Air-Stable Dinuclear Iodine-Bridged Pd(I) Complex - Catalyst, Precursor, or Parasite? The Additive Decides. Systematic Nucleophile-Activity Study and Application as Precatalyst in Cross-Coupling

Dinuclear Pd(I) complexes were recently shown to potentially adopt various possible roles in catalysis, being capable of functioning as catalyst via dinuclear catalysis cycles, precatalyst for Pd(0), or inhibitor. This report examines the factors that control the mechanistic role in catalysis. Our data suggest that the transformation to Pd(0) occurs via nudeophile-induced fragmentation of Pd(I) Pd(I). A systematic study examining the nudeophilicity of additive versus activity was undertaken that revealed the minimum nudeophilicity necessary to activate two structurally very similar Pd(I) dimers, [Pd(mu-X) (PtBu3)](2) (X = I or Br). While the more labile bromine-bridged Pd(I) dirner is converted to Pd(0) with nudeophiles N >= 10.5, the iodine-bridged analogue requires N >= 16.1 (N according to Mayr's scale). Too strong nudeophiles generate a high concentration of unstable monoligated Pd(0) rapidly, leading to Pd loss (i.e., Pd black). On the other hand, careful tuning of nudeophilicity allows for a controlled release of well-defined Pd(0) species. These insights have led to the first application of the air-stable and previously thought unreactive iodine-bridged dimer [Pd(mu-I) (PtBu3)](2) as a precatalyst for monoligated Pd(0) in cross-coupling reactions. The reactions were performed without exclusion of oxygen-all reagents were handled in air without special precautions. Highly efficient Kumada couplings of aryl iodides and bromides were achieved in <5 min at room temperature.