Zwitterionic hydrogen-bonded organic framework (ZHOF): a metal-free water oxidation proton relay in alkaline and neutral media

Abstract

This work reports the synthesis of a single crystalline zwitterionic hydrogen-bonded organic framework (ZHOF) by utilizing 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (H₄TATA) ligand. A modified solvothermal method employs a non-bulky polar solvent system, allowing maximum interpenetration of zwitterionic skeletons (ZSs). The results of single-crystal X-ray diffraction analysis display a two-dimensional (2D) extension of ZSs through strong H-bonding (HB), asymmetric Speakman-type, between carboxyl and carboxylate groups. The sheets are latched and knotted through two-per-tecton zwitterionic intermolecular interactions of the ammonium–carboxylate motif. The 2D sheets are assembled in a third dimension through an AB pattern in the form of a pseudo-laminated 3D framework with an interlayer spacing of 3.64 Å, providing a water-included rigid proton-conduction channel in the course of the water oxidation reaction. The metal-free graphite-supported ZHOF (FGF/ZHOF) shows striking performance as a water oxidation proton relay in alkaline and neutral media. The proton/deuterium isotopic studies unraveled accelerating proton transfer (PT) on FGF/ZHOF with a secondary kinetic isotopic effect (k_H/k_D, KIE) as low as 1.15. The post-metalation of ZHOF affords a primary KIE value of 3.3, with a reversible overtaking of the electron transfer against PT. The observation of Gerischer impedance approves an efficient protonic species translocation at exchange sites through a practically blocked natural diffusion. The FGF/ZHOF displays a promising charge storage ability with a specific capacitance of 903 F g⁻¹.