Noncooperative guest binding by metal-free [2 + 2] Schiff-base macrocycles

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Macrocyclic Chemistry

Background:

• Salphen-based macrocycles are well-studied for their coordination chemistry with metal ions.
• Metal-free salphen macrocycles exhibit distinct reactivity but are less explored.
• Schiff-base macrocycles offer unique structural and binding properties.

Purpose of the Study:

• To investigate the host-guest binding properties of a metal-free [2 + 2] Schiff-base macrocycle (host 3).
• To characterize the binding affinity and stoichiometry of host 3 with various cationic guests.
• To compare the binding behavior of metal-free host 3 with its metal-coordinated analogs.

Main Methods:

• Synthesis and characterization of the [2 + 2] Schiff-base macrocycle host 3.
• Spectrophotometric titration in dichloromethane/acetonitrile (DCM/MeCN) to determine binding constants (K11).
• Analysis of host-guest complex stoichiometry and binding models.

Main Results:

• Host 3 exhibits strong binding for spherical alkali metal cations (K+, Na+, Li+, Rb+) with association constants (K11) in the range of 10^3–10^4 M^-1.
• Binding affinity is highest for K+ and Na+, with significantly weaker binding observed for sterically demanding guests like dibenzylammonium (DBA+).
• The macrocycle's cavity size, slightly smaller than 18-crown-6, facilitates 2:1 host-guest complex formation, where two host molecules bind a single guest.
• All host-guest interactions follow a noncooperative 2:1 binding model, contrasting with the predominantly 1:1 binding of metal-coordinated salphen macrocycles.

Conclusions:

• Metal-free [2 + 2] Schiff-base macrocycle host 3 demonstrates selective binding towards spherical cations.
• The unique 2:1 noncooperative binding mode highlights the distinct supramolecular behavior of metal-free macrocycles.
• These findings expand the understanding of salphen-based macrocycle chemistry beyond metal coordination.