably, the electrochemical performances of 1a and 1c are better than those of 2a and 2c, respectively. This work explores new strategies, i.e., tuning the electronic structure of metal atoms and coordination halide anions, for boosting the pseudocapacitive performance of CNs.