Metal-Ligand Cooperativity Promotes Reversible Capture of Dilute CO₂ as a Zn(II)-Methylcarbonate

In this study, a series of thiosemicarbazonato-hydrazinatopyridine metal complexes were evaluated as CO₂ capture agents. The complexes incorporate a non-coordinating, basic hydrazinatopyridine nitrogen in close proximity to a Lewis acidic metal ion allowing for metal-ligand cooperativity. The coordination of various metal ions with (diacetyl-2-(4-methyl-thiosemicarbazone)-3-(2-hydrazinopyridine) (H₂L¹) yielded ML¹ (M = Ni(II), Pd(II)), ML¹(CH₃OH) (M = Cu(II), Zn(II)), and [ML¹(PPh₃)₂]BF₄ (M = Co(III)) complexes. The ML¹(CH₃OH) complexes reversibly capture CO₂ with equilibrium constants of 88 ± 9 and 6900 ± 180 for Cu(II) and Zn(II), respectively. Ligand effects were evaluated with Zn(II) through variation of the 4-methyl-thiosemicarbazone with 4-ethyl (H₂L²), 4-phenethyl (H₂L³), and 4-benzyl (H₂L⁴) derivatives. The equilibrium constant for CO₂ capture increased to 11,700 ± 300, 15,000 ± 400, and 35,000 ± 200 for ZnL²(MeOH), ZnL³(MeOH), and ZnL⁴(MeOH), respectively. Quantification of ligand basicity and metal ion Lewis acidity shows that changes in CO₂ capture affinity are largely associated with ligand basicity upon substitution of Cu(II) with Zn(II), while variation of the thiosemicarbazone ligand enhances CO₂ affinity by tuning the metal ion Lewis acidity. Overall, the Zn(II) complexes effectively capture CO₂ from dilute sources with up to 90%, 86%, and 65% CO₂ capture efficiency from 400, 1000, and 2500 ppm CO₂ streams.