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

The reaction of [NiArBr(PPh3)2] with AgBF4 brings about the abstraction of both the halide and phosphine from the nickel center by silver. When the reaction is carried out in CH2Cl2/toluene a mixture of the cationic aquo derivatives [NiAr(H2O)(PPh3)2]BF4 (2) and [NiAr(H2O)2(PPh3)]BF4 (3) is formed, along with AgBr and [Ag(PPh3)n]BF4. When the same reaction is carried out in acetone as the solvent, it leads to the completely different complex [NiAr(κ2-O, O-MeC(O)CH2C(OH)Me2)(PPh3)] (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 AgBF4 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.

A different polynorbornene backbone by combination of two polymer growth pathways: vinylic addition and ring opening via ß-C elimination.

A new polynorbornene skeleton has been found that contains bicyclic norbornane units and cyclohexenyl methyl linkages. The polymers have been synthesized using a nickel catalyst in the presence of a controlled amount of ligands with low or moderate coordination ability. The backbone structure is the result of a vinylic addition polymerization, via sequential insertions of norbornene into a Ni-C bond (bicyclic units) combined with an unusual ring opening of the norbornene structure by a ß-C elimination (cyclohexenyl methyl units) to give a new Ni-C(alkyl) bond that continues the polymerization. The ring opening events are favored when the rate of propagation of the vinylic addition polymerization decreases, and this can be modulated by making the coordination of norbornene to the metal center less favorable using additional ligands.

Trispyrazolylborate Ligands Supported on Vinyl Addition Polynorbornenes and Their Copper Derivatives as Recyclable Catalysts.

Polynorbornenes prepared by vinyl addition polymerization and bearing pendant alkenyl groups serve as skeletons to support trispyrazolylborate ligands (Tpx ) built at those alkenyl sites. Reaction with CuI in acetonitrile led to VA-PNB-Tpx Cu(NCMe) (VA-PBN=vinyl addition polynorbornene) with a 0.8-1.4 mmol incorporation of Cu per gram of polymer. The presence of tetracoordinated copper(I) ions was been assessed by FTIR studies on the corresponding VA-PNB-Tpx Cu(CO) adducts, in agreement with those on discrete Tpx Cu(CO). The new materials were employed as heterogeneous catalysts in several carbene- and nitrene-transfer reactions, showing a behavior similar to that of the homogeneous counterparts but also being recycled several times maintaining a high degree of activity and selectivity. This is the first example of supported Tpx ligands onto polymeric supports with catalytic applications.