Pseudocryptand Hosts for Paraquats and Diquats

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry

Background:

• Crown ethers are macrocyclic polyethers known for their ability to bind cations.
• Inclusion complex formation is crucial in supramolecular chemistry for molecular recognition and assembly.
• The synthesis and study of pseudorotaxanes and cryptands are areas of active research in host-guest chemistry.

Purpose of the Study:

• To investigate the effect of hydrogen bonding interactions on the complexation of crown ethers with paraquat derivatives.
• To explore the formation of pseudocryptands and pseudopseudorotaxanes through anion-templated assembly.
• To compare the binding affinities of different functionalized crown ethers with diquat and paraquat dications.

Main Methods:

• Synthesis of functionalized bis(phenylene) crown ethers and their derivatives.
• Spectrophotometric titration to determine association constants of inclusion complexes.
• Anion-templated binding studies using various anions, including trifluoroacetate (TFA).

Main Results:

• Hydrogen bonding between acidic moieties (CH2OH, COOH) on crown ethers and anions enhances complex formation with paraquat derivatives.
• Trifluoroacetate (TFA) as a templating anion dramatically increases association constants, approaching those of covalent cryptands.
• Functionalization position and ester type (normal vs. reverse) influence binding affinities, with some reverse esters showing stronger paraquat complexation than normal esters.

Conclusions:

• Anion-templated hydrogen bonding is a powerful strategy to enhance host-guest complexation in supramolecular chemistry.
• The formation of pseudocryptands and pseudopseudorotaxanes can be effectively controlled by judicious design of crown ether hosts and templating anions.
• Structural modifications of crown ethers significantly impact their binding selectivity and strength towards specific guests like paraquat and diquat.