Palladium(II) Complexes of a Sterically Bulky, Benzannulated N-Heterocyclic Carbene with Unusual Intramolecular C−H···Pd and Ccarbene···Br Interactions and Their Catalytic Activities

Abstract

The sterically bulky carbene precursor 1,3-diisopropylbenzimidazolium bromide (iPr2-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(iPr2-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)(iPr2-bimy)] (2), (iPr2-bimyH)[PdBr3(iPr2-bimy)] (3), trans-[PdBr2(iPr2-bimy)(Ph3P)] (trans-4), and cis-[PdBr2(iPr2-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···Pd interactions. The large downfield shift of the C−H protons in the 1H NMR spectrum compared to the precursor A indicates that these C−H···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 Ccarbene···Br interactions. The nature of these interactions can be attributed to a form of back-bonding to the formally vacant p-orbital of the Ccarbene 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.