Computational Study of Formic Acid Dehydrogenation Catalyzed by Al^III–Bis(imino)pyridine

Abstract

The mechanism of formic acid dehydrogenation catalyzed by the bis(imino)pyridine-ligated aluminum hydride complex (PDI(2-))Al(THF)H (PDI=bis(imino)pyridine) was studied by density functional theory calculations. The overall transformation is composed of two stages: catalyst activation and the catalytic cycle. The catalyst activation begins with O-H bond cleavage of HCOOH promoted by aluminum-ligand cooperation, followed by HCOOH-assisted Al-H bond cleavage, and protonation of the imine carbon atom of the bis(imino)pyridine ligand. The resultant doubly protonated complex ((H,H) PDI)Al(OOCH)3 is the active catalyst for formic acid dehydrogenation. Given this, the catalytic cycle includes β-hydride elimination of ((H,H) PDI)Al(OOCH)3 to produce CO2, and the formed ((H,H) PDI)Al(OOCH)2 H mediates HCOOH to release H2.

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

“…[23] Also the last s-bond metathesis step in the cycle seemed questionable.Acomprehensive calculational study, however, demonstrated that the metal hydride mechanism shown in Scheme 1isfeasible. [15,18,[31][32][33][34][35][36][37][38][39][40][41][42] Especially noteworthy are two very recent investigations on LiAlH 4 -catalyzed alkene and aldehyde/ketone hydroboration. Acrucial step is the hydrogenolysis of the metal À Cbond by H 2 ,for which the rate is known to decrease with decreasing bond ionicity:N a À C > Mg À C > Al À C. [25] since it is well-known that RNH 2 and R 2 NH react smoothly with LiAlH 4 to give aluminium amide products and H 2 , [26][27][28][29][30][31] hydrogenolysis of AlÀNbonds to give amines is anticipated to be even more cumbersome.S ubstitution of H 2 for polar borane or silane reductants was therefore al ogical step.…”

“…Indeed, hydroboration and hydrosilylation reactions were found to be more promising. [15,41,42] Herein, we introduce LiAlH 4 , which is normally applied as astoichiometric reducing agent, as as imple catalyst for imine reduction using the bulk commodity H 2 at ac onvenient 1bar pressure (Table 1). [15,18,[31][32][33][34][35][36][37][38][39][40][41][42] Especially noteworthy are two very recent investigations on LiAlH 4 -catalyzed alkene and aldehyde/ketone hydroboration.…”

Recent Developments on the Use of Group 13 Metal Complexes in Catalysis

“…Essentially a reverse reaction to the CO 2 reduction discussed above is the recently reported aluminum‐catalyzed dehydrogenation of formic acid to H 2 and CO 2 using Al catalyst 15 (Scheme ) . An “aluminum‐ligand cooperative activation” operative mechanism was proposed on the basis of experimental data and DFT calculations . The initial turnover frequency is high with 5200 h −1 , and this work is of interest with regards to hydrogen storage.…”

An Update on Formic Acid Dehydrogenation by Homogeneous Catalysis

“…According to the analysis of NMR, IR, and X‐ray, a plausible mechanism was proposed to proceed via an Al‐(OOCH) 2 species (unlike other monoformate intermediates) followed by β‐hydride elimination and FA‐enabled protonation (Scheme ). Further insights about these mechanistic pathways have been scrutinized computationally by Yu and Fu …”

“…Further insights about these mechanistic pathways have been scrutinized computationally by Yu and Fu. [40] Enthaler's group reported the first Ni-based PCP pincer complex 20 for the FA decomposition to CO 2 and H 2 with the TON of up to 626 (Figure 7). [25b] However, the efficiency is not as good as those of others complexes.…”