An Electrochemical Study of Frustrated Lewis Pairs: A Metal-Free Route to Hydrogen Oxidation

Frustrated Lewis pairs have found many applications in the heterolytic activation of H2 and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H2 can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H2 oxidation by 610 mV (117.7 kJ mol–1). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology.


. Na[HB(C 6 F 5 ) 3 ]
To a solution of 1.0 M Na[HBEt 3 ] in toluene (3.7 mL, 3.7 mmol) was added a solution of B(C 6 F 5 ) 3 (1.71 g, 3.3 mmol) in toluene (30 mL). The reaction mixture was left to stir under N 2 at room temperature for 2 h, during which time a white precipitate formed. The precipitate was left to settle before it was filtered and triturated with toluene (2 × 10 mL). The residue was dried in vacuo to yield Na[HB(C 6 F 5 ) 3 ] (1.15 g, 2.1 mmol) as a fine white powder in 64 % yield. 1

S1.2. [ n Bu 4 N][HB(C 6 F 5 ) 3 ] ([ n Bu 4 N]1)
A solution of n Bu 4 NCl (0.45 g, 1.6 mmol) in CH 2 Cl 2 (20 mL) was added to a white suspension of Na[HB(C 6 F 5 ) 3 ] (0.86 g, 1.6 mmol) in CH 2 Cl 2 (20 mL) at room temperature, with stirring under N 2 . This resulted in the formation of a fine flocculent precipitate with the simultaneous breakup of the suspended material. The reaction mixture was left to stir overnight. The precipitate was then allowed to settle before it was filtered. The filtrate was concentrated in vacuo until a minimum quantity of solvent remained. A white precipitate was obtained at room temperature by layering the solution carefully with light petroleum ether (40/60, approximately twice the volume of solution was added). The precipitate was filtered and dried in vacuo to afford [ n Bu 4 N]1 (0.89 g, 1.2 mmol) as a white powder in 74 % yield.
Crystals suitable for X-ray crystallography (colourless plates) were grown by dissolving [ n Bu 4 N]1 in a minimum quantity of CH 2 Cl 2 , warming to ca. 35 °C, adding an equal quantity of light petroleum ether and slow-cooling to room temperature. 1

S1.4. [ n Bu 4 N][DB(C 6 F 5 ) 3 ] ([ n Bu 4 N]1 D )
A clear colourless solution of [TMPD][DB(C 6 F 5 ) 3 ] (0.31 g, 0.47 mmol) in toluene (20 mL) was added to NaH (11 mg, 0.47 mmol) to give some effervescence. The reaction mixture was left to stir at room temperature under N 2 overnight. The reaction mixture was then filtered and the filtrate was concentrated in vacuo. The residue was dissolved in CH 2 Cl 2 (10 mL) to give a clear colourless solution. To this was added a clear colourless solution of N n Bu 4 Cl (0.13 g, 0.47 mmol) in CH 2 Cl 2 (10 mL). A very fine precipitate rapidly formed. The reaction mixture was left to stir at room temperature for 1 h before it was filtered. The filtrate was concentrated to ca. 2 mL to give a white precipitate. This was filtered and the filtrated was concentrated in vacuo to yield a colorless viscous oil that solidified overnight to give   and refined by full-matrix least-squares methods, on F 2 's, in SHELXL-97. 2 In the preliminary refinement, all non-hydrogen atoms were refined with anisotropic thermal parameters; hydrogen atoms were included in idealized positions and their Uiso values were set to ride on the Ueq values of the parent S16 carbon and boron atoms. The array of difference peaks (highest peak 0.8 eÅ −3 ) suggested disorder in the anion, where an alternative orientation for each of the three C 6 F 5 rings (by rotation in the ring plane) was clear; the minor fluorine component atoms were readily identified and refined (isotropically) well, all with site occupancies of 0.081(1); the minor carbon sites were less well resolved and the three C 6 rings were refined with geometrical constraints. The carbon and nitrogen atoms of the cation were refined anisotropically and hydrogen atoms were included as described above; only at the end of one butyl chain was any disorder found -an alternative site for C74 was refined. At the conclusion of the refinement, wR 2 = 0.121 and R 1 = 0.061 2 for all 6049 In the final difference map, the highest peak (ca 0.4 eÅ −3 ) was close to H(73B).
Scattering factors for neutral atoms were taken from the literature. 3  open circles are best fit simulated data (see main text, Table 1).

S5. DFT Computational modeling
DFT calculations were performed using the Gaussian 09 computational package 1