Self aggregation of supramolecules of nitroxides@cucurbit[8]uril revealed by EPR spectra

Abstract

Supramolecular complexation behavior of cucurbiturils with paramagnetic nitroxide spin probes was examined by ¹H NMR, X-ray diffraction studies of crystals, computation, and EPR. Both cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) form a 1:1 complex with 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide (CAT1). The structure of the complex in the solid state was inferred by X-ray diffraction studies and in the gas phase by computation (B3LYP/6-31G(d)). Whereas ESI-MS data provided evidence for the existence of the complex in solution, indirect evidence was obtained through ¹H NMR studies with a structural diamagnetic analogue, 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethyl-N-methylpiperidine iodide (DCAT1). The EPR spectrum of the CAT1@CB7 complex consisting of three lines suggested that probe CAT1 is associated with host CB7 such that the nitroxide part is exposed to water. The spectral pattern was independent of the concentration of the complex and the presence of salt such as NaCl. The most interesting observation was made with CB8 as the host. In this case, in addition to the expected three-line spectrum, an additional spectrum consisting of seven lines was recorded. The contribution of the seven-line spectrum to the total spectrum was dependent on the concentration of the complex and added salt (NaCl) to the aqueous solution. The coupling constant for the seven-line spectrum for ¹⁴N-substituted CAT1 is 5 G, and that for the four-line spectrum for ¹⁵N-substituted CAT1 is 7.15 G. The only manner by which we could reproduce the observed spectra by simulation for both ¹⁴N- and ¹⁵N-substituted CAT1@CB8 was by assuming a spin exchange among three nitroxide radicals. To account for this observation, we hypothesize that three CAT1 molecules included within CB8 interact in such a way that there is an association of three supramolecules of CAT1@CB8 (i.e., [CAT1@CB8]₃) in a triangular geometry that leads to spin exchange between the three radical centers. We have established, with the help of 13 additional examples, that this is a general phenomenon. We are in the process of understanding this unusual phenomenon.