Versatility of Amide-Functionalized Co(II) and Ni(II) Coordination Polymers: From Thermochromic-Triggered Structural Transformations to Supercapacitors and Electrocatalysts for Water Splitting.

The new 2D coordination polymers (CPs) [M(L)(HO)] [M = Co () and Ni (); L = 4-(pyridin-3-ylcarbamoyl)benzoate] were synthesized from pyridyl amide-functionalized benzoic acid (). They were characterized by elemental, Fourier transform infrared, thermogravimetric, powder X-ray diffraction (PXRD), and single-crystal X-ray diffraction (XRD) structural analyses. Single-crystal XRD analysis revealed the presence of a 2D polymeric architecture, and topological analyses disclose a 2,4-connected binodal net. A thermochromic effect leads to the production of two new CPs, and , by heating at ca. 220 °C, accompanied by a color change from orange to purple in the case of and from blue to green in the case of . The transformation of to takes place through an intermediate () with a different twist of the L ligand, leading to the formation of a 1D polymeric architecture, as proven by single-crystal XRD analysis. The addition of water or keeping or in air for several days leads to regeneration of or , respectively. The thermochromic-triggered structural transformations of and were further substantiated by PXRD and UV-vis ground-state diffuse-reflectance absorption studies. The supercapacitance ability of the CPs and and a Ni-Co composite (made from mixing the CPs and ) was investigated by electroanalytical techniques, such as cyclic voltammetry and electrochemical impedance spectroscopy. The CP exhibits the highest specific capacity of 273.8 C g at an applied current density of 1.5 A g. These newly developed CPs further act as electrocatalysts for the water-splitting reaction.