Palladium(II) complexes of a sterically bulky, benzannulated N-heterocyclic carbene with unusual intramolecular C-H center dot center dot center dot Pd and C-carbene center dot center dot center dot Br interactions and their catalytic activities

摘要

The sterically bulky carbene precursor 1,3-diisopropylbenzimidazolium bromide (Pr-i(2)-bimyH(+)Br(-)) (A) has been prepared by an improved method in 84% yield. Reaction of A with Pd(OAc)(2) and NaBr gave the dimeric Pd(II) benzimidazolin-2-ylidene complex [PdBr2(iPr(2)-bimy)](2) (1), which can be easily cleaved by CH3CN, another equivalent of salt A, and triphenylphosphine to afford the novel benzannulated monocarbene complexes trans-[PdBr2(CH3CN)(Pr-i(2)-bimy)] (2), (Pr-i(2)-bimyH)[PdBr3(Pr-i(2)-bimy)] (3), trans-[PdBr2(Pr-i(2)-bimy)(Ph3P)] (trans-4), and cis-[PdBr2(Pr-i(2)-bimy)(Ph3P)] (cis- 4), respectively. All compounds have been fully characterized by multinuclei NMR spectroscopies and mass spectrometries (FAB, ESI). X-ray diffraction studies on single crystals of 1-3 and cis-4 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H group pointing to the metal center to maximize C-H center dot center dot center dot Pd interactions. The large downfield shift of the C-H protons in the H-1 NMR spectrum compared to the precursor A indicates that these C-H center dot center dot center dot Pd interactions are retained in solution and better described as weak hydrogen bonds, rather than as agostic interactions. Furthermore, the molecular structures of especially complexes 2 and 3 clearly show a bending of the bromo ligands toward the carbene carbon atom in order to maximize intramolecular C-carbene center dot center dot center dot Br interactions. The nature of these interactions can be attributed to a form of back-bonding to the formally vacant p-orbital of the C-carbene atom with the electron density originating from the bromo ligands' lone pairs. A detailed study on the trans-cis isomerization of the mixed NHC-phosphine complexes 4 revealed that a cis arrangement in such complexes is thermodynamically favored. Furthermore, a preliminary catalytic study shows that complex 1 is highly active in the Suzuki-Miyaura coupling of aryl bromides and chlorides in pure water as environmentally benign solvent.