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) ZnLn(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.