Mechanochemistry Meets Catalysis: Metal Complexes for Greener Organic Transformations

Mechanochemistry and transition-metal catalysis are converging into a platform in which mechanical energy reshapes catalyst formulation, speciation, and operative state, rather than simply replacing solvent. Through intense mixing, continuously renewed interfaces, liquid-assisted grinding (LAG), and rheological control, milling can direct metal-complex assembly, activation, reactivity, and selectivity in ways difficult to reproduce in solution. These attributes streamline catalyst preparation, lessen dependence on stringent inert-atmosphere protocols, and open access to transformations that are inefficient, selective only under milling, or inaccessible by conventional methods. This Review examines the mechanochemical synthesis of transition-metal complexes and their direct deployment in catalytic organic transformations, from earth-abundant first-row metals to selected noble-metal systems. Quantitative benchmarks, including enantioselectivities up to 99% ee, turnover frequencies above 100 h^-1, and cross-electrophile couplings completed within minutes, demonstrate that mechanocatalysis can deliver not only greener variants of known reactions but also distinct reactivity regimes. Mechanistic uncertainty, reproducibility, and scalable technologies such as twin-screw extrusion (TSE) and resonant acoustic mixing (RAM) are assessed, framing mechanocatalysis as both an enabling methodology and a conceptual basis for next-generation green catalysis.