Dianionic pincer ligands of the type C^N^C are a highly effective way of stabilizing gold(III) complexes. Complexes derived from 2,6-diphenylpyridine in particular have been intensely investigated. The development of gold pincer chemistry is however limited by the failure of synthetic routes for heterocycles other than pyridine. We have developed a protocol for the synthesis of pyrazine-based pincers which has opened the route to an unexpectedly large range of photoluminescent complexes, of the type (C^N(pz)^C)AuX. In particular, pyrazine complexes are luminescent at room temperature where their pyridine analogues are not, and the photoemissions can easily be modified by protonation, N-alkylation, Lewis acids and metal ions, to cover the emission range from white to blue and deep red. Complexes where X = thiolate constitute a new class of photoluminescent materials with unexpectedly varied emission behaviour, from green ligand-based triplet emissions to intense red. They form both red and yellow crystal polymorphs which differ only in type of pi-stacking of the planar molecules. Pyrazine-pyrazine pi-stacking is shown to be responsible for the red emission. These pi-interactions persist in solution. DFT modelling has traced this emission to a bimolecular excited stated, which extends over the two stacked pyrazine rings, whereas other stacking types lead to non-emissive compounds. Intriguingly, on sample manipulation the emissions can be switched reversibly from red to green. Other types of (C^N(pz)^C)AuX complexes show good in-vitro anti-cancer activity, including towards cisplatin resistant cell lines, and their cell uptake can be monitored by luminescence microscopy.

Pyrazine-based gold(III) pincer complexes: Synthesis and Luminescence

253rd National Meeting (2017)

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