Palladium(II) and gold(I) complexes of a new O-functionalized N-heterocyclic carbene ligand: Synthesis, structures, and catalytic application

Synthetic and structural studies of Pd(II) and Au(I) complexes of a new O-functionalized N-heterocyclic carbene ligand, namely, 1-(o-methoxybenzyl)-3-tert-butylimidazol-2-ylidene, are reported. Specifically, the N-heterocyclic carbene precursor 1-(o-methoxybenzyl)-3-tert-butylimidazolium bromide (1a) was synthesized by the reaction of 2-methoxybenzyl bromide and tert-butylimidazole in 44% yield. The Au(I) and Pd(II) complexes [1-(o-methoxybenzyl)-3-tert-butylimidazol-2-ylidene]AuCl (1c) and [1-(o-methoxybenzyl)-3-tert-butylimidazol-2-ylidene](2)PdCl2 (1d) were prepared in 77% and 89% yields, respectively, by the commonly used silver carbene transfer route from the silver complex {[1-(o-methoxybenzyl)-3-tert-butylimidazol-2-ylidene](2)Ag}Br-+(-) (1b). The silver complex 1b was in turn synthesized from the reaction of 1a with Ag2O. The molecular structures of the complexes 1c and 1d have been determined by X-ray diffraction studies, which revealed that the gold complex 1c possessed a linear geometry while the palladium complex 1d had a trans-square-planar geometry at their respective metal centers. The Pd(II) complex 1d was found to be an efficient catalyst for Suzuki-Miyaura type cross-coupling reactions of phenylboronic acid and aryl halides (ArX, X = Br, I) in high yields and turnover numbers (up to 109 600).