Controlling the Extent of Spin Exchange Coupling in 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) Biradicals via Molecular Recognition with Cucurbit[n]uril Hosts

Abstract

The binding interactions between two paramagnetic cobaltocenium guests and the hosts cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) were investigated using a combination of electronic absorption, NMR, and electron paramagnetic resonance (EPR) spectroscopies, mass spectrometry, and X-ray crystallography. Guest 1, (4-amido-2,2,6,6-tetramethylpiperidine-1-oxyl)cobaltocenium, forms very stable inclusion complexes with CB7 and CB8. However, CB7 interacts with 1 by including the organometallic cobaltocenium unit, while CB8 engulfs the TEMPO residue. The corresponding equilibrium association constant (K) values are 2.8 ± 0.3 × 10(6) M(-1) for CB7•1 and 2.1 ± 1.0 × 10(8) M(-1) for CB8•1. Biradical guest 2, 1,1'-bis(4-amido-2,2,6,6-tetramethylpiperidine-1-oxyl)cobaltocenium, forms a very stable ternary complex with two CB8 hosts, in which each 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) residue is encircled by a host molecule. The structure of this ternary complex was confirmed in the solid state using single-crystal X-ray diffraction. Binding of the TEMPO side arms by the CB8 hosts gradually decreases the observed level of spin exchange coupling between the two nitroxide groups. In the final 2:1 complex, no spin exchange coupling was observed, but the initial levels of spin exchange coupling could be regenerated in a reversible fashion by adding a competing guest, adamantyltrimethylammonium (AdTMA), to the solution. The binding interactions between 2 and CB7 are similar but the stabilities of the 1:1 and 2:1 complexes are much lower than those of the corresponding CB8 complexes.

High binding yet accelerated guest rotation within a cucurbit[7]uril complex. Toward paramagnetic gyroscopes and rolling nanomachines

“…High resolution mass spectra of equimolar solutions of bPTO and CB [7] (1 mM) in water showed one peak at m/z 708.2644 corresponding to a doubly charged cation of formulae C 55 2+ . 15,16 This broadening was not observed with CB[7]@bPTO ( Figure 1), and as discussed below this result can be accounted for by an anisotropic rotational diffusion tensor for the included bPTO. EPR spectra of PTO and bPTO show a typical three line pattern with a width at half height of 0.26 mT and 0.33 mT respectively and nitrogen coupling constants a N of 1.57 and 1.53 mT respectively (gPTO = 2.0058, gbPTO = 2.0061, Figure 1).…”

Supramolecular Chemistry of Cucurbiturils: Tuning Cooperativity with Multiple Noncovalent Interactions from Positive to Negative

“…On account of their high affinities, CB[n]-mediated host-guest interactions are of great interest in the field of supramolecular chemistry and chemical biology. [25][26][27][28][29][30][31][32] While the ability to exhibit multiple non-covalent interactions, CB [8]-mediated heteroternary and homoternary complexes have been exploited in the construction of supramolecular polymers, 33-36 supramolecular hydrogels, [37][38] supra-amphiphiles [39][40] and other supramolecular functional self-assembled systems [41][42][43][44][45][46] .…”

Supramolecular host–guest interaction of trityl-nitroxide biradicals with cyclodextrins: modulation of spin–spin interaction and redox sensitivity

“…Since cyclodextrins have a relatively weak binding with TN biradicals, new studies can be directed to investigate the supramolecular host-guest interaction of TN biradicals with more efficient hosts such as cucurbit[ n ]uril. 32, 33, 35 …”