Molecular Assembly Directed by Metal-Aromatic Interactions: Control of the Aggregation and Photophysical Properties of Zn-Salen Complexes by Aromatic Mercuration

There is widespread interest in non-covalent bonding and weak interactions, such as electrostatic interactions, hydrogen bonding, solvophobic/hydrophobic interactions, metalmetal interactions, and pp stacking, to tune the molecular assembly of planar p-conjugated organic and inorganic molecules. Inspired by the roles of metalaromatic interaction in biological systems, such as in ion channels and metalloproteins, herein, we report the first example of the use of Hg2+aromatic interactions to selectively control the assembly and disassembly of zincsalen complexes in aqueous media; moreover, this process exhibited significant turn on fluorescent properties. UV/Vis and fluorescence spectroscopic analysis of the titration of Hg2+ ions versus complex ZnL1 revealed that the higher binding affinity of Hg2+ ions (compared to 13 other metal ions) was ascribed to specific interactions between the Hg2+ ions and the phenyl rings of ZnL1; this result was also confirmed by 1H NMR spectroscopy and HRMS (ESI). Further evidence for this type of interaction was obtained from the reaction of small-molecule analogue L1 with Hg2+ ions, which demonstrates the proximity of the N-alkyl group to the aromatic protons during Hg2+-ion binding, which led to the consequential H/D exchange reaction with D2O. DFT modeling of such interactions between the Hg2+ ions and the phenyl rings afforded calculated distances between the C and Hg atoms (2.29 angstrom) that were indicative of C?Hg bond-formation, under the direction of the N atom of the morpholine ring. The unusual coordination of Hg2+ ions to the phenyl ring of the metallosalen complexes not only strengthened the binding ability but also increased the steric effect to promote the disassembly of ZnL1 in aqueous media.