AAA-DDD triple hydrogen bond complexes.

Area of Science:

• Supramolecular Chemistry
• Chemical Physics
• Organic Chemistry

Background:

• The AAA-DDD pattern represents the optimal arrangement for three contiguous hydrogen bonding centers, maximizing association between molecular species.
• Previous studies established lower limits for neutral and cationic AAA-DDD systems, indicating significant binding strengths.

Purpose of the Study:

• To synthesize and precisely quantify the association constant of a novel cationic AAA-DDD system.
• To investigate the structural features contributing to the enhanced binding strength in this system.

Main Methods:

• Synthesis of a new cationic AAA-DDD complex (6*10+).
• Determination of the association constant (K(a)) using (1)H NMR spectroscopy.
• X-ray crystallography to elucidate the structural arrangement of hydrogen bonds and electrostatic interactions.

Main Results:

• The cationic complex 6*10+ demonstrated a record-breaking association constant (K(a)) of 3 x 10(10) M(-1) in dichloromethane at room temperature.
• X-ray structural analysis revealed a planar array of three short, parallel primary hydrogen bonds (NH...N distances 1.95-2.15 Å).
• These primary hydrogen bonds are reinforced by significant electrostatic interactions between protons and adjacent acceptor atoms.

Conclusions:

• The AAA-DDD arrangement, particularly in cationic systems, can achieve unprecedented binding strengths.
• The precise structural configuration, including short, parallel hydrogen bonds and electrostatic reinforcement, is crucial for maximizing molecular association.