Lack of Cooperativity in the Triangular X₃ Halogen-Bonded Synthon?

Area of Science:

• Crystallography
• Computational Chemistry
• Supramolecular Chemistry

Background:

• Halogen bonding is a key non-covalent interaction.
• The X3 synthon, featuring three halogen-halogen contacts, is hypothesized to be stabilized by cooperative effects.
• Previous theoretical studies suggested weak synergy only for iodine-based systems.

Purpose of the Study:

• To computationally investigate cooperativity in type II halogen-halogen contacts within X3 synthons in crystal structures.
• To evaluate the stabilizing role of synergistic interactions in these motifs.
• To test the hypothesis of cooperative effects in triangular halogen-bonded motifs.

Main Methods:

• Analysis of 44 crystal structures from the Cambridge Structural Database.
• Computational investigation of X3 synthons, including halomesitylene structures.
• Theoretical calculations to quantify synergistic interaction energies.

Main Results:

• No significant cooperativity was found for halomesitylene trimers with X3 synthons.
• Very weak or weak synergy (less than -0.40 kcal mol⁻¹) was observed in only two other structures containing the I3 synthon.
• The iodoform crystal structure exhibited the most pronounced cooperativity, approximately 10% of the total interaction energy.

Conclusions:

• The hypothesis of significant cooperative effects stabilizing X3 synthons is largely unsupported by computational analysis of crystal structures.
• Synergistic interactions in these halogen-bonded motifs are minimal, except for specific iodine-containing structures.
• The findings refine our understanding of halogen bond cooperativity and its role in crystal packing.