Design of cationic mixed phosphine/N-Heterocyclic carbene palladium(II) pi-Allyl complexes as monoligated phosphine Pd(0) precatalysts: synthesis, structural studies, catalysis, and reactivity

Compounds of general formula [Pd(L)(NHC)(L′)]BF4 (where L = allyl or crotyl, NHC = tetramethylimidazolin-2-ylidene (tmiy) or 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (dipdmiy), and L′ = tertiary phosphine) have been prepared from the parent [Pd(L)Cl(NHC)(L′)] complexes and fully characterized. A perpendicular orientation of the NHC is observed in the solid state (X-ray) and in solution (NOESY). The crystal structures of these complexes reveal the double-bond character of the allyl trans to the phosphine ligand; for example, [Pd(η3-C3H5)(tmiy)(PPh3)]BF4 (4d) displays C−C distances of 1.198 Å (trans to P) and 1.374 Å (trans to the NHC). The NHC-Pd-allyl precatalysts are thought to generate monoligated phosphine-Pd(0) active species by reductive elimination of 2-allylimidazolium following η3−η1 isomerization of the allyl group. This was observed in the case of [Pd(η3-C3H5)(tmiy)(PCy3)]BF4 (4b), which successfully catalyzed the coupling of imidazolium salts with ethylene. The intriguing reactivity of 4b with PhI, yielding 2-phenylimidazolium selectively, is also reported