A new pathway of electrocatalytic transfer hydrogenation with neutral water as the H-donor was discovered using [(tBuPCP)Ir(H)(Cl)] (1) as the catalyst and styrene as a model substrate in THF electrolyte. Cyclic voltammetry experiments with 1 revealed that two subsequent reductions at –2.55 and –2.84 V vs. Fc+/Fc trigger the elimination of Cl– and afford the highly reactive anionic Ir(I) hydride complex [(tBuPCP)Ir(H)]– (2). The identity of 2 and its reactivity were further investigated by LIFDI-MS, which confirmed 2 as reactive species in the alkene hydrogenation cycle. Bulk electrolysis and chronoamperometry for electro-hydrogenation of styrene established ethylbenzene as the only product, formed with high faradaic efficiency of 96% and a turnover frequency of 1670 h–1 at an electrolysis potential of –3.1 V, with insignificant H2 formation. Importantly, the electro-hydrogenation performance of 1 remained constant upon addition of KOH to the electrolyte, which suggests a reaction mechanism that is independent of free H+. Instead, the reactive Ir-hydrides are regenerated by oxidative addition of H2O to the complex, which creates a reaction cascade that is reminiscent of metal-hydride formation in classical transfer hydrogenation systems. As such, the herein presented study on electrocatalytic transfer hydrogenation (e-TH) with H2O as the H-donor is different from the plethora of other electro-hydrogenation studies that operate via H+ reduction, often in low-pH electrolyte. These findings may inspire the general, pH independent use of H2O as H-donor in conjunction with electrochemistry, to replace isopropanol or formate as intrinsically reducing H-donors in the many existing examples of classical transfer hydrogenation.
Supporting information containing experimental details, synthesis procedures, relevant NMR data, additional electrochemical experiments and LIFDI-MS details.