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Issue 13, 2018
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Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c2e terminal hydride, 3c2e bridging hydride, and 3c4e bridging hydride

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Abstract

Metal hydrides play important roles in catalysis for sustainable energy, the environment, the petrochemical industry, and many important chemical processes. Despite this significance, the mystery behind metal hydride reactivity still remains. This theoretical study reveals a surprising reactivity discrepancy for different types of terminal hydrides and bridging hydrides, with Lewis acid–transition metal (LA–TM) hydride complex promoted alkene hydrogenations as model reactions, using density functional theory (DFT) studies. PBP(μ-H)CoH and DPB(μ-H)NiH complexes were chosen as representative models for the boryl type and the borane type LA–TM hydride, respectively. The bridging hydride is less reactive than the terminal hydride in the borane type complex DPB(μ-H)NiH. However, in sharp contrast, the bridging hydride is more reactive than the terminal hydride in the boryl type complex PBP(μ-H)CoH. Typical features of the electronic structure are unfolded to rationalize the origin of the reactivity discrepancy. The bridging hydride in the sp3 borane type DPB(μ-H)NiH complex forms a typical three-center two-electron (3c2e) B–H–Ni bond. In the sp2 boryl PBP(μ-H)CoH complex, the bridging hydride forms an unusual three-center four-electron (3c4e) B–H–Co bond. The 3c2e bridging hydride is stabilized by two LA sites, leading to a lower nucleophilicity than that of a normal 2c2e terminal hydride. Meanwhile the 3c4e bridging hydride shows a stronger free-hydride character, resulting in a higher nucleophilicity than that of a 2c2e terminal hydride. A general hydride nucleophilic trend is proposed: 3c4e bridging hydride > 2c2e terminal hydride > 3c2e bridging hydride. These fundamental aspects of metal hydride reactivity should be helpful for mechanistic understanding and catalyst/material design involving metal hydride complexes.

Graphical abstract: Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c–2e terminal hydride, 3c–2e bridging hydride, and 3c–4e bridging hydride

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Publication details

The article was received on 17 Apr 2018, accepted on 04 Jun 2018 and first published on 08 Jun 2018


Article type: Paper
DOI: 10.1039/C8CY00766G
Citation: Catal. Sci. Technol., 2018,8, 3395-3405
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    Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c2e terminal hydride, 3c2e bridging hydride, and 3c4e bridging hydride

    Y. Li, J. Liu, C. Hou, Y. Shao, L. Qu, C. Zhao and Z. Ke, Catal. Sci. Technol., 2018, 8, 3395
    DOI: 10.1039/C8CY00766G

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