Herein, we present the thermodynamic analysis of a cobalt complex with a new pentadentate N‐donor ligand bearing four ionisable pyrazole protons in aqueous solution. A detailed analysis of the Co^II complex [Co(L)(X)]^+/2+ in the solid state revealed that the 6^th ligand X at the metal centre depends on the cobalt source employed. Small anions such as Cl^– and NO₃^– coordinate to the metal ion, while larger anions that are weaker hydrogen‐bond acceptors are found in the second coordination sphere of the complex and instead a solvent molecule coordinates. However, in aqueous KCl solution, the sixth ligand is always chloride forming [Co(L)Cl]Cl, 1^Cl. pH dependent species distribution studies revealed a pK_a of 7.3(3) for the first ionisable pyrazole proton in the cobalt(II) complex and 6.0(3) in the cobalt(III) complex (methanol/H₂O mixture). That is the oxidation state has a fairly minor influence on the pK_a of the pyrazole proton. The Co^III/Co^II redox pair of the complex with the fully protonated ligand exhibits a potential of 0.78 V vs. NHE. The BDFE of the hypothetical H‐atom abstraction step of [Co^II(L)Cl]⁺ forming [Co^III(LH_–1)Cl]⁺ was determined to equal 336 kJ mol^–1.

中文翻译：

---

含吡咯质子的钴配合物的热化学

在本文中，我们介绍了一种含新的五齿N供体配体的钴配合物的热力学分析，该配体在水溶液中带有四个可电离的吡唑质子。对固态的Co ^II配合物[Co（L）（X）] ^{+ / 2 +}的详细分析表明，金属中心的第六^个配体X取决于所用的钴源。小的阴离子如Cl ^-和NO ₃ ^-配位到金属离子，而较大的阴离子是较弱的氢键受体在复杂和代替溶剂分子坐标的第二配位球中找到。但是，在KCl水溶液中，第六个配体始终是氯化物，形成[Co（L）Cl] Cl，1^氯。pH依赖性物种分布研究表明，钴（II）配合物中第一个可电离的吡唑质子的ap K _a值为7.3（3），钴（III）配合物（甲醇/ H ₂ O混合物）中的ap K _a为6.0（3 ）。也就是说，氧化态对吡唑质子的p K _a具有相当小的影响。具有完全质子化的配体的配合物的Co ^III / Co ^II氧化还原对相对于NHE表现出0.78 V的电势。假设的[Co ^II（L）Cl] ⁺形成[Co ^III（L H _–1）Cl]的H原子抽象步骤的BDFE⁺被确定为等于336 kJ mol ^–1。