Chowdhury Srijan Narayan, Biswas Sachidulal, Das Saikat, Biswas Achintesh N
Department of Chemistry, National Institute of Technology Sikkim, Ravangla, South Sikkim 737139, India.
Dalton Trans. 2023 Aug 22;52(33):11581-11590. doi: 10.1039/d3dt02194g.
A pentadentate Cu(II) complex, Cu(dpaq) (1), featuring a redox active ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino--quinolin-8-yl-acetamidate), catalyses four-electron reduction of dioxygen by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CFCOOH) in acetone at 298 K. No catalytic oxygen reduction was observed in the presence of stronger Brønsted acids than CFCOOH, such as perchloric acid (HClO) or trifluoromethanesulphonic acid (HOTf). In contrast, facile catalytic reduction of O occurs by Fc* with 1 and HClO or HOTf in dimethylformamide (DMF). The use of CFCOOH as the proton source in DMF results in the suppression of O reduction under otherwise identical reaction conditions. While the O reduction reactions in DMF are linearly dependent on the p of Brønsted acids, the acid dependence on catalytic O-reduction reactivity by 1 in acetone showed complete reversal. Cyclic voltammetry studies using -chloranil as the probe substrates in the presence of acids in the solvents reveal that the strengths of the protonic acids increase significantly in acetone compared to that in DMF. The amidate-N in Cu(dpaq) (1) undergoes protonation in the presence of HClO or HOTf in DMF to form [Cu(H-dpaq)] (1-H+), but not in the presence of CFCOOH. Enhanced acid strength of CFCOOH in acetone, however, effectively protonates 1 and triggers O reduction. Protonation of 1 with HClO or HOTf in acetone results in the change of its coordination environment, and this protonated species does not trigger O reduction. Detailed kinetic studies indicate that 1-H+ undergoes reduction by two-electrons and the reduced species binds O to form a Cu(II)-superoxo intermediate. This is followed by a rate-determining proton-coupled electron-transfer (PCET) reduction to generate the Cu(II)-hydroperoxo intermediate. While catalytic O reduction in acetone occurs predominantly a 4e/4H pathway, product selectivity (HO HO) in DMF depends upon the concentration of the reductant (Fc*). While dioxygen reduction to HO is favoured at low [Fc*], mechanistic studies suggest that O reduction with high [Fc*] proceeds a [2e + 2e] mechanism, where the released HO during catalysis is further reduced to water.
一种五齿铜（II）配合物Cu(dpaq)（1），其具有氧化还原活性配体H-dpaq（H-dpaq = 2-[双（吡啶-2-基甲基）]氨基-喹啉-8-基-乙酰胺），在298 K的丙酮中，在三氟乙酸（CF₃COOH）存在下，催化十甲基二茂铁（Fc*）对氧气进行四电子还原。在比CF₃COOH更强的布朗斯特酸存在下，如高氯酸（HClO₄）或三氟甲磺酸（HOTf），未观察到催化氧还原。相反，在二甲基甲酰胺（DMF）中，Fc与1以及HClO₄或HOTf能轻松催化O₂还原。在DMF中使用CF₃COOH作为质子源会导致在其他相同反应条件下抑制O₂还原。虽然DMF中的O₂还原反应与布朗斯特酸的pKa呈线性相关，但在丙酮中酸对1催化O₂还原反应活性的依赖性却完全相反。在溶剂中存在酸的情况下，以-氯醌为探针底物进行循环伏安研究表明，与DMF相比，质子酸在丙酮中的强度显著增加。Cu(dpaq)（1）中的酰胺-N在DMF中HClO₄或HOTf存在下会发生质子化形成[Cu(H-dpaq)]（1-H⁺），但在CF₃COOH存在下不会。然而，CF₃COOH在丙酮中增强的酸强度有效地使1质子化并引发O₂还原。在丙酮中用HClO₄或HOTf使1质子化会导致其配位环境发生变化，且这种质子化物种不会引发O₂还原。详细的动力学研究表明，1-H⁺通过双电子还原，还原后的物种与O₂结合形成Cu（II）-超氧中间体。随后是速率决定步骤的质子耦合电子转移（PCET）还原以生成Cu（II）-氢过氧中间体。虽然丙酮中的催化O₂还原主要通过4e/4H途径进行，但DMF中的产物选择性（H₂O₂/HO₂）取决于还原剂（Fc）的浓度。在低[Fc*]时，氧气还原为H₂O₂更有利，但机理研究表明，高[Fc*]时的O₂还原通过[2e + 2e]机制进行，催化过程中释放的H₂O₂会进一步还原为水。
