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

Abstract

There is widespread interest in non-covalent bonding and weak interactions, such as electrostatic interactions, hydrogen bonding, solvophobic/hydrophobic interactions, metal-metal interactions, and π-π stacking, to tune the molecular assembly of planar π-conjugated organic and inorganic molecules. Inspired by the roles of metal-aromatic interaction in biological systems, such as in ion channels and metalloproteins, herein, we report the first example of the use of Hg(2+) -aromatic interactions to selectively control the assembly and disassembly of zinc-salen complexes in aqueous media; moreover, this process exhibited significant "turn on" fluorescent properties. UV/Vis and fluorescence spectroscopic analysis of the titration of Hg(2+) ions versus complex ZnL(1) revealed that the higher binding affinity of Hg(2+) ions (compared to 13 other metal ions) was ascribed to specific interactions between the Hg(2+) ions and the phenyl rings of ZnL(1); this result was also confirmed by (1)H NMR spectroscopy and HRMS (ESI). Further evidence for this type of interaction was obtained from the reaction of small-molecule analogue L(1) with Hg(2+) ions, which demonstrates the proximity of the N-alkyl group to the aromatic protons during Hg(2+)-ion binding, which led to the consequential H/D exchange reaction with D(2) O. DFT modeling of such interactions between the Hg(2+) ions and the phenyl rings afforded calculated distances between the C and Hg atoms (2.29 Å) that were indicative of C-Hg bond-formation, under the direction of the N atom of the morpholine ring. The unusual coordination of Hg(2+) 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 ZnL(1) in aqueous media.

Two-photon luminescent metal complexes for bioimaging and cancer phototherapy

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A sensitive and quantitative autolysosome probe for detecting autophagic activity in live and prestained fixed cells

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