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

Novel methylpalladium(II) complexes of the general composition [PdCl(CH 3){N∧D∧N}] ({N∧D∧N}) = (mim) 3COCH 3 (1a), (mim) 2CHCH 2C(O)Bu-t (3a) were synthesized from [PdCl(CH 3)(COD)] and the respective N-methylimidazole (mim) 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} = (mim) 2CHCH 2C(O)Bu-t) were obtained by halide abstraction with AgBF 4. 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(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 σ 2-N∧N coordination mode in the former and a σ 3-N∧N∧N coordination 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 complex has two square-planar N 3X (X = CH 3, 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 σ 2-P∧N 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 σ 2-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 turnover numbers of up to 800 000 being obtained for 1b.