Lone-Pair-π Bond Strength Unveiled by a Combined Experimental and Computational Study

Area of Science:

• Organometallic Chemistry
• Supramolecular Chemistry
• Photochemistry

Background:

• N-heterocyclic carbenes (NHCs) are versatile ligands in organometallic chemistry.
• Perylene-diimide (PDI) and naphthalene-diimide (NDI) are important chromophores with unique electronic properties.
• Understanding non-covalent interactions is crucial for designing functional molecular materials.

Purpose of the Study:

• To synthesize and characterize novel iridium(III) complexes featuring NDI and PDI-based NHC ligands.
• To investigate the presence and influence of lone-pair-π interactions in these complexes.
• To quantify the strength of these non-covalent interactions in solution.

Main Methods:

• Synthesis and full characterization of iridium(III)-NHC complexes.
• Nuclear Magnetic Resonance (NMR) spectroscopy (including variable-temperature 1H NMR).
• X-ray crystallography for molecular structure determination.
• Computational studies (e.g., DFT) to support experimental findings.

Main Results:

• Successful preparation of iridium(III) bis-NHC complexes linked to NDI and PDI units.
• Evidence of lone-pair-π interactions between chloride ligands and NDI/PDI moieties.
• Formation of stable atropisomers in solution due to restricted rotation around the Ir-Ccarbene bond.
• Quantification of the lone-pair-π interaction strength (ΔH = 6.6–10 kcal/mol).

Conclusions:

• The study demonstrates the occurrence of significant lone-pair-π interactions in solution for iridium(III)-NHC complexes.
• These interactions play a key role in dictating the conformational behavior and stability of the complexes.
• The findings provide valuable insights into the design principles for advanced organometallic materials with tunable properties.