Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

Abstract

Electrocatalytic hydrogenation of 1-octene as non-activated model substrate with neutral water as H-donor is reported, using [(^tBuPCP)Ir(H)(Cl)] (1) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a k_obs of 87 s^-1. Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl^- and afford the highly reactive anionic Ir(I) hydride complex [(^tBuPCP)Ir(H)]^- (2), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [(^tBuPCP)Ir(H)₂] (3) in 88 % FE from the reaction of 2 with H₂O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro-hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H₂ formation. Even in strongly alkaline conditions (Bu₄NOH added), the electro-hydrogenation of 1-octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H₂O activation, reminiscent to transfer hydrogenation pathways, instead of classical H⁺ reduction. DFT calculations confirmed oxidative addition of H₂O as a key step in this context.

Keywords: Ir(III)-pincer; anionic Ir(I) hydride; cyclic voltammetry; electro-synthesis; electrochemical hydrogenation.