Novel methylpalladium(II) complexes bearing tridentate imidazole-based chelate ligands:: Synthesis, structural characterization, and reactivity

Novel methylpalladium(II) complexes of the general composition [PdCl(CH3){N boolean ANDD boolean ANDN}] ({N boolean ANDD boolean ANDN} = (mim)(3)COCH3 (1a), (mim)(2)CHCH2C(O)Bu-t (3a)) were synthesized from [PdCl(CH3)(COD)] and the respective N-methylimidazole (mim) ligands. The cationic derivatives [Pd-2(CH3)(2){N boolean ANDD boolean ANDN}(2)](BF4)(2) (1b; {N boolean ANDD boolean ANDN} = (mim)(3)COCH3) and [Pd(CH3){N boolean ANDD boolean ANDN}(CH3CN)](BF4) (3b; {N boolean ANDD boolean ANDN} = (mim)(2)CHCH2C(O)Bu-t) were obtained by halide abstraction with AgBF4. The effect of varying the third donor moiety (D = "O", "N", "P") in the structurally very similar ligands on the coordination chemistry and reactivity of 1 and 3 and the previously reported [PdCl(CH3){N boolean ANDD boolean ANDN}] (2a; {N boolean ANDD boolean ANDN} = (mim)(2)CHCH2PPh2) was investigated in detail. Thus, a neutral monomeric and an ionic dimeric isomer were identified for la, with the ligand adopting a sigma (2)-N boolean ANDN coordination mode in the former and a sigma (3)-N boolean ANDN boolean ANDN coordination mode in the latter. Exclusive formation of the ionic dimer occurs in methanol. Crystal structures are recorded for 1b and [Pd2Cl(CH3){N boolean ANDN boolean ANDN}(2)]Cl-2 (1c). Each complex has two square-planar N3X (X = CH3, Cl) coordinated Pd centers with the coordination planes in a quasi-parallel orientation but laterally displaced in projection. Two nitrogens from the three imidazole rings of one ligand chelate one Pd center, whereas the third is bridging to the neighboring Pd center. Complex 2a displays sigma (2)-P boolean ANDN coordination and fluxional behavior of the ligand at room temperature. At lower temperatures, various isomers, possibly involving five-coordinate compounds, are observed in the NMR spectrum of 2a. The predominant mode of coordination in complexes 3 is sigma (2)-N boolean ANDN; 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 turnover numbers of up to 800 000 being obtained for 1b.