Novel Methylpalladium(II) Complexes Bearing Tridentate Imidazole-Based Chelate Ligands; Synthesis, Structural Characterization, and Reactivity

摘要

Novel methylphalladium(II) complexes of the general composition[PdCl(CH_3){N^D^N}](N^D^N)=(mim)_3COCH_3(1a), (min)_2CHCH_2C(O)Bu-t(3a) were synthesized from[PdCl(CH)_3(COD)] and the respective N-methylimidazole(min)ligands. The cationic derivatives [Pd_2(CH_3)_2{N^D^N}_2(BF_4)_2(1B;{N^D^N}=(mim_3COCH_3)and [Pd(CH_3){N^D^N}(CH_3CN)](BF_4)(3b;{N^D^N}=(min)_2CHCH_2C(0)Bu-t) were obtained by halide asbtraction with AgBF_4. The effect of varying the third donor moiety (D="O","N","P") in the strucration very similar ligands on the coordination chemistry and reactivity of 1 and 3 and the previously reported [PdCl(CH_3){N^D^N}](2A;{N^D^N}=(mim)_2CHCH_2PPh_2) was investigated in detail. Thus, a neutral monomeric and an ionic dimeric isomer were identified for 1a, with the ligand adopting a #sigma#~2-N^N coordination mode in the former and a #sigma#~3-N^N^N coodination mode in the latter . Exclusive formation of the ionic dimer occurs in methanol. Crystal structures are recorded for 1b and [Pd_2Cl(CH_3){N^N^N}_2]Cl_2 (1c) . Each comple has two square-planar N_3X(X=CH_3, Cl)coordinated Pd centers with the coordination planes in a quasi-parallel orientation out laterally displaced in projection.Two nitrogens from the tree imidazole rings of one ligand chelate one pd center, whereas the third is ridging to the neighboring Pd center. Complex 2a displays #sigma#~2-P^N coordination and fluxional behavior of the ligand at room temperature. At lower temperatures, various isomers, possibly involving five-coordination in complexes 3 is #sigma#-N^N; their NMR spectra provide evidence for weak O…Pd interactions. The complexes 1a-3a readily insert CO, resulting in the formation of the respective Pd acyl complexes.The methylpalladium complexes give rise to active and stable catalysts for the Heck coupling reaction ,with turnove numbers of up to 800 000 being obtained for 1b