The structure of Cu(II) and Hg(II) complexes of bispyrenyl azine revisited

Azine ligands show preference for the antiperiplanar conformation, but their 2:2 ligand-metal complexes can exhibit a central N₄M₂ core in the most stable chair arrangement, as in the case of the model lithium complex, with the ligand displaying a synclinal conformation that is not stable in the free ligand. According to DFT calculations, complexation of Cu²⁺ with bis(1-pyrenyl)azine (1) affords a C₂-symmetric [1·Cu²⁺]₂ species with a planar central N₄Cu₂ core exhibiting a weak cuprophilic interaction. The pendant pyrenyl substituents are brought close and parallel to each other, therefore accounting for the π-stacking that is responsible for excimer fluorescence. In case of Hg²⁺, complexation by the same ligand affords a C₂-symmetric complex with an essentially linear N-Hg-C coordination environment at every metal center and without intermetallic interaction. T-Stacking interactions between pyrenyl groups explain the overall rigidity required for enhancement of fluorescence. Graphical abstract The structures of 2:2 complexes of Hg(II) and Cu(II) with bispyrenyl azine are reported.