Mechanochemical one-pot synthesis and solid-state transformation of cobalt(ii) Schiff base complexes: a green route to tailored coordination architecture

Abstract

Schiff base complexes, traditionally synthesized via time-consuming, solvent-intensive solution methods, are pivotal in coordination chemistry but face limitations in accessing diverse architecture and sustainable scalability. Mechanochemistry has emerged as a solvent-free alternative, yet its potential to drive multicomponent reactions with precise control over metal–ligand coordination modes remains underexplored. Herein, we propose a mechanochemical one-pot synthesis strategy that synergistically integrates condensation, metal coordination, and deprotonation–dehalogenation reactions to fabricate Schiff base Co(II) complexes. Utilizing adamantylamine, 5-halosalicylaldehyde, and CoCl₂·6H₂O as precursors, mechanical forces drive the self-ordering of building blocks, enabling ultrahigh selective coordination and complex chemical processes. The approach efficiently yielded 12 Co(II) complexes, including the κ¹-O-monodentate CoCl₂(HL)₂ and κ²-O, N-bidentate CoL₂, which are typically challenging to access via conventional solution methods. Remarkably, the reactions achieved full conversion within 10 minutes, underscoring the rapidity and sustainability of mechanochemistry. Mechanical activation unlocked dormant reactivity in reactants, facilitating pathways otherwise inaccessible in solution. Furthermore, reversible solid-state transformations between complexes were demonstrated through dehydrohalogenation–hydrohalogenation processes. Specifically, CoCl₂(HL)₂ is converted to CoL₂via cleavage of N–H and Co–Cl bonds and subsequent Co–N bond formation, while CoL₂ reverts upon HCl absorption during grinding. This work highlights the utility of mechanochemistry in simplifying synthetic procedures, enhancing reaction complexity, and enabling green, solvent-free syntheses. By elucidating pathways for one-pot synthesis and solid-state transformations, it establishes mechanochemistry as a versatile and sustainable route for designing advanced coordination complexes with tailored architecture.