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.