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 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.

“Homoleptic” Tetracoordinate Boron Compounds

“…In 2023, Zhang and Chen developed a pincer-type palladium hydride complex that can catalyze the reduction of carbon dioxide at ambient temperature and atmospheric pressure by employing NaBH 4 as a reductant (Scheme ). The tetracoordinate formatoboron compound NaB­[OC­(O)­H] 4 was isolated as a powdery white solid and characterized by multinuclear NMR spectroscopy, crystallography analysis, HRMS, and elemental analysis.…”

Palladium-anchored donor-flexible pyridylidene amide (PYA) electrocatalysts for CO2 reduction

“…2). 29–32 The resonance adaptation, inductive effect of various substituents on the PYA spectator ligand, and effect of different labile ligands (OAc, Cl) were investigated for palladium-centered CO 2 activation.…”

Understanding the Influence of Lewis Acids on CO2 Hydrogenation: The Critical Effect Is on Formate Rotation

“…Formic acid is a valuable feedstock employed in agriculture as well as pharmaceutical and textile industries. − It also has potential as a hydrogen storage medium. − There is increasing focus on developing synthetic strategies for the production of formate from sustainable resources such as CO 2 , a widely accessible and cheap carbon source. − However, utilization of CO 2 in hydrogenations or other chemical transformations remains challenging, due to the inertness and thermal stability of CO 2 . Strategies for CO 2 activation are therefore of great importance, with one promising approach involving use of Lewis acid (LA) additives. − …”

“…Despite the large number of experimental studies reporting that LA additives are beneficial for metal-mediated CO 2 hydrogenation (and formate dehydrogenation), ,,,− ,,,− the underlying mechanistic details are not fully understood. For insertion of CO 2 into metal–R bonds (R = hydride or alkyl/aryl), generally two mechanisms can be considered (Scheme A): i ) An inner sphere path , involving an interaction between CO 2 and the metal at the transition state for H–CO 2 bond formation (TS inn ). ,− Note that a precoordination of CO 2 is not required …”