Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

Abstract

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b·2Cl were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b·2Br or a six-step sequence for propylene dications 7a,b·2Br in ≥ 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17–20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]⁺ (25–26) complexes with the same ligand bridge were synthesized. [PdX₂bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]₂ and µ-bis(carbene)(AgX)₂ complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd₂Cl₂(allyl)₂(µ-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]⁺ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]⁺ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF₆ complexes were inactive as catalytic precursors in the allylic substitution reaction.