Evaluating the Catalytic Activities of PNCNP Pincer Group 10 Metal Hydride Complexes: Pd-Catalyzed Reduction of CO 2 to the Formic Acid Level with NH 3 ·BH 3 and NaBH 4 under Ambient Conditions.

In order to develop efficient protocols for CO 2 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-( t Bu 2 PNH) 2 C 6 H 3 ]MH (M = Ni, 1a ; Pd, 1b ; Pt, 1c ), against the hydroboration of CO 2 with NH 3 ·BH 3 and NaBH 4 have been explored. Both 1a and 1b readily react with CO 2 at room temperature to form the corresponding formato complexes, [2,6-( t Bu 2 PNH) 2 C 6 H 3 ]MOC(O)H (M = Ni, 2a ; Pd, 2b ), in nearly quantitative yields. Treatment of NH 3 ·BH 3 with CO 2 (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 , NaBH 4 reacts with CO 2 (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 CO 2 into the Pd-H bond in 1b to form 2b , which sequentially reacts with the hydrides in NH 3 ·BH 3 or NaBH 4 to produce boron formato species and regenerate 1b . This work represents one of the rare examples of catalytic transfer hydrogenation of CO 2 with NH 3 ·BH 3 to the formic acid level under very mild conditions without any additives and also the first example of 4 equiv of CO 2 uptake by NaBH 4 in a reaction.