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Structural Characterization, Solution Dynamics, and Reactivity of Palladium Complexes with Benzimidazolin- 2-ylidene N-Heterocyclic Carbene Ligands



Mononuclear and halide-bridged dinuclear palladium(II) complexes with symmetrically (1 and 3) and nonsymmetrically (2, 46) substituted benzimidazolin-2-ylidene N-heterocyclic carbene ligands were synthesized and characterized by 1H and 13C NMR spectroscopy, elemental analysis, and X-ray crystallography. The mononuclear complexes exist as a mixture of cis and trans isomers, and the identity of these was ascertained by NMR spectroscopy and structural characterization. The dinuclear complexes 46 with nonsymmetrically substituted benzimidazolin-2-ylidene ligands form rotamers (cis/anti, cis/syn, trans/anti, and trans/syn) at room temperature, which can be transformed to a single rotamer at higher temperatures as evidenced by 1H NMR spectroscopy. Crystal structure analyses of representative examples show that the PdII centers are bound in a distorted square-planar environment by halides and carbene C ligands, and the carbene ligands are perpendicular to the Pd–halide plane. All of the complexes were tested for their efficiency as (pre)catalysts in the Suzuki–Miyaura cross-coupling reaction, as well as in hydrodehalogenation reactions, and they show good activity. The Suzuki–Miyaura coupling reactions can be performed under air and in water as an environmentally benign solvent. The most active catalyst for the Suzuki–Miyaura coupling was also used in a sequence together with a “click reaction” to synthesize valuable sterically demanding tripodal triazole ligands.