Dinuclear Au(i) N-heterocyclic carbene complexes derived from unsymmetrical azolium cyclophane salts: Potential probes for live cell imaging applications

We have synthesized a new series of azolium cyclophanes and used them as precursors of inherently luminescent dinuclear Au(i)-N-heterocyclic carbene (NHC) complexes. The azolium cyclophanes contained two azolium groups (either imidazolium or benzimidazolium), an o-xylyl group, and an alkyl linker chain (either C₂, C₃ or C₄). All of the azolium cyclophanes were characterised by X-ray diffraction studies and VT NMR studies, and all were fluxional in solution on the NMR timescale. The C₃- and C₄-linked azolium cyclophanes served as precursors of Au₂L₂ ²⁺ complexes (L is a cyclophane bis(NHC) ligand). Due to the unsymmetrical nature of the azolium cyclophanes, the Au₂L₂ ²⁺ complexes each existed as cis and trans isomers. X-ray diffraction studies showed that the Au₂L₂ ²⁺ complexes had short intramolecular Au⋯Au distances, in the range 2.9-3.3 Å, suggestive of an aurophilic attraction, presumably as a consequence of the geometrical constraints imposed by the cyclophane bis(NHC) ligands. The complexes having the shortest Au⋯Au distances (i.e., those based on C₃-linked cyclophanes) exhibited intense luminescence in solution. The uptake of one of the dinuclear Au-NHC complexes by tumorigenic cells, and its subsequent distribution and toxicity in the cells, was monitored by luminescence microscopy over 6 h and proliferation measurements, respectively.