Synthesis and characterization of novel Pd(ii) complexes with chelating and non-chelating heterocyclic iminocarbene ligands

Abstract

The imidazolium salts [3-R¹-1-{2-Ar-imino)-2-R²-ethyl}imidazolium] chloride (C–N; Ar = 2,6-ⁱPr₂C₆H₃; R¹/R² = Me/Me (a), Me/Ph (b), Ph/Me (c), 2,4,6-Me₃C₆H₂ (d), 2,6-ⁱPr₂C₆H₃ (e)) react with Ag₂O to give Ag(I) iminocarbene complexes (C–N)AgCl (4a–e) in which the iminocarbene ligand is bonded to Ag via the imidazoline-2-ylidene carbon atom. The solid-state structures of 4b and 4d were determined by X-ray crystallography and revealed the presence of monomeric (carbene)AgCl units with Z and E configurations at the imine CN bonds, respectively. Carbene transfer to Pd occurs when compounds 4b–e are treated with (COD)PdCl₂ to yield bis(carbene) complexes (C–N)₂PdCl₂ (6b–e) containing two κ¹-C bonded iminocarbene moieties. NMR spectroscopic data indicated a trans coordination geometry at Pd. This conclusion was supported by an X-ray structure determination of 6b which clearly demonstrated the non-chelating nature of the iminocarbene ligand system. EXSY ¹H NMR spectroscopy suggests that the non-chelating structures undergo E/Z isomerization at the imine CN double bonds in solution. The preparative results contrast our earlier report that the reaction between 4a and (COD)PdCl₂ results in a chelating κ²-C,N bonded iminocarbene complex (C–N)PdCl₂. The coordination mode and dynamic behavior of the iminocarbene ligand systems have been found to be dramatically affected by changes in the substitution pattern of the ligand system. Sterically unencumbered systems (a) favor the formation of κ²-C,N chelate structures containing one iminocarbene moiety per metal upon coordination at Pd(II); these complexes were demonstrated to engage in reversible, solvent-mediated chelate ring-opening reactions. Sterically encumbered systems (b–e) form non-chelating κ¹-C iminocarbene Pd(II) complexes containing two iminocarbene ligands per metal. Transannular repulsions across the chelate ring are believed to be the origin of these structural differences.