A Co^III complex (2) of a bispyridine-dioxime ligand (H₂L^NMe2) containing a tertiary amine group in the proximity of the Co center is synthesized and characterized. One of the oxime protons of the ligand is deprotonated, and the amine group remains protonated in the solid-state structure of the Co^II complex (2a). The acid–base properties of 2 showed pK_a values of 5.9, 8.4, and 9.6, which are assigned to the dissociation of two consecutive oxime protons and amine protons, respectively. The electrocatalytic proton reduction of 2 was investigated in an aqueous phosphate buffer solution (PBS), revealing a catalytic hydrogen evolution reaction (HER) at an E_cat/2 of −1.01 V vs. the SHE, with an overpotential of 673 mV and a k_obs value of 2.6 × 10³ s⁻¹ at pH 7. For comparison, the HER of the Co complex (1) lacking the tert-amine group at the secondary sphere was investigated in PBS, which showed a k_obs of 1.3 × 10³ s⁻¹ and an overpotential of 577 mV. At pH 4, however, 2 revealed a ∼3 times higher k_obs value than 1, which suggests that the protonated amine group likely works as a proton relay site. Notably, no significant change in the reaction rate was observed at different pH values for 1, implying that oxime protons may not be involved in the intramolecular proton-coupled electron transfer reaction in the HER. The k_obs values for Co complexes at pH 7.0 are significantly higher than those of the [Co(dmgH)₂(pyridine)(Cl)] complex, implying that the primary coordination sphere around 1 or 2 enhances the HER and offers better catalyst stability in acidic buffer solutions.

中文翻译：

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探索钴络合物二次配位层上的侧胺基对电催化析氢反应的影响

合成并表征了在 Co 中心附近含有叔胺基团的双吡啶-二肟配体 (H ₂ L ^NMe2 ) 的Co ^III配合物 ( 2 )。配体的肟质子之一被去质子化，并且胺基在 Co ^II络合物 ( 2a )的固态结构中保持质子化。 2的酸碱性质显示 p K _a值为 5.9、8.4 和 9.6，分别归因于两个连续肟质子和胺质子的解离。在磷酸盐缓冲水溶液 (PBS) 中研究了2的电催化质子还原，揭示了催化析氢反应 (HER)，相对于SHE，E _cat/2为 -1.01 V ，超电势为 673 mV， pH 7 时k _obs值为 2.6 × 10 ³ s ^-1。为了进行比较，在 PBS 中研究了二级球上缺少叔胺基团的 Co 配合物 ( 1 )的 HER ，结果显示k _obs为 1.3 × 10 ³ s ^-1和577 mV的过电势。然而，在 pH 4 时，2显示出比1高约 3 倍的k _obs值，这表明质子化胺基团可能充当质子中继位点。值得注意的是，在1的不同pH值下没有观察到反应速率的显着变化，这意味着肟质子可能不参与HER中的分子内质子耦合电子转移反应。 Co 配合物在 pH 7.0 时的k _obs值显着高于 [Co(dmgH) ₂ (pyridine)(Cl)] 配合物，这意味着1或2附近的主配位层增强了 HER 并提供了更好的催化剂稳定性在酸性缓冲溶液中。