Halogen Bond-Driven Ligand Displacement: Co-Crystal Lattice Versus Coordination Bonds

Area of Science:

• Supramolecular Chemistry
• Coordination Chemistry
• Crystallography

Background:

• Coordination bonds typically exhibit greater strength than halogen bonds.
• Specific electronic configurations, like the Jahn-Teller effect in Cu(II) and trans influence in vanadyl complexes, can modulate M-N bond strengths.
• Halogen bonding (XB) involves interactions between electron-deficient regions of halogen atoms and electron-rich sites.

Purpose of the Study:

• To investigate the competitive interactions between metal-ligand coordination bonds and halogen bonds.
• To explore the formation of co-crystals between transition metal acetylacetonate complexes and halogen bond donors.
• To understand the factors influencing ligand displacement in these systems.

Main Methods:

• Co-crystallization experiments using M(acac)2(L) complexes (M=Cu(II), V(IV)=O; L=amine) and 1,4-diiodotetrafluorobenzene (1,4-DITFB).
• X-ray diffraction analysis to characterize the resulting crystalline structures.
• Computational studies to elucidate the energetic contributions to observed phenomena.

Main Results:

• Unexpected formation of separate M(acac)2·1,4-DITFB and L·1,4-DITFB co-crystals, instead of the anticipated M(acac)2(L)·1,4-DITFB adducts.
• Evidence of amine ligand displacement driven by halogen bonding.
• Computational analysis indicated that while halogen bonds alone might not break M-N bonds, lattice stabilization of halogen-bonded products facilitates displacement.

Conclusions:

• Halogen bonding, particularly when coupled with lattice stabilization, can overcome weaker coordination bonds in specific transition metal complexes.
• The Jahn-Teller distortion in Cu(II) and the trans influence in vanadyl complexes play a crucial role in enabling ligand displacement.
• This study highlights the nuanced interplay between coordination strength, halogen bonding, and crystal packing effects.