Synthesis of a flexible macrocyclic tetraimidazolium salt-precursor for a tetracarbene ligand with metal dependent coordination modes

The cyclic tetraimidazolium salt (H-4-4)(PF6)(4) with flexible linkers between the imidazolium groups has been synthesized following a stepwise synthetic approach. In situ deprotonation of (H-4-4)(PF6)(4) in the presence of different metal ions leads to complexes where the tetracarbene ligand shows differing coordination modes depending on the metal center. Due to its high flexibility the tetracarbene ligand folds around d(8) transition metal ions such as Ni-II, Pd-II and Pt-II to yield mononuclear tetracarbene complexes of type [M(4)](PF6)(2) featuring a square-planar coordinated metal center. Reaction of 4 with metal centers that prefer a linear coordination mode such as Ag-I yields the tetranuclear silver(I) octacarbene complex [Ag-4(4)(2)](PF6)(4) featuring four silver(I) ions sandwiched in between two tetra-NHC ligands.