Mechanochemical One-Pot Transformation of Indium from ITO into Functional Materials: A Coordination-Based Approach to...

Recycling of rare metals with lower environmental burden is increasingly required. Herein, we report that an azobenzene ligand selectively complexes specific metal species through mechanochemical reactions among multiple metal species. We established a one-pot grinding method comprising acid-mediated chlorination of indium oxide (In₂O₃) to InCl₃ (Step 1), followed by mechanochemical metal complexation with an azobenzene ligand to afford a functional luminescent azobenzene-indium complex (AzInCl) (Step 2). The reaction is facilitated by the presence of H₂O/pyridine, which promotes quantitative complexation (>99%) with a stoichiometric amount of hydrochloric acid (HCl). In addition, we achieved high indium selectivity from the mixture of In₂O₃ and tin (IV) oxide (SnO₂) by exploiting the differences in chemical reactivity of In₂O₃ and SnO₂ toward acid (Step 1) and chemical stability between the resulting indium and tin complexes (Step 2), while the mechanochemical process provides an efficient reaction field for heterogeneous solid-state complexation with minimal solvent use. Finally, from an indium tin oxide (ITO) substrate, AzInCl was selectively obtained as the main product, with indium reaction rate of 79% even in the presence of other metal species. These results should be the potential approach for realizing a foundational technology for selective metal recycling and upcycling via complexation.