MECHANOCHEMISTRY: A SELECTIVE TOOL FOR THE SYNTHESIS OF METAL COMPLEXES

Mechanochemistry, defined by IUPAC as “any chemical reaction induced by the direct absorption of mechanical energy,” has earned a place among the “Ten Chemical Innovations That Will Change Our World” [1]. Its appeal lies in its low environmental impact, enabled by significant solvent reduction, and in its capacity to access compounds otherwise unattainable through conventional methods [2]. The mechanochemical synthesis of molecular metal complexes is well-documented and increasingly popular due to these benefits, including reduced solvent use, accelerated reaction kinetics, and improved sustainability. Yet, mechanochemical reactions are often perceived as unpredictable and highly heterogeneous, giving rise to the notion that products form more by chance than through a well-defined pathway. To challenge this perception, we set out to demonstrate that mechanochemistry can achieve clean, rapid, and selective reactions, particularly in synthesizing metal complexes with monodentate ligands, where chelation is absent. In this study, we synthesized a series of heteroleptic mononuclear complexes using pyridine and a substituted benzoic acid as ligands, achieving distinct coordination geometries under varying mechanochemical conditions. When hydrated zinc acetate was used, the reaction led to the formation of an octahedral water-coordinated complex, while using anhydrous zinc acetate plus “dry milling” conditions yielded a tetrahedral complex. This outcome highlights not only the adaptability of mechanochemistry in controlling product geometry but also its unique capacity to produce moisture-sensitive products without requiring specialized setups, such as Schlenk lines, gloveboxes, or dry solvents. By enabling such control in a simplified, solvent-free environment, mechanochemistry offers a practical and environmentally friendly alternative to traditional synthetic routes for air- or moisture-sensitive compounds. Building on these findings, we then replaced pyridine with isonicotinamide as the ligand. This substitution led, unexpectedly, to the selective formation of hexanuclear assemblies, underscoring mechanochemistry’s potential to drive complex self-assembly in a controlled, selective manner. The ability to access such higher-order structures from simple starting materials and without solution-based protocols expands the scope of mechanochemical synthesis, offering new avenues for the sustainable, scalable production of complex metal-organic architectures.