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

A series of thiosemicarbazonato-hydrazinatopyridine zinc(II) complexes were evaluated as direct air CO₂ capture agents. The complexes sequester CO₂ in a methanol solution as a metal-coordinated methylcarbonate. The reaction is reversible upon sparging of solutions with an inert gas (N₂ or Ar). The capture process involves metal-ligand cooperativity with the noncoordinating nitrogen of the hydrazinatopyridine functional group serving as a Brønsted-Lowry base and the zinc acting as a Lewis acid. In this study, the pendent amine of the thiosemicarbazonato group was varied to include 4-phenyl (ZnL⁵), 4-(trifluoromethyl)phenyl (ZnL⁶), 4-cyanophenyl (ZnL⁷), 4-tolyl (ZnL⁸), and 4-naphthyl (ZnL⁹). Hyperconjugation between the pendent group and the ligand core resulted in modulation of the metal ion acidity, as quantified by ligand exchange equilibrium constants (K₃ = 193-511) and ligand basicity (pK_a,MeOH = 11.09-11.94). Variations in electronic structure that decreased ligand basicity were more than offset by increases in Lewis acidity. The equilibrium constant (K₁) for CO₂ capture varied from 46300 to 73700. Overall, the value of K₁ was directly related to the relative Lewis acidity of the complexes (K₃). Notably, there was an overall inverse relationship between K₁ and the ligand basicity. The results provide insights into ligand design to further improve CO₂ capture.