precatalyst

A precatalyst is a chemical species that is not itself the active catalyst but generates the active catalytic species under the reaction conditions. Precatalysts are often designed to be more stable, easier to handle, and less prone to deactivation than the true catalyst, enabling shelf stability and better reaction control. They are widely used in homogeneous catalysis, especially for metal-catalyzed processes.

Activation of a precatalyst typically occurs in situ through a transformation that produces the active metal complex. Common pathways include reduction of a higher-oxidation-state metal center to the active oxidation state, removal or exchange of ligands to generate the true catalytic species, halide abstraction to form more reactive cationic complexes, or substrate- and base-induced activation. Activation can be driven thermally, photochemically, or by a chemical reductant or base present in the reaction mixture.

In practice, precatalysts are frequently employed for palladium- and nickel-catalyzed cross-coupling reactions, hydrofunctionalization, and related organometallic transformations. Examples include well-defined palladium precatalysts that generate active Pd(0) species under typical Suzuki–Miyagawa, Heck, or Negishi conditions, as well as precatalysts for other metals that release the active metal complex upon activation. The use of precatalysts can improve handling, reproducibility, and selectivity, but may introduce an induction period related to the activation step.

Overall, precatalysts are a strategic class of catalyst precursors that balance practical handling with in situ generation of the active catalytic species.