Metal-free carbon dioxide reduction and acidic C–H activations using a frustrated Lewis pair

Activation of CO2 and acidic C–H bonds by the lutidine-tris(pentafluorophenyl)borane [Lut/B(C6F5)3] frustrated Lewis pair (FLP) are described (lutidine = 2,6-dimethylpyridine). Lut/B(C6F5)3 reacts with CO2 and H2 at ambient temperature and 4 atm of pressure to form the lutidinium boro-formate salt [LutH+][HC(O)OB(C6F5)3−]. This salt has been fully characterized including an X-ray crystal structure and independent synthesis from formic acid and Lut/B(C6F5)3. Attempts to activate a C–H bond in methane by Lut/B(C6F5)3, analogous to its heterolytic cleavage of H2, were unsuccessful, which are consistent with published calculations showing significant barriers to this reaction. Lut/B(C6F5)3 does react with more acidic C–H bonds, including acetone and nitroalkanes. With nitromethane, the boro-nitrone anion H2CNO2B(C6F5)3− is formed, as indicated by NMR and mass spectral analyses.

Graphical abstractReduction of CO2 by H2 by the lutidine-tris(pentafluorophenyl)borane [Lut/B(C6F5)3] frustrated Lewis pair (FLP) at ambient temperature and 4 atm of pressure gives the lutidinium-boroformate salt [LutH+][HC(O)OB(C6F5)3−]. Attempts to activate the C–H bond of methane with the same FLP are described; but no activation of methane is observed, consistent with published calculations of the significant barrier for this reaction. Lut/B(C6F5)3 is shown to be capable of reacting with more acidic C–H bonds such as acetone and nitroalkanes.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► We use a frustrated Lewis pair (FLP) to reduce CO2, with H2. ► A boroformate complex with lutidinium counter ion was characterized by X-ray crystallography. ► This FLP does not activate methane, and studies show a large barrier for this process. ► C–H activation of acidic hydrocarbons by the FLP was studied.