Palladium Carbene Complexes for Selective Alkene Di- and Oligomerization

A series of palladium complexes were synthesized that comprise three sterically different C,N-bidentate coordinating NHC-pyridine ligands (NHC = N-heterocyclic carbene). In one set, the pyridine and the carbene are linked by a flexible CH2 group (a); in the other two sets, the two ligand units are directly linked and feature a shielding mesityl substituent on the carbene and either an unsubstituted pyridine (b) or a xylyl-substituted pyridine unit (c). Investigation of the reactivity of cationic complexes [Pd­(C∧N)­Me­(NCMe)]+ (6), analogues to Brookhart’s α-diimine system, toward alkenes showed a strong correlation between the catalytic activity and selectivity and the ligand setting. While 6a was inactive in ethylene conversion, 6b afforded low-molecular-weight olefins (oligomerization), and 6c produced exclusively butene (dimerization). With styrene as substrate, exclusive dimerization occurred with all three complexes. Steric and electronic factors were identified that govern the disparate activity and selectivity and that allow for efficient tailoring of the catalytic performance.