Templating Schiff-base lateral macrobicycles: an experimental and theoretical structural study of the intermediates

Area of Science:

• Coordination Chemistry
• Supramolecular Chemistry
• Organic Synthesis

Background:

• The synthesis of macrocyclic compounds often relies on template effects, where metal ions guide the assembly of organic ligands.
• Barium complexes with N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L(2)) were investigated for their structural properties and templating ability.

Purpose of the Study:

• To elucidate the structural characteristics of barium complexes with L(2) in both solid and solution states.
• To rationalize the role of the barium(II) ion as a template in the synthesis of Schiff-base lateral macrobicycles.

Main Methods:

• X-ray crystallography was used to determine the solid-state structures of barium complexes.
• Variable temperature NMR spectroscopy ((1)H and (13)C) was employed to study the conformational behavior in solution.
• Density Functional Theory (DFT) calculations (B3LYP/LanL2DZ, GIAO method) were performed to predict conformational stability and NMR shielding tensors.

Main Results:

• Solid-state structures revealed an anti conformation of the L(2) receptor induced by the barium(II) ion.
• In solution, complexes exist as a mixture of syn and anti isomers, with the syn isomer being crucial for templating.
• DFT calculations predicted the syn conformation to be more stable and experimental NMR data supported its predominance in solution.

Conclusions:

• The presence of the syn isomer in solution, irrespective of the counterion, explains the effectiveness of barium(II) as a template agent.
• The study assigns a syn stereochemistry to the major species in solution, facilitating macrobicycle synthesis.
• This work provides insights into the structure-activity relationship of barium complexes in template-assisted organic synthesis.