Evaluating the Catalytic Activities of PNCNP Pincer Group 10 Metal Hydride Complexes: Pd-Catalyzed Reduction of CO2 to the Formic Acid Level with NH3·BH3 and NaBH4 under Ambient Conditions.

Abstract

In order to develop efficient protocols for CO2 reduction with less expensive and more convenient hydrogen sources, the catalytic reactivities of group 10 metal hydride complexes supported by a PNCNP pincer ligand, [2,6-(tBu2PNH)2C6H3]MH (M = Ni, 1a; Pd, 1b; Pt, 1c), against the hydroboration of CO2 with NH3·BH3 and NaBH4 have been explored. Both 1a and 1b readily react with CO2 at room temperature to form the corresponding formato complexes, [2,6-(tBu2PNH)2C6H3]MOC(O)H (M = Ni, 2a; Pd, 2b), in nearly quantitative yields. Treatment of NH3·BH3 with CO2 (1 atm) in 1,4-dioxane or THF at room temperature in the presence of 0.05-1.0 mol % of 1b followed by hydrolysis of the resulting mixtures produces formic acid in 105-186% yields, and initial turnover frequencies of up to 2000 h-1 are observed. In the presence of 1.0 mol % of 1b, NaBH4 reacts with CO2 (1 atm) in THF at room temperature to form NaB[OC(O)H]4 (3) in 87% isolated yield. In situ NMR spectroscopy indicates that the reactions proceed through the insertion of the C═O bond in CO2 into the Pd-H bond in 1b to form 2b, which sequentially reacts with the hydrides in NH3·BH3 or NaBH4 to produce boron formato species and regenerate 1b. This work represents one of the rare examples of catalytic transfer hydrogenation of CO2 with NH3·BH3 to the formic acid level under very mild conditions without any additives and also the first example of 4 equiv of CO2 uptake by NaBH4 in a reaction.