A bio-inspired zinc finger analogue anchored in 2D hexagonal mesoporous silica for room temperature CO2 activation via a hydrogenocarbonate route

Abstract

Bio-inspired diethylenetriamine–zinc(II) complexes were anchored into the nanopores of hexagonal mesoporous MCM41-like silicas targeting a carbamate free and low temperature CO₂ recycling process. A step-by-step approach was adopted to perform an in situ synthesis in order to mimic the zinc finger of carbonic anhydrases, the fastest family of enzymes. In the presence of a surface-masking pattern of TMA⁺ ions, some silanol groups were capped using grafted trimethylsilyl functions, TMS_gr, (gr for grafted). After removing the masking ions, a tridentate diethylenetriamine ligand was anchored using diethylenetriamine propyltrimethoxysilane. The so-called DETA_an ligands (an for anchored) were partially monoprotonated using either cyclohexane or isopropanol as a solvent. Nonetheless, up to two thirds of them were metallated by Zn(II) ions, leading to the targeted anchored zinc finger mimic [Zn(DETA_an)L]⁺(L = Cl⁻ or OH⁻). CO₂ is then adsorbed at room temperature and in humid ambient air by the formation of an intermediate hydrogenocarbonate–zinc complex. Specific IR signatures at 1330 and 1400 cm⁻¹ together with characteristic C 1s and Zn 2p_3/2 XPS binding energies at 286.4 and 1024.6 eV advocate for a rather symmetrical bidentate [η²-CO₃] structural unit in the anchored complex [Zn(DETA_an)(η²-HCO₃*)]⁺, where the Zn(II) ion is most likely pentacoordinated. The internal pH value varied by less than 0.5 depending on the metal reacting with the DETA_an ligand and its ability to generate HCO₃⁻, due to the buffering effect of surface silanol and amino groups according to the level of protonation of the DETA moieties measured from the N 1s XPS spectra. In contrast to nitrate ions, chloride ions were found to inhibit the formation of hydrogenocarbonate.