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

• Published in: Chemphyschem Vol. 25; no. 16; p. e202400324
• Main Authors: Murillo, Fernando, Zarate, Ximena, Fernandez-Herrera, María A, Merino, Gabriel
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 19.08.2024
• Subjects: Argon; Basicity; Carbon dioxide; Density functional theory; Exchanging; Ligands; Methanol; Protonation; Reaction mechanisms; CO2 capture, reaction mechanism, solvent effects, tautomerization, zinc complexes
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.202400324

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. This 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. Furthermore, we evaluated the ligand basicity through the pKa 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.