Palladium complex with coexisting [PdCl₆] and [PdCl₄] polyhedra: toward sustainable solar-assisted wastewater treatment

A organic–inorganic hybrid compound, (C₆H₈N₂)₂(C₆H₉N₂)₆[Pd^IICl₄]₂[Pd^IVCl₆] [abbreviated as (2A4P)₂(2A4P-H)₆[PdCl₄]₂[PdCl₆] (1)], was synthesized and structurally characterized through single-crystal X-ray diffraction, FTIR, Raman spectroscopy, and thermal analysis. The crystal exhibits a triclinic structure with discrete tetrahedral [Pd^IICl₄]²⁻ and octahedral [Pd^IVCl₆]²⁻ units stabilized by N–H···Cl hydrogen bonding and π–π stacking interactions. The compound demonstrated notable thermal stability up to 200 °C and underwent controlled decomposition at higher temperatures. Its photocatalytic performance was evaluated in the degradation of Orange G (OG) under simulated solar irradiation using a photo-Fenton process. Optimal conditions (catalyst dose: 0.15 g L⁻¹, pH 3, [H₂O₂]: 200 mg L⁻¹) led to complete OG mineralization within 5 h, following pseudo-first-order kinetics. Chromatographic analyses identified key aromatic and carboxylic acid intermediates, allowing the proposal of a detailed degradation pathway. Moreover, the catalyst showed excellent reusability over multiple cycles with minimal loss in activity. These findings highlight the potential of (1) as an efficient and stable photocatalyst for advanced oxidation processes, offering a sustainable approach for the remediation of hazardous organic pollutants.