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