From Classical Adducts to Frustrated Lewis Pairs: Steric Effects in the Interactions of Pyridines and B(C6F5)3

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• 作者：Stephen J. Geier ; Austin L. Gille ; Thomas M. Gilbert ; Douglas W. Stephan
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：November 2, 2009
• 年：2009
• 卷：48
• 期：21
• 页码：10466-10474
• 全文大小：947K
• 年卷期：v.48,no.21(November 2, 2009)
• ISSN：1520-510X

文摘

The pyridine adducts of B(C₆F₅)₃, (4-tBu)C₅H₄NB(C₆F₅)₃ 1, ((2-Me)C₅H₄N)B(C₆F₅)₃ 2, ((2-Et)C₅H₄N)B(C₆F₅)₃ 3, ((2-Ph)C₅H₄N)B(C₆F₅)₃ 4, ((2-C₅H₄N)C₅H₄N)B(C₆F₅)₃ 5, (C₉H₇N)B(C₆F₅)₃ 6, and ((2-C₅H₄N)NH(2-C₅H₄N))B(C₆F₅)₃ 7, were prepared and characterized. The B−N bond lengths in 2−7 reflect the impact of ortho-substitution, increasing significantly with sterically larger and electron-withdrawing substituents. In the case of 2-amino-6-picoline, reaction with B(C₆F₅)₃ affords the zwitterionic species (5-Me)C₅H₃NH(2-NH)B(C₆F₅)₃ 8. In contrast, lutidine/B(C₆F₅)₃ yields an equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me₂C₅H₃N)B(C₆F₅)₃ 9. This equilibrium has a ΔH of −42(1) kJ/mol and ΔS of −131(5) J/mol·K. Addition of H₂ shifts the equilibrium and yields [2,6-Me₂C₅H₃NH][HB(C₆F₅)₃] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C₆F₅)₃ showed no evidence of adduct formation and upon exposure to H₂ afforded [(2,6-Ph₂)C₅H₃NH][HB(C₆F₅)₃] 11 and [(2-tBu)C₅H₄NH][HB(C₆F₅)₃] 12, respectively. The energetics of adduct formation and the reactions with H₂ are probed computationally. Crystallographic data for compounds 1−10 are reported.