Multifaceted role of silver salts as ligand scavengers and different behavior of nickel and palladium complexes: beyond halide abstraction

The reaction of [NiArBr(PPh 3 ) 2 ] with AgBF 4 brings about the abstraction of both the halide and phosphine from the nickel center by silver. When the reaction is carried out in CH 2 Cl 2 /toluene a mixture of the cationic aquo derivatives [NiAr(H 2 O)(PPh 3 ) 2 ]BF 4 ( 2 ) and [NiAr(H 2 O) 2 (PPh 3 )]BF 4 ( 3 ) is formed, along with AgBr and [Ag(PPh 3 ) n ]BF 4 . When the same reaction is carried out in acetone as the solvent, it leads to the completely different complex [NiAr(κ 2 -O, O-MeC(O)CH 2 C(OH)Me 2 )(PPh 3 )] ( 5 ), bearing a chelating ligand formed by the aldol self-condensation of acetone. Phosphine abstraction by silver is less favorable for the analogous palladium( ii ) complexes and only occurs if a large excess of AgBF 4 is used. Thus, silver salts can be safely used as halide scavengers for palladium derivatives. However, the generation of cationic Ni complexes from neutral precursors by halide extraction with a silver salt may produce naked species, different than those expected, and highly reactive in certain media. What do you get when in situ preparing your cationic nickel catalyst? Silver salts are halide scavengers but they also trap phosphine ligands to form reactive nickel complexes, which can either bind water or induce the aldol condensation of acetone.