Palladium(II) complexes containing a bulky pyridinyl N-heterocyclic carbene ligand: Preparation and reactivity

Coordination chemistry of a pyridine imidazole-2-ylidene ligand (pyN ˆC) with sterically hindered substituents toward palladium(II) metal ions has been investigated. The palladium carbene complex [(C-pyN ˆC)Pd(η3-allyl)Cl] (3) is prepared via the transmetallation from the corresponding silver carbene complexes with [ClPd(η3-allyl)]2. Upon the abstraction of chloride, coordination of pyridinyl-nitrogen becomes feasible to form [C,N-(pyN ˆC)Pd(η3-allyl)](BF4) (4). Ligand substitution reaction of 4 with triphenylphosphine results in the formation of [(C-pyN ˆC)Pd(PPh3)(η3-allyl)](BF4)], which the pyridinyl-nitrogen donor is substituted by the phosphine. This palladium complex appears to be base sensitive. Treatment of 4 with t-butoxide causes the decomposition to yield the metal nano-particles. Furthermore, de-complexation of 4 takes place under hydrogen atmosphere to generate the carbene precursor, 1-(6-mesityl-2-picolyl)-3-mesitylimidazolium salt. Nevertheless, the palladium complex 4 shows good catalytic activity on the Suzuki–Miyaura and Mizoroki–Heck reactions.