Hydration-Driven Structural Modulation of Cd(II)–2-Amino-5-nitrobenzoate Complexes: Enhancing Charge-Transfer Kinetics for Supercapacitor Applications

Abstract

The rational design of redox-active supramolecular networks for high-performance energy storage demands a fundamental understanding of how subtle structural modifications dictate interfacial charge-storage kinetics. In this work, we report the synthesis, comprehensive structural characterization, and electrochemical evaluation of two d10 Cd(II) coordination complexes [Cd(ANB)2(H2O)3]·H2O (Cd1) and [Cd(ANB)2(H2O)2] (Cd2), derived from 2-amino-5-nitrobenzoic acid. Single-lattice water variation alters the coordination geometry, transforming the monohydrated Cd1 lattice into the compact network in Cd2, highlighting hydration as a decisive structural parameter. Single crystal X-ray diffraction, powder X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis confirm this hydration-driven dimensionality. Spectroscopic-computational correlation between UV–vis absorption and time-dependent density functional theory calculations reveals a narrowed optical energy gap of 3.65 eV upon metalation, driving efficient ligand-to-metal charge transfer. Advanced topological analyses (Hirshfeld, quantum theory of atoms in molecules, and noncovalent Interaction) demonstrate that Cd2 compensates for reduced hydrogen bonding through dense, dispersive van der Waals interactions and an asymmetric coordination sphere. Electrostatic potential mapping further reveals an unshielded, intensely positive Cd(II) center in Cd2, which lowers the activation barrier for electron transfer. Electrochemical evaluations (CV, GCD, EIS) establish Cd2 outperforming Cd1, delivering a mass-specific capacitance of 7.91 F/g, a low charge-transfer resistance (Rct = 7.662 Ω), an energy density of 0.126 mWh/cm2, and a power density of 453.32 mW/cm2. Collectively, these findings establish hydration control as a vital paradigm for engineering high-power-density supercapacitors.