Precatalysts

Precatalysts are chemical species that are not themselves the active catalyst under the reaction conditions but furnish the active catalytic species after a preactivation step. They are used in homogeneous catalysis to improve handling, stability, and reproducibility of catalytic systems, especially for air- and moisture-sensitive metal catalysts.

Activation is typically triggered by the reaction medium or additives and can involve reduction of a higher oxidation state to the active metal(0) or metal(II) species, ligand rearrangement, or decarboxylation. The exact pathway depends on the metal, the ligands, and the substrate; activation may be driven by base, reductants, heat, light, or coordination with substrates.

Commonly, palladium precatalysts are reduced in situ to Pd(0) species for cross-coupling reactions, while nickel and rhodium precatalysts follow analogous activation routes. Well-defined precatalysts include air-stable palladium(II) complexes that generate Pd(0) upon activation with base and substrate. In addition to cross-couplings, precatalysts are used in hydroformylation, hydrogenation, and other homogeneous catalytic processes.

Advantages include safer handling of air-sensitive catalysts, simplified storage, and more reproducible activity. Limitations include the need for an activation step, possible induction periods, and potential sensitivity to reaction conditions that influence the activation rate or pathway.