Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology (BIST), Av. Països Catalans 16, 43007, Tarragona, Spain.
Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057, Zurich, Switzerland.
ChemSusChem. 2020 May 22;13(10):2745-2752. doi: 10.1002/cssc.202000283. Epub 2020 Apr 6.
Cobalt complexes containing equatorial tetraazamacrocyclic ligands are active catalysts for the hydrogen evolution reaction in pure aqueous conditions. We investigated the effect of different groups directly linked to the macrocyclic ligand (-NH-, -NCH -, or -N(CH OH)-). In electrochemically induced hydrogen evolution catalysis at pH 4, the rate determining step is the protonation of the reduced Co species that gives a cobalt hydride (Co -H), a key intermediate towards the H-H bond formation. In sharp contrast, under photochemical conditions using [Ru(bpy) ] (bpy=2,2'-bipyridine) as a photosensitizer and ascorbate as sacrificial electron donor, the formation of a Co species that quickly protonates to give a Co -H is proposed. In this scenario, the rate determining step is the H-H bond formation that occurs in an intermolecular fashion from the Co -H species and a water molecule. Both mechanisms are supported by DFT calculations, which allowed us to estimate the pK values of the Co -H and Co -H species and transition states based on intramolecular and intermolecular H-H bond formation from Co -H.
含有赤道四氮大环配体的钴配合物是在纯水溶液中进行析氢反应的活性催化剂。我们研究了直接与大环配体相连的不同基团（-NH-、-NCH-或-N(CHOH)-）的影响。在 pH 值为 4 时的电化学诱导析氢催化中，速率决定步骤是还原的 Co 物种的质子化，生成钴氢化物（Co-H），这是形成 H-H 键的关键中间体。相比之下，在使用 [Ru(bpy)]（bpy=2,2'-联吡啶）作为光敏剂和抗坏血酸作为牺牲电子供体的光化学条件下，提出了形成一种钴物种的假设，该物种迅速质子化生成 Co-H。在这种情况下，速率决定步骤是 H-H 键的形成，它以分子间的方式从 Co-H 物种和水分子中发生。这两种机制都得到了 DFT 计算的支持，该计算允许我们根据 Co-H 从分子内和分子间形成 H-H 键来估计 Co-H 和 Co-H 物种以及过渡态的 pK 值。
