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

Diacetyl-2-(4-methyl-3-thiosemicarbazonato)-3-(2-hydrazinatopyridine)(methanol) zinc(II) (ZnL¹⁰(MeOH)) and related structures use metal-ligand cooperativity to capture atmospheric CO₂ under ambient conditions. However, the low solubility in protic solvents limits their practical use in direct air capture systems. This study reports the synthesis, characterization, and CO₂ binding affinity of a series of new alkylthiocarbamato-hydrizinato(pyridine) ZnL(MeOH) complexes (n = 1-9) and assesses the solubility and CO₂ binding affinity of each complex. Replacement of the thiosemicarbazonato functional group with alkylthiocarbamato groups leads to increased Lewis acidity and CO₂ binding affinity relative to ZnL¹⁰(MeOH). Additionally, the solubility of the complexes increased as a function of the alkylthiocarbamato group. In comparison to the structurally related thiosemicarbazonato complex, ZnL¹¹(MeOH), the solubility of the ZnL¹(MeOH) to ZnL⁹(MeOH) complexes was more than 100 times higher, accompanied by excellent binding affinities. The CO₂ equilibrium binding constant (K₁) showed an increase from 33,600 ± 1700 for ZnL¹(MeOH) to 69,000 ± 7900 for ZnL⁸(MeOH) with the addition of the backbone phenyl group. Overall, the study revealed that the total amount of CO₂ captured per unit volume is influenced by both the CO₂ binding constant (K₁) and the solubility of the complex, with the solubility being the dominant factor.