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Metal-free dihydrogen oxidation by a borenium cation: a combined electrochemical/frustrated Lewis pair approach.

Lawrence EJ, Herrington TJ, Ashley AE, Wildgoose GG - Angew. Chem. Int. Ed. Engl. (2014)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ (UK) http://wildgooseresearch.com.

No MeSH data available.


a) Overlaid cyclic voltammograms showing the complete potential window for 1-H (2.0 mm, CH2Cl2) over the voltage scan-rate range 200–1000 mV s−1. b) Cyclic voltammograms comparing the oxidation potentials of 1-H (solid line, 2.0 mm), [HB(C6F5)3]− (dotted line, 2.0 mm), H2 (dashed line, 2.0 mm) in CH2Cl2 at voltage scan rates of 100 mV s−1.
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f1: a) Overlaid cyclic voltammograms showing the complete potential window for 1-H (2.0 mm, CH2Cl2) over the voltage scan-rate range 200–1000 mV s−1. b) Cyclic voltammograms comparing the oxidation potentials of 1-H (solid line, 2.0 mm), [HB(C6F5)3]− (dotted line, 2.0 mm), H2 (dashed line, 2.0 mm) in CH2Cl2 at voltage scan rates of 100 mV s−1.

Mentions: We prepared 1-H according to the method of Farrell et al.6, and its redox properties were explored using cyclic voltammetry. A single oxidation wave was observed at +(0.58±0.01) V versus Cp2Fe0/+ (Figure 1) with no corresponding reduction wave at scan rates up to 5 V s−1. The observed voltammetric behavior of 1-H is very similar to that of [HB(C6F5)3]−[2] (Figure 1 b) and therefore we propose the mechanism shown in Scheme 2 to account for the observed voltammetry: upon the application of an oxidizing potential, 1-H undergoes a one-electron oxidation to form a transient [1-H].+ species. This then undergoes rapid dissociation in solution to give H+ and a neutral radical, 1. As the applied potential is very positive compared to the formal potential for the 1+/1. couple (see below), this radical undergoes a second one-electron oxidation to generate 1+. This second oxidation occurs in competition with side reactions: 1) the decomposition of 1. by the solvent, and 2) the reaction between electrogenerated H+ and a second incoming molecule of 1-H to regenerate 1+ and H2 (see below).


Metal-free dihydrogen oxidation by a borenium cation: a combined electrochemical/frustrated Lewis pair approach.

Lawrence EJ, Herrington TJ, Ashley AE, Wildgoose GG - Angew. Chem. Int. Ed. Engl. (2014)

a) Overlaid cyclic voltammograms showing the complete potential window for 1-H (2.0 mm, CH2Cl2) over the voltage scan-rate range 200–1000 mV s−1. b) Cyclic voltammograms comparing the oxidation potentials of 1-H (solid line, 2.0 mm), [HB(C6F5)3]− (dotted line, 2.0 mm), H2 (dashed line, 2.0 mm) in CH2Cl2 at voltage scan rates of 100 mV s−1.
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Related In: Results  -  Collection

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f1: a) Overlaid cyclic voltammograms showing the complete potential window for 1-H (2.0 mm, CH2Cl2) over the voltage scan-rate range 200–1000 mV s−1. b) Cyclic voltammograms comparing the oxidation potentials of 1-H (solid line, 2.0 mm), [HB(C6F5)3]− (dotted line, 2.0 mm), H2 (dashed line, 2.0 mm) in CH2Cl2 at voltage scan rates of 100 mV s−1.
Mentions: We prepared 1-H according to the method of Farrell et al.6, and its redox properties were explored using cyclic voltammetry. A single oxidation wave was observed at +(0.58±0.01) V versus Cp2Fe0/+ (Figure 1) with no corresponding reduction wave at scan rates up to 5 V s−1. The observed voltammetric behavior of 1-H is very similar to that of [HB(C6F5)3]−[2] (Figure 1 b) and therefore we propose the mechanism shown in Scheme 2 to account for the observed voltammetry: upon the application of an oxidizing potential, 1-H undergoes a one-electron oxidation to form a transient [1-H].+ species. This then undergoes rapid dissociation in solution to give H+ and a neutral radical, 1. As the applied potential is very positive compared to the formal potential for the 1+/1. couple (see below), this radical undergoes a second one-electron oxidation to generate 1+. This second oxidation occurs in competition with side reactions: 1) the decomposition of 1. by the solvent, and 2) the reaction between electrogenerated H+ and a second incoming molecule of 1-H to regenerate 1+ and H2 (see below).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ (UK) http://wildgooseresearch.com.

No MeSH data available.