Hooe Shelby L, Cook Emma N, Reid Amelia G, Machan Charles W
Department of Chemistry, University of Virginia PO Box 400319 Charlottesville VA 22904-4319 USA
Chem Sci. 2021 Jun 17;12(28):9733-9741. doi: 10.1039/d1sc01271a. eCollection 2021 Jul 21.
The two-electron and two-proton -hydroquinone/-benzoquinone (HQ/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH) ]˙. The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH) ]˙ + AH + e ⇌ [HQ(AH)(A)]), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O). The Mn electrocatalyst is selective for HO with only TFEOH and O present, however, with BQ present under sufficient concentrations of TFEOH, an electrogenerated [HQ(AH)(A)] adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% HO in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to HQ.
双电子和双质子-对苯二酚/苯醌（HQ/BQ）氧化还原电对在电子传递链中与泛醌的功能具有机制上的相似性。这种质子依赖的氧化还原行为已在催化需氧氧化反应、氧化还原液流电池和共电催化氧还原中得到应用。在非水溶剂中名义上的非质子条件下，BQ可在单独的电化学可逆反应中被还原多达两个电子。在高浓度弱酸（AH）存在下，由于与中间体单阴离子[BQ(AH)]˙形成有利的氢键相互作用，可能会发生电位反转。这些相互作用产生的溶剂化壳可以介导在更正的电位下与质子转移耦合的第二个单电子还原（[BQ(AH)]˙ + AH + e ⇌ [HQ(AH)(A)]），从而在中等酸浓度下在单个电位处实现整体双电子还原。在这里，我们表明还原醌与质子供体2,2,2-三氟乙醇（TFEOH）形成的氢键加合物可以在电催化还原氧气（O）过程中介导电子转移到基于锰的配合物。在仅存在TFEOH和O的情况下，锰电催化剂对HO具有选择性，然而，在足够浓度的TFEOH存在下BQ存在时，电生成的[HQ(AH)(A)]加合物（其中AH = TFEOH）以共电催化方式将产物选择性改变为96(±0.5)% HO。这些结果表明，氢键结合的醌阴离子可以以类似于HQ的共电催化方式发挥作用。
