Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach

Abstract

In order to use H2 as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H2 into 2 H+ and 2 e−. Herein, we report a metal-free approach to catalyze the oxidation of H2 by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H2 with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC-stabilized borenium cation [(IiPr2)(BC8H14)]+ (IiPr2=C3H2(NiPr)2, NHC=N-heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H2 oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol−1. The NHC–borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C6F5)3, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach.

Exploring structural and electronic effects in three isomers of tris{bis(trifluoromethyl)phenyl}borane: towards the combined electrochemical-frustrated Lewis pair activation of H2

“…Whilst electrochemical studies show that all three isomers are less electrophilic than the archetypal Lewis acid B(C 6 F 5 ) 3 the link between Lewis acidity, activity towards H 2 in an FLP, and electrophilicity, as shown in our previous studies 20,25,26 and those of other groups [33][34][35] is complex and requires further study. This lack of reactivity is due to a combination of, kinetic effects resulting from in part steric shielding of the boron centre; but also quenching of the boranes' electrophilicity through B⋯F bonding interactions, which are quantified by DFT calculations and electrochemical measurements.…”

A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

“…[29–33] Aside from our recent studies using carbon electrodes, the non-aqueous redox chemistry of bulkier borohydrides, such as [ 1 -H] − ,[27, 34] and their parent electron-deficient, Lewis acidic boranes,[35, 36] remain largely unexplored. Therefore, to better understand the oxidation mechanism of important intermediates, such as [ 1 -H] − in combined electrochemical–FLP systems, herein, we seek to explore and understand the electrochemical behaviour of [ 1 -H] − at Pt electrode surfaces.…”

Mono- and Polynuclear Boron Dications

“…However, the hydrogendeuterium exchange is not observed in [6] 2+ . Besides, treatment of [6] 2+ with LiNMe 2 , LiOMe, and pyridine all result in dissociation of the phosphine.…”

“…However, the PEt 3 analogue of [7] 2+ cannot be prepared by the same method, potentially due to the increased steric repulsion between PEt 3 ligands. The 11 B NMR chemical shifts of [6] 2+ (¹32.3 ppm), [7] 2+ (¹47.1 ppm), and [8] 2+ (¹53.5 ppm) are considerably upfield shifted compared to that of pyridine coordinated derivatives (813 ppm) due to the strong σ-donating ability of phosphine. The structural characterization of [7][Br] 2 and [8][Br] 2 have also been determined by X-ray diffraction studies.…”

“…Besides, treatment of [6] 2+ with LiNMe 2 , LiOMe, and pyridine all result in dissociation of the phosphine. However, both [3] 2+ and [6] 2+ are stable towards strong bases including LiH, n-BuLi, and KHMDS.…”