Metal-Ligand Cooperativity Promotes Reversible Capture of Dilute CO 2 as a Zn(II)-Methylcarbonate.

In this study, a series of thiosemicarbazonato-hydrazinatopyridine metal complexes were evaluated as CO 2 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) (H 2 L 1 ) yielded ML 1 (M = Ni(II), Pd(II)), ML 1 (CH 3 OH) (M = Cu(II), Zn(II)), and [ML 1 (PPh 3 ) 2 ]BF 4 (M = Co(III)) complexes. The ML 1 (CH 3 OH) complexes reversibly capture CO 2 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 (H 2 L 2 ), 4-phenethyl (H 2 L 3 ), and 4-benzyl (H 2 L 4 ) derivatives. The equilibrium constant for CO 2 capture increased to 11,700 ± 300, 15,000 ± 400, and 35,000 ± 200 for ZnL 2 (MeOH), ZnL 3 (MeOH), and ZnL 4 (MeOH), respectively. Quantification of ligand basicity and metal ion Lewis acidity shows that changes in CO 2 capture affinity are largely associated with ligand basicity upon substitution of Cu(II) with Zn(II), while variation of the thiosemicarbazone ligand enhances CO 2 affinity by tuning the metal ion Lewis acidity. Overall, the Zn(II) complexes effectively capture CO 2 from dilute sources with up to 90%, 86%, and 65% CO 2 capture efficiency from 400, 1000, and 2500 ppm CO 2 streams.