Abstract
The reaction mechanisms for the reduction of carbon dioxide to formaldehyde catalyzed by bis(tricyclopentylphosphine) metal complexes, [RuH2(H2)(PCyp3)2] (1Ru), [FeH2(H2)(PCyp3)2] (1Fe) and [OsH4(PCyp3)2] (1Os), were studied computationally by using the density functional theory (DFT). 1Ru is a recently reported highly efficient catalyst for this reaction. 1Fe and 1Os are two analogues of 1Ru with the Ru atom replaced by Fe and Os, respectively. The total free energy barriers of the reactions catalyzed by 1Ru, 1Fe and 1Os are 24.2, 24.0 and 29.0 kcal/mol, respectively. With a barrier close to the experimentally observed Ru complex, the newly proposed iron complex is a potential low-cost catalyst for the reduction of carbon dioxide to formaldehyde under mild conditions. The electronic structures of intermediates and transition states in these reactions were analyzed by using the natural bond orbital theory.
Highlights
IntroductionAs an abundant and non-toxic C1 -building block, carbon dioxide can be reduced to various chemicals, such as carbon monoxide [1], methanol [2], formaldehyde [3], acetals [4,5], formic acid [5], formate [6,7], formamides [8], methylamines [8], formamidines [8], imines [3] and methane [9]
We report a density functional theory (DFT) study of the reaction mechanisms of the reduction of CO2 to formaldehyde catalyzed by [RuH2 (H2 )(PCyp3 )2 ]
Scheme 1 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru
Summary
As an abundant and non-toxic C1 -building block, carbon dioxide can be reduced to various chemicals, such as carbon monoxide [1], methanol [2], formaldehyde [3], acetals [4,5], formic acid [5], formate [6,7], formamides [8], methylamines [8], formamidines [8], imines [3] and methane [9]. Catalysts 2017, 7, 5 an organocatalyst 1-Bcat-2-PPh2 –C6 H4 (Bcat = catecholboryl) Their mechanistic study showed that the simultaneous activation of both the reducing agent and CO2 plays an important role in the catalytic reaction. Hill and co-workers [23] reported a selective reductive hydroboration of CO2 to a methanol equivalent, CH3 OBpin, using B(C6 F5 )3 -activated alkaline earth compounds as catalysts. They proposed a mechanism with the formation of formaldehyde as a byproduct.
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