US EPA

3065086 

Journal Article 

Mechanistic studies on the pH-controllable interconversion between hydrogen and formic acid in water: DFT insights 

Zhang, D; Chen, X; Liu, H; Huang, X 

2015 

1 

New Journal of Chemistry
ISSN: 1144-0546
EISSN: 1369-9261 

39 

10 

8060-8072 

10.1039/c5nj01740h 

WOS:000362355200073 

A complete reaction mechanism for interconversion between hydrogen and formic acid catalyzed by [C,N] cyclometallated organoiridium complex [Ir-III(Cp*)(4-(1H-pyrazol-1-yl-kappa N-2)benzoic acid-kappa C-3)(H2O)](2)center dot SO4, i.e. [Ir-1](2)center dot SO4, has been revealed by density functional theory (DFT) calculations. For both the hydrogen storage catalytic cycle I and hydrogen evolution catalytic cycle II, the detailed reaction profiles with the key transition states and intermediates are revealed. Catalytic cycle I shows that the dihydrogen heterolysis facilitated by OH gives the considerable stable iridium hydride intermediate M-4, followed by an outer-sphere hydrogen transfer to afford a metal-formate complex M-6. Upon the increasing of pH, catalytic cycle II occurs via the generation of the metal-formate complex M-7, followed by the outer-sphere beta-H elimination to form a metal-hydride complex M-9, which is subsequently protonated by the hydrated proton H3O+ to afford dihydrogen. The decomposition of bicarbonate and the beta-hydride elimination of formate are believed to be the rate-determining steps for cycle I and II, respectively. The acid-base equilibrium between the hydroxy and oxyanion form on the catalyst [C, N] ligand has a considerable influence on the catalytic hydrogen transfer. Our studies are in good agreement with experimental results. Remarkably, the new theoretically designed low-cost cobalt(III) complex, as a promising catalyst, exhibits catalytic activity for the interconversion between hydrogen and formic acid.