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A Cucurbit[8]uril 2:2 Complex with a Negative p K a Shift
Author(s) -
Yin Hang,
Cheng Qian,
Rosas Roselyne,
Viel Stéphane,
Monnier Valérie,
Charles Laurence,
Siri Didier,
Gigmes Didier,
Ouari Olivier,
Wang Ruibing,
Kermagoret Anthony,
Bardelang David
Publication year - 2019
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201902057
Subject(s) - protonation , chemistry , imidazole , deprotonation , pyridinium , proton , stereochemistry , molecule , crystallography , benzimidazole , medicinal chemistry , organic chemistry , ion , physics , quantum mechanics
A viologen derivative carrying a benzimidazole group ( V‐P‐I 2+ ; viologen–phenylene–imidazole V‐P‐I ) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest p K a , by more than 1 pH unit, contrasting with the positive p K a shift usually observed for CB‐based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V‐P‐ I 3+ indeed released protons as monitored by pH‐metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V‐P‐I 3+ ⋅ CB[7] and a CB[8] complex of 1‐methyl‐4‐(4‐pyridyl)pyridinium. The origin of this negative p K a shift is proposed to stand in an ideal charge state, and in the position of the two pH‐responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.