Dipalladium Complexes with Triazolidin-Diylidene Bridges and Their Catalytic Activities

The 1,2,4-trimethyltriazolidin-3,5-diylidene (ditz) bridged dipalladium heterotetracarbene complex [PdBr2(Pr-i(2)-birny)]2(mu-ditz) (3) (Pr-i(2)-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 Pr-i(2)-bimyH)[PdBr3(Pr-i(2)-bimy)] (2). Bromido substitution of 3 with AgO2CCH3 and AgO2CCF3 afforded the carboxylato complexes [Pd(O2CCH3)(2)('Pr(2)bimy)](2)(p-ditz) (4) and [Pd(O2CCF3)(2)(Pr-i(2)-bimy)](2)(y-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)(Pr-i(2)-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.