Mechanically induced nickel catalyst activation in cross-coupling reactions via abrasion

In this study, we unravel the non-innocent role of mechanical abrasion in mechanochemical cross-coupling reactions catalyzed by nickel salts, demonstrating that abrasion of stainless steel in ball milling efficiently activates nickel catalysts even in the absence of piezoelectric redox-materials or intentionally added zerovalent-metal reductants. The abrasion is facilitated by the addition of solid additives, which include Celite or commonly used BaTiO3. Detailed STEM, XRD, EDX, and XPS analyses revealed the formation of zerovalent iron- and chromium-coated heterogeneous particles during ball milling, which activate nickel salts catalyzing C-N cross-coupling or cross-electrophile C-C coupling reactions. These findings challenge the common view of ball-milling equipment as being an inert material, demonstrating the importance of mechanical phenomena along with the chemical composition of ball miling materials typically used in catalytic reactions.