A series of thiosemicarbazonato-hydrazinatopyridine zinc(II) complexes were evaluated as direct air CO₂ capture agents. The complexes sequester CO₂ in a methanol solution as a metal-coordinated methylcarbonate. The reaction is reversible upon sparging of solutions with an inert gas (N₂ or Ar). The capture process involves metal–ligand cooperativity with the noncoordinating nitrogen of the hydrazinatopyridine functional group serving as a Brønsted–Lowry base and the zinc acting as a Lewis acid. In this study, the pendent amine of the thiosemicarbazonato group was varied to include 4-phenyl (ZnL⁵), 4-(trifluoromethyl)phenyl (ZnL⁶), 4-cyanophenyl (ZnL⁷), 4-tolyl (ZnL⁸), and 4-naphthyl (ZnL⁹). Hyperconjugation between the pendent group and the ligand core resulted in modulation of the metal ion acidity, as quantified by ligand exchange equilibrium constants (K₃ = 193–511) and ligand basicity (pK_a,MeOH = 11.09–11.94). Variations in electronic structure that decreased ligand basicity were more than offset by increases in Lewis acidity. The equilibrium constant (K₁) for CO₂ capture varied from 46300 to 73700. Overall, the value of K₁ was directly related to the relative Lewis acidity of the complexes (K₃). Notably, there was an overall inverse relationship between K₁ and the ligand basicity. The results provide insights into ligand design to further improve CO₂ capture.

中文翻译：

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通过金属-配体协同作用增强 CO2 捕获：调节配体碱度和 Zn(II) Lewis 酸度


一系列氨基硫脲-肼基吡啶锌 (II) 络合物作为直接空气 CO ₂ 捕获剂进行了评估。该配合物以金属配位的碳酸甲酯的形式在甲醇溶液中螯合CO ₂ 。当用惰性气体（N ₂ 或 Ar）喷射溶液时，该反应是可逆的。捕获过程涉及金属-配体与作为布朗斯台德-洛瑞碱的肼基吡啶官能团的非配位氮和作为路易斯酸的锌的协同作用。在本研究中，氨基硫脲基团的侧胺变化为包括 4-苯基 (ZnL ⁵ )、4-(三氟甲基)苯基 (ZnL ⁶ )、4-氰基苯基 ( ZnL ⁷ ）、4-甲苯基（ZnL ⁸ ）和 4-萘基（ZnL ⁹ ）。悬垂基团和配体核心之间的超共轭导致金属离子酸度的调节，通过配体交换平衡常数 (K ₃ = 193–511) 和配体碱度 (pK _a,MeOH 捕获的平衡常数 (K ₁ ) 在 46300 到 73700 之间变化。总体而言，K ₁ 的值与相对路易斯酸度直接相关配合物 (K ₃ )。值得注意的是，K ₁ 与配体碱度之间总体呈反比关系。结果为配体设计提供了见解，以进一步提高 CO ₂ 捕获。