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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

Abstract

Metal hydrides play important roles in catalysis for sustainable energy, environment, petrochemical, and many important chemical processes. Despite this significance, the mystery behind metal hydride reactivity is still little known. This theoretical study reveals a surprising reactivity discrepancy for different types of terminal hydride and bridging hydride, with Lewis acid - transition metal (LA-TM) hydrides complexes promoted alkene hydrogenations as model reactions, using density function theory (DFT) studies. The PBP(μ-H)CoH and the DPB(μ-H)NiH complexes were chosen as representative models for the boryl type and the borane type LA-TM hydrides, 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 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 (3c-2e) B-H-Ni bond. In the sp2 boryl PBP(μ-H)CoH complex, the bridging hydride forms an unusual three-center four-electron (3c-4e) B-H-Co bond. The 3c-2e bridging hydride is stabilized by two LA sites, leading to lower nucleophilicity than that of a normal 2c-2e terminal hydride. While the 3c-4e bridging hydride shows more free-hydride characteristic, resulting in higher nucleophilicity than that of a 2c-2e terminal hydride. A general hydride nucleophilic trend is proposed: the 3c-4e bridging hydride > the 2c-2e terminal hydride > the 3c-2e bridging hydride. These fundamental aspects of metal hydride reactivity should be helpful for mechanistic understanding and catalyst/material design involving metal hydrides complexes.

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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, Accepted Manuscript
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    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

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

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