Ion-Exchange-Free Gold Recovery via Reductive Ligand Exchange to a Dual-Nitrogen Coordinated Au(I)

Abstract The recovery of Au(III) from aqueous solutions is often hampered by the inherent drawbacks of conventional mechanisms, for instance, anion exchange inevitably introduces secondary pollution, while redox-based methods lead to irreversible degradation of the adsorbent. To overcome these limitations, we designed a cationic piperazine−triazine−based porous organic polymer (I-COP). Its synergistic structure incorporates quaternary ammonium centers for electrostatic attraction and a conjugated diamine–triazine moiety where the electron-deficient triazine fine-tunes the electron density of the adjacent piperazine nitrogen (Npip). This design not only suppresses excessive reducibility but also creates a cooperative dual-nitrogen microenvironment. I-COP exhibits exceptional performance: substantial adsorption capacity at high initial concentrations (e.g., c0 = 2500 mg/L, Qmax = 4870.37 mg/g), outstanding selectivity among precious metals (SFAu/Ag = 3009, SFAu/Pd = 1789, SFAu/Pt = 751), and excellent reusability (>93.35% retention after 17 cycles) without any detectable ion exchange (Br−). A combination of spectroscopic, chromatographic, and DFT studies reveals a novel “capture-reduction-exchange-stabilization” mechanism. After rapid electrostatic pre-concentration, Au(III) undergoes controlled single-electron reduction to Au(I), followed by partial ligand exchange in which two chloride ligands are replaced by the synergistically acting Npip and triazine nitrogen (Ntri), forming an extraordinarily stable mixed-ligand [Au(I)Cl2(Npip)(Ntri)]-type complex. This pathway completely avoids counter‑ion release and minimizes oxidative adsorbent damage. I-COP efficiently recovers trace Au(III) from challenging real matrices including electronic waste(e-waste) leachates and seawater. Beyond presenting a superior adsorbent, this work establishes a new design paradigm: steering metal adsorption toward a thermodynamically favored, ligand-exchange-driven coordination pathway by constructing a cooperative hetero-dinitrogen binding pocket. This principle offers a general strategy for developing robust, sustainable materials for critical metal recovery.