Isegawa Miho, Matsumoto Takahiro, Ogo Seiji
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395, Japan.
Dalton Trans. 2021 Dec 20;51(1):312-323. doi: 10.1039/d1dt03582g.
Hydrogen fuel is a promising alternative to fossil fuel. Therefore, efficient hydrogen production is crucial to elucidate the distinctive reactivities of metal hydride species, the intermediates formed during hydrogen activation/evolution in the presence of organometallic catalysts. This study uses density functional theory (DFT) to investigate the isomerizations and reactivities of three nickel-iron (NiFe) hydride isomers synthesized by mimicking the active center of NiFe hydrogenase. Hydride transfer within these complexes, rather than a chemical reaction between the complexes, converts the three hydrides internally. Their reactivities, including their electron-transfer, hydride-transfer and proton-transfer reactions, are investigated. The bridging hydride complex exhibits a higher energy level for the highest occupied molecular orbital (HOMO) than the terminal hydride during the electron-transfer reaction. This energy level indicates that the bridging hydride is more easily oxidized and is more susceptible to electron transfer than the terminal hydride. Regarding the hydride-transfer reaction between the NiFe hydride complex and methylene blue, the terminal hydrides exhibit larger hydricity and lower reaction barriers than the bridging hydride complexes. The results of energy decomposition analysis indicate that the structural deformation energy of the terminal hydride in the transition state is smaller than that of the bridging hydride complex, which lowers the reaction barrier of hydride transfer in the terminal hydride. To produce hydrogen, the rate-determining step is represented by the protonation of the hydride, and the terminal hydrides are thermodynamically and kinetically superior to the bridging ones. The differences in the reactivities of the hydride isomers ensure the precise control of hydrogen, and the theoretical calculations can be applied to design catalysts for hydrogen activation/production.
氢燃料是一种很有前景的化石燃料替代品。因此，高效制氢对于阐明金属氢化物物种的独特反应性至关重要，金属氢化物物种是在有机金属催化剂存在下氢活化/析出过程中形成的中间体。本研究使用密度泛函理论（DFT）来研究通过模拟镍铁氢化酶的活性中心合成的三种镍铁（NiFe）氢化物异构体的异构化和反应性。这些配合物内部的氢化物转移而非配合物之间的化学反应使这三种氢化物发生内部转化。研究了它们的反应性，包括电子转移、氢化物转移和质子转移反应。在电子转移反应中，桥连氢化物配合物的最高占据分子轨道（HOMO）能级高于端基氢化物。该能级表明桥连氢化物比端基氢化物更容易被氧化，更易发生电子转移。关于NiFe氢化物配合物与亚甲基蓝之间的氢化物转移反应，端基氢化物比桥连氢化物配合物表现出更大的氢负离子给予能力和更低的反应势垒。能量分解分析结果表明，过渡态中端基氢化物的结构变形能小于桥连氢化物配合物，这降低了端基氢化物中氢化物转移的反应势垒。为了产生氢气，速率决定步骤由氢化物的质子化表示，端基氢化物在热力学和动力学上优于桥连氢化物。氢化物异构体反应性的差异确保了对氢的精确控制，并且理论计算可应用于设计用于氢活化/生产的催化剂。
