Heterolysis of H2 Across a Classical Lewis Pair, 2,6-Lutidine∙BCl3: Synthesis, Characterization, and Mechanism
In this study, we report that 2,6-lutidine∙trichloroborane (Lut∙BCl3) reacts with H2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut∙BHCl2. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut∙BCl3 was calculated to react with H2 and form the ion pair, [LutH+][HBCl3–], with a barrier of ΔH≠=24.7 kcal mol–1 (ΔG≠=29.8 kcal mol–1). Metathesis with a second molecule of Lut∙BCl3 produced Lut∙BHCl2 and [LutH+][BCl4–]. The overall reaction is exothermic by 6.0 kcal mol–1 (ΔrG°=–1.1). Alternate pathways were explored involving the borenium cation (LutBCl2+) and the four-membered boracycle [(CH2{NC5H3Me})BCl2]. Barriers for addition of H2 across the Lut/LutBCl2+ pair and the boracycle B—C bond are substantially higher (ΔG≠=42.1 and 49.4 kcal mol–1, respectively), such that these pathways are excluded. The barrier for addition of H2 to the boracycle B—N bond is comparable (ΔH≠=28.5 and ΔG≠=32 kcal mol–1). Conversion of the intermediate 2-(BHCl2CH2)-6-Me(C5H3NH) to Lut∙BHCl2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl3. Intramolecular protodeboronation, which could form Lut∙BHCl2 directly, is prohibited by a high barrier (ΔH≠=52, ΔG≠=51 kcal mol–1).
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; USDOE Office of Science (SC), Biological and Environmental Research (BER)
- Grant/Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1734875
- Report Number(s):
- PNNL-SA-109925
- Journal Information:
- Chemistry - A European Journal, Vol. 21, Issue 44; ISSN 0947-6539
- Publisher:
- ChemPubSoc EuropeCopyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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journal | January 2016 |
Replacing C 6 F 5 groups with Cl and H atoms in frustrated Lewis pairs: H 2 additions and catalytic hydrogenations
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journal | January 2017 |
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