DFT Study on the Mechanism of Formic Acid Decomposition by a Well-Defined Bifunctional Cyclometalated Iridium(III) Catalyst: Self-Assisted Concerted Dehydrogenation via Long-Range Intermolecular Hydrogen Migration

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• 作者：Jingjing Li ; Jinghua Li ; Dongju Zhang ; Chengbu Liu
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：July 1, 2016
• 年：2016
• 卷：6
• 期：7
• 页码：4746-4754
• 全文大小：625K
• 年卷期：0
• ISSN：2155-5435

文摘

DFT calculations have been performed to gain insight into the mechanism of formic acid (HCOOH) decomposition into H₂ and CO₂, catalyzed by a well-defined bifunctional cyclometalated iridium(III) complex (a Ir–H hydride) based on a 2-aryl imidazoline ligand with a remote NH functionality. It is shown that the reaction features the direct protonation of the Ir–H hydride by HCOOH with the hydrogen shuttling between the NH group and the carbonyl group of HCOOH. Importantly, the simultaneous presence of two HCOOH molecules is proposed to be important for the dehydrogenation, where one works as a hydrogen source and the other acts as a hydrogen shuttle to assist the long-range intermolecular hydrogen migration. The dehydrogenation mechanism is referred to as HCOOH self-assisted concerted hydrogen migration. With such a mechanism, the energetic span, i.e. the apparent activation energy of the catalytic cycle, is calculated to be 17.3 kcal/mol, which is consistent with the observed rapid dehydrogenation of HCOOH under mild conditions (40 °C). On one hand, the effectiveness of the self-assisted catalytic system is attributed to the d–pπ conjugation between the Ir center and the proximal nitrogen, which increases the electron density at the Ir center and hence promotes the Ir–H bond cleavage. On the other hand, the effectiveness is also closely related to the hydrogen-shared three-center–four-electron (3c-4e) bond between formate and formic acid, which stabilizes the transition states and hence reduces the free energy barriers of the reaction. In addition, calculated results also emphasize the importance of the concerted catalysis of the bifunctional catalyst: when the γ-NH functional group does not participate in the reaction or is replaced by an O atom, the reaction becomes remarkably less favorable. The present work rationalizes the experimental findings and provides important insights into understanding the catalysis of the bifunctional cyclometalated iridium(III) complexes.