Dipalladium Complexes with Triazolidin-Diylidene Bridges and Their Catalytic Activities

The 1,2,4-trimethyltriazolidin-3,5-diylidene (ditz) bridged dipalladium heterotetracarbene complex [PdBr2(iPr2-bimy)]2(μ-ditz) (3) (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was prepared by Ag–carbene transfer involving the 1,2,4-trimethyltriazolium dication (C), Ag2O, and the precursor complex (iPr2-bimyH)­[PdBr3(iPr2-bimy)] (2). Bromido substitution of 3 with AgO2CCH3 and AgO2CCF3 afforded the carboxylato complexes [Pd­(O2CCH3)2(iPr2-bimy)]2(μ-ditz) (4) and [Pd­(O2CCF3)2(iPr2-bimy)]2(μ-ditz) (5). Multinuclei NMR spectroscopies and X-ray diffraction analyses showed that the all-trans isomers of 3, 4, and 5 are the predominant products in all three cases. In addition, the decomposition product of complex 4, trans-[Pd­(O2CCH3)2(iPr2-bimy)2] (trans-6) was structurally determined by X-ray crystallography. A comparative catalytic study revealed the superiority of complexes 3, 4, and 5 over the previously reported mononuclear bis­(benzimidazolin-2-ylidine) complexes without ditz bridges in the Mizoroki–Heck coupling reaction. Overall, complex 4 bearing acetato coligands showed the best catalytic performance.