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∧D∧N}]
({N∧D∧N} = (mim)3COCH3 (1a), (mim)2CHCH2C(O)Bu-t (3a)) were synthesized from [PdCl(CH3)(COD)] and the respective N-methylimidazole (mim) ligands. The cationic derivatives
[Pd2(CH3)2{N∧D∧N}2](BF4)2 (1b; {N∧D∧N} = (mim)3COCH3) and [Pd(CH3){N∧D∧N)(CH3CN)](BF4) (3b; {N∧D∧N} = (mim)2CHCH2C(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∧D∧N}] (2a; {N∧D∧N} = (mim)2CHCH2PPh2) was investigated in
detail. Thus, a neutral monomeric and an ionic dimeric isomer were identified for 1a, with
the ligand adopting a σ2-N∧N coordination mode in the former and a σ3-N∧N∧N coordination
mode in the latter. Exclusive formation of the ionic dimer occurs in methanol. Crystal
structures are recorded for 1b and [Pd2Cl(CH3){N∧N∧N}2]Cl2 (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 σ2-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 σ2-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.