Evaluating the Catalytic Activities of PNCNP Pincer Group 10 Metal Hydride Complexes: Pd-Catalyzed Reduction of CO to the Formic Acid Level with NH·BH and NaBH under Ambient Conditions.

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