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

Abstract

A series of thiosemicarbazonato-hydrazinatopyridine zinc(II) complexes were evaluated as direct air CO 2 capture agents. The complexes sequester CO 2 in a methanol solution as a metal-coordinated methylcarbonate. The reaction is reversible upon sparging of solutions with an inert gas (N 2 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 (ZnL 5 ), 4-(trifluoromethyl)phenyl (ZnL 6 ), 4-cyanophenyl (ZnL 7 ), 4-tolyl (ZnL 8 ), and 4naphthyl (ZnL 9 ). Hyperconjugation between the pendent group and the ligand core resulted in modulation of the metal ion acidity, as quantified by ligand exchange equilibrium constants (K 3 = 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 (K 1 ) for CO 2 capture varied from 46300 to 73700. Overall, the value of K 1 was directly related to the relative Lewis acidity of the complexes (K 3 ). Notably, there was an overall inverse relationship between K 1 and the ligand basicity. The results provide insights into ligand design to further improve CO 2 capture.