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EPR Spectroscopy: A Powerful Tool to Analyze Supramolecular Host•Guest Complexes of Stable Radicals with Cucurbiturils

1
The State Key Laboratory of Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China
2
Aix Marseille Univ, CNRS, ICR, 13013 Marseille, France
3
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, 1117 Budapest, Hungary
4
Department of Physics, Budapest University of Technology and Economics, Budafoki ut 8, 1111 Budapest, Hungary
*
Authors to whom correspondence should be addressed.
Academic Editor: Andreas Hennig
Molecules 2020, 25(4), 776; https://doi.org/10.3390/molecules25040776 (registering DOI)
Received: 21 December 2019 / Revised: 1 February 2020 / Accepted: 7 February 2020 / Published: 11 February 2020
(This article belongs to the Special Issue Characterization Techniques in Supramolecular Chemistry)
Stable organic free radicals are increasingly studied compounds due to the multiple and unusual properties imparted by the single electron(s). However, being paramagnetic, classical methods such as NMR spectroscopy can hardly be used due to relaxation and line broadening effects. EPR spectroscopy is thus better suited to get information about the immediate surroundings of the single electrons. EPR has enabled obtaining useful data in the context of host•guest chemistry, and a classical example is reported here for the stable (2,2,6,6-tetramethyl-4-oxo-piperidin-1-yl)oxyl or 4-oxo-TEMPO nitroxide (TEMPONE) inside the macrocycle host cucurbit[7]uril (CB[7]). Generally and also observed here, a contraction of the spectrum is observed as a result of the reduced nitrogen coupling constant due to inclusion complexation in the hydrophobic cavity of the host. Simulations of EPR spectra allowed determining the corresponding binding constant pointing to a weaker affinity for CB[7], compared to TEMPO with CB[7]. We complement this work by the results of EPR spectroscopy of a biradical: bis-TEMPO-bis-ketal (bTbk) with cucurbit[8]uril (CB[8]). Initial investigations pointed to very weak effects on the spectrum of the guest and incorrectly led us to conclude an absence of binding. However, simulations of EPR spectra combined with NMR data of reduced bTbk allowed showing inclusion complexation. EPR titrations were performed, and the corresponding binding constant was determined. 1H NMR spectra with reduced bTbk suggested a shuttle mechanism, at nearly one equivalent of CB[8], for which the host moves rapidly between two stations. View Full-Text
Keywords: EPR spectroscopy; supramolecular chemistry; host•guest chemistry; nitroxides; cucurbiturils EPR spectroscopy; supramolecular chemistry; host•guest chemistry; nitroxides; cucurbiturils
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MDPI and ACS Style

Liu, F.; Karoui, H.; Rockenbauer, A.; Liu, S.; Ouari, O.; Bardelang, D. EPR Spectroscopy: A Powerful Tool to Analyze Supramolecular Host•Guest Complexes of Stable Radicals with Cucurbiturils. Molecules 2020, 25, 776.

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