Homoleptic crown N-heterocyclic carbene complexes

Two ways about it: The first metal N-heterocyclic carbene complexes derived from a cyclic tetraimidazolium salt show a remarkable versatility of ligand conformation and coordination geometry. With Pd ^II, a mononuclear square-planar complex is obtained, but with Cu ^I and Ag^I, an unprecedented dinuclear motif with a short metal-metal interaction is observed (see structure; N blue, C white ellipsoids, H white circles).