Pd(II) complexes of a sterically bulky, benzannulated N-heterocyclic carbene and their catalytic activities in the Mizoroki-Heck reaction

Abstract

The bis(N,N′-diisopropylbenzimidazolin-2-ylidene)Pd(II) complexes trans-[PdBr2(iPr2-bimy)2] (trans-1) and trans-[PdI2(iPr2-bimy)2] (trans-2) have been prepared in good yields by in situ deprotonation of the corresponding N,N′-diisopropylbenzimidazolium salt (iPr2-bimyH+X-) (A: X = Br, B: X = I) with Pd(OAc)2 in DMSO at elevated temperature. Salt metathesis of trans-1 or trans-2 with AgO2CCF3 in refluxing CH3CN afforded the novel mixed carbene-carboxylato complex cis-[Pd(O2CCF3)2(iPr2-bimy)2] (cis-3). This halo/trifluorocarboxylato ligand substitution can be regarded as a selective method for the synthesis of cis-configured bis(carbene) complexes. All compounds have been fully characterized by multinuclei NMR spectroscopies and ESI mass spectrometry. X-ray diffraction studies on single crystals of trans-1, trans-2 and cis-3 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H protons pointing to the metal center to maximize rare C-H⋯Pd preagostic interactions. These interactions are also retained in solution as indicated by the large downfield shift of the isopropyl C-H protons in the 1H NMR spectrum compared to those in precursor salts A or B. A preliminary catalytic study revealed that all complexes are highly active in the Mizoroki-Heck coupling of aryl bromides and chlorides. However, these complexes gave slower conversions as compared to catalysts with less bulky benzimidazolin-2-ylidenes. This is most likely due to the steric bulk of the ligands, which hamper a fast reductive formation of catalytically active Pd(0) species. © 2007 Elsevier B.V. All rights reserved.