Enhancing CO₂ Capture via Metal-Ligand Cooperativity: Tuning Ligand Basicity and Zn(II) Lewis Acidity

Area of Science:

• Coordination Chemistry
• Materials Science
• Environmental Chemistry

Background:

• Direct air capture of carbon dioxide (CO₂) is crucial for mitigating climate change.
• Metal-organic complexes offer potential for efficient CO₂ sequestration.
• Understanding structure-activity relationships is key to designing effective capture agents.

Purpose of the Study:

• To synthesize and evaluate novel thiosemicarbazonato-hydrazinatopyridine zinc(II) complexes for direct air CO₂ capture.
• To investigate the role of ligand modification on the CO₂ capture performance.
• To elucidate the mechanism of CO₂ sequestration involving metal-ligand cooperativity.

Main Methods:

• Synthesis of a series of zinc(II) complexes with varying thiosemicarbazonato ligands (ZnL⁵-ZnL⁹).
• Evaluation of CO₂ capture efficiency in methanol solution.
• Reversibility studies using inert gas sparging.
• Quantification of metal ion Lewis acidity and ligand basicity through equilibrium constants (K₃) and pK_a,MeOH measurements.

Main Results:

• The zinc(II) complexes effectively sequester CO₂ as metal-coordinated methylcarbonate.
• CO₂ capture is reversible, allowing for potential regeneration and reuse.
• Ligand modification significantly impacted CO₂ capture efficiency, with equilibrium constants (K₁) ranging from 46300 to 73700.
• CO₂ capture efficacy correlated positively with the Lewis acidity of the zinc center and inversely with ligand basicity.

Conclusions:

• The studied zinc(II) complexes demonstrate promising performance as direct air CO₂ capture agents.
• Metal-ligand cooperativity, involving the zinc Lewis acid and the hydrazinatopyridine base, is the key mechanism.
• Tuning ligand electronic properties, particularly enhancing Lewis acidity, is a viable strategy for designing superior CO₂ capture materials.