Reversible Capture Mechanism of CO₂ as a Zn(II)-Methylcarbonate

In this study, we elucidate the reaction mechanism for capturing CO₂ with the ZnL¹(MeOH) complex (L¹=diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazinatopyridine)) in a methanol solution, using density functional theory calculations. One pathway involves the protonation of ZnL¹(MeOH) by methylcarbonic acid, followed by ligand exchange of MeOH with MeOCO₂ ^-. An alternative mechanism suggests a tautomerization between ZnL¹(MeOH) and Zn(HL¹)(OMe), followed by CO₂ insertion. The latter pathway is energetically more favorable than the former and more complex than initially proposed. In fact, we unveiled that the solvent catalyzes tautomerization, as one explicit methanol molecule acts as a proton transfer agent. Then, Zn(HL¹)(OMe) captures CO₂, yielding a methylcarbonate bound to the metal center. The final step involves a rearrangement that leads to the cleavage of the Zn-O(Me)(COO) bond and the formation of a new Zn-O(COOMe) bond, along with the rotation of the methylcarbonate group. We consider an additional mechanism that combines tautomerization and ligand exchange but is endergonic and requires a high activation barrier for the ligand exchange. Furthermore, we evaluate the ligand basicity through the pK_a calculated values of the Zn(II) complexes, the effects of varying the ligand from 4-methyl-thiosemicarbazone to 4-ethyl (L²), 4-phenethyl (L³), and 4-benzyl (L⁴) derivatives, and reversibility of the reaction in an argon environment.