Anion Control of Lanthanoenediyne Cyclization

Area of Science:

• Coordination Chemistry
• Organometallic Chemistry
• Materials Science

Background:

• Enediyne compounds are known for their unique reactivity, including Bergman cyclization.
• Lanthanide complexes offer tunable electronic and magnetic properties.
• Macrocyclic ligands provide structural control and stability in coordination complexes.

Purpose of the Study:

• To synthesize and characterize a series of lanthanide-enediyne macrocyclic complexes.
• To investigate the influence of counter-anions on the Bergman cyclization temperature of these complexes.
• To explore the electronic perturbations within the macrocycle upon metal coordination and anion interaction.

Main Methods:

• Synthesis of lanthanide-enediyne complexes using a hexaaza-macrocycle ligand.
• Differential Scanning Calorimetry (DSC) to determine solid-state Bergman cyclization temperatures.
• Carbon-13 Nuclear Magnetic Resonance (¹³C NMR) spectroscopy for electronic characterization.
• Computational modeling to analyze metal-macrocycle interactions and coordination numbers.

Main Results:

• Bergman cyclization temperatures decrease with decreasing anion denticity (NO₃^- > Cl^- > OTf^-).
• ¹³C NMR chemical shifts indicate alkyne depolarization upon metal inclusion, with the effect increasing for less coordinating anions.
• Computational studies reveal increased lanthanide-macrocycle coordination number and decreased Ln-N bond lengths with decreasing anion coordination.

Conclusions:

• The Bergman cyclization barrier of lanthanide-enediyne macrocycles is primarily governed by the coordinating ability of the counter-anion.
• Anion coordination strength modulates the electronic properties and metal-macrocycle interaction.
• This study provides insights into controlling the reactivity of enediyne ligands through metal coordination and anion effects.