Synthesis and characterization of palladium(ii) complexes with a novel chelating iminocarbene ligand

Abstract

The reaction between 1-chloro-2-(2,6-diisopropylphenylimino)propane and N-methylimidazole yields 3-methyl-1-{2-(2,6-diisopropylphenylimino)propyl}imidazolium] chloride, (C–N)·HCl (2). A silver(I) iminocarbene complex formulated as (C–N)₂Ag⁺AgCl₂ ⁻ is formed in the reaction between 2 and Ag₂O. Carbene transfer to Pd occurs when 2 is treated with (COD)Pd(Cl)(X) to yield (C–N)Pd(Cl)(X) (4a, X = Cl; 4b, X = CH₃). Chloride abstraction from 4a and 4b with AgPF₆ in MeCN yields (C–N)Pd(NCMe)(X)⁺PF₆ ⁻ (5a and 5b). A second chloride abstraction can be done with 5a to yield (C–N)Pd(NCMe)₂ ²⁺(PF₆ ⁻)₂ (6). An X-ray structure determination of 4a verifies the chelating nature of the iminocarbene ligand system and shows that the molecule adopts a boat conformation with Pd and the CH₂ link between the carbene and the imine occupying the two apical positions. The species 4–6 have been subjected to extensive analysis by variable-temperature, EXSY, and NOESY NMR spectroscopy in various solvents. In the weakly coordinating solvent nitromethane, 4a and 4b exist as square planar species in which the iminocarbene acts as a C,N-chelating ligand. NMR experiments reveal a dynamic behavior of 4a that involves a boat “flip”. This process is fast even at −40 °C for 4b but considerably slower for 4a. In more coordinating solvents, evidence is seen for solvent-dependent equilibra that involve rapid interconversion between the chelating κ²(C,N) structures (κ²-(C–N))Pd(Cl)(X) and non-chelating κ¹(C) iminocarbene structures (κ¹-(C–N))Pd(Cl)(X)(Solvent). Complex 5a exists as two interconverting species, possibly cis/trans isomers by interchange of the MeCN and Cl ligands, whereas no evidence for dynamic processes is seen for 5b and 6.