Solubility-Driven Ligand Design of Zn(II) Complexes for Enhanced CO₂ Capture in Methanol

Area of Science:

• Coordination Chemistry
• Materials Science
• Environmental Chemistry

Background:

• Metal-ligand cooperativity enables zinc complexes to capture atmospheric CO2 under ambient conditions.
• Low solubility of existing complexes in protic solvents hinders practical application in direct air capture (DAC) systems.
• Need for improved materials with enhanced solubility and CO2 binding affinity for efficient DAC.

Purpose of the Study:

• To synthesize and characterize novel zinc complexes with alkylthiocarbamato ligands.
• To evaluate the CO2 binding affinity and solubility of these new complexes.
• To investigate the structure-property relationships influencing CO2 capture efficiency.

Main Methods:

• Synthesis of a series of alkylthiocarbamato-hydrizinato(pyridine) zinc(II) complexes (ZnL(n)(MeOH), n=1-9).
• Characterization using spectroscopic and analytical techniques.
• Measurement of CO2 binding affinity (equilibrium binding constant, K1) and solubility in protic solvents.

Main Results:

• Replacement of thiosemicarbazonato with alkylthiocarbamato groups increased Lewis acidity and CO2 binding affinity.
• Solubility significantly increased (over 100 times) compared to thiosemicarbazonato analogues, dependent on the alkylthiocarbamato chain length.
• CO2 binding constant (K1) increased from 33,600 ± 1700 for ZnL(1)(MeOH) to 69,000 ± 7900 for ZnL(8)(MeOH).

Conclusions:

• Novel zinc complexes exhibit enhanced solubility and CO2 binding affinity, overcoming limitations of previous materials.
• Solubility is identified as the dominant factor for total CO2 captured per unit volume in DAC systems.
• These findings present promising candidates for developing efficient direct air capture technologies.