Reversible CO₂ Fixation and Release by a Trinuclear Zn(II) Cryptate Complex and Operando Analysis of the Complex...

Metal complexes inspired by carbonic anhydrase (CA), which is a metalloenzyme containing Zn(II), have been investigated as alternatives for CO₂ fixation systems operating under ambient temperature and pressure conditions. In this study, we designed a trinuclear Zn(II) cryptate complex (Zn₃ L) and demonstrated rapid CO₂ fixation with carbonation of CO₂ using Zn₃ L. The CO₂ fixation performance of Zn₃ L surpassed that of a standard CO₂ absorbent, KOH(aq) solution, under conditions of the same solute concentration. In addition, the reaction achieved operation without support addition of base, which has been often required in systems of CA-inspired complexes. Fixed CO₂ was released by protonating polyazacryptate ligand (L) and breaking the complex structure, and deprotonation of L induced the reconstruction of Zn₃ L, allowing it to refix CO₂ . This reaction mechanism was proposed based on the analysis of operando extended X-ray absorption fine structure spectroscopy. Zn₃ L also demonstrated the ability to capture dilute CO₂ from air, and the volume of CO₂ captured by Zn₃ L was approximately 2.6 times that captured by the KOH(aq) solution. Our Zn₃ L exhibited three valuable properties: rapid CO₂ fixation without a base, reversibility, and ability to capture dilute CO₂ ; thus Zn₃ L is a promising candidate as CO₂ fixatives.