Dipalladium complexes with triazolidin-diylidene bridges and their catalytic activities

Abstract

The 1,2,4-trimethyltriazolidin-3,5-diylidene (ditz) bridged dipalladium heterotetracarbene complex [PdBr 2( iPr 2-bimy)] 2(μ-ditz) (3) ( iPr 2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was prepared by Ag-carbene transfer involving the 1,2,4-trimethyltriazolium dication (C), Ag 2O, and the precursor complex ( iPr 2-bimyH)[PdBr 3( iPr 2-bimy)] (2). Bromido substitution of 3 with AgO 2CCH 3 and AgO 2CCF 3afforded the carboxylato complexes [Pd(O 2CCH 3) 2( iPr 2-bimy)] 2(μ-ditz) (4) and [Pd(O 2CCF 3) 2( iPr 2-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(O 2CCH 3) 2( iPr 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. © 2012 American Chemical Society.