Macrocyclic High‐Spin (S=2) Molecule: Spin Identification of a Sterically Rigid Metacyclophane‐Based Nitroxide Tetraradical by Two‐Dimensional Electron Spin Transient Nutation Spectroscopy

Abstract

Macrocyclic molecules, such as cyclophanes and calixarenes, have attracted much interest because of their ability to include various guests and the possibility of forming supramolecular assemblies.[1] Calixarenes functionalized by nitroxide radicals, which are widely used as stable spin sources, have been documented, [2][3][4][5] revealing their peculiar properties; especially, remarkable changes in continuous-wave (cw)-ESR spectra with and without guests were observed. [3] Calixarene-based high-spin entities, nevertheless, have rarely been well-characterized, particularly in terms of their electronic-spin properties versus their molecular structure. The design and syntheses of well-characterized macrocyclic high-spin systems are an important issue not only for exotic molecular magnetic materials formed by supramolecular assembly, [4] but also for magnetic chemical sensors with multi-sensing sites. The difficulty in studying macrocyclic high-spin systems results from the lack of well-established experimental methods which allow the apparently small exchange or spin-spin interactions to be characterized in supramolecular species which have sizable inclusion pockets. In addition, synthetic macrocyclic systems are frequently contaminated by lower spin species having similar molecular frames. Rajca et al. reported pioneering work on a calix[4]-arene derivative having four nitroxide radicals in the upper (wider) rim of the calixarene skeleton.[5] The X-ray crystal structure of that calixarene derivative revealed that the two diagonally arranged radical sites formed an intramolecular dimeric structure undergoing a strong antiferromagnetic interaction. The spin state of the derivative was only characterized from magnetic susceptibility measurements, which did not give clear and straightforward evidence for the molecular-structure-related microscopic magnetic properties of calixarene-based exchange-coupled systems.Herein, we show for the first time that nitroxide tetraradical 1 (Scheme 1) with four nitroxide radical sites in the upper rim of the calixarene skeleton is in a quintet state. We have exploited the sterically rigid molecular structure of the macrocyclic skeleton to control intramolecular through-space exchange interactions.[6] Molecule 1 has a unique structure with three methyl substituents in each benzene ring (mesityl groups), giving rise to steric rigidity, and suppressing the flexibility of the skeleton. To unequivocally identify the spin multiplicity of 1 and possibly occurring lower-spin chemical species in an organic glass, 2-dimensional pulse-based electron spin transient nutation (2D-ESTN) spectroscopy developed in our group has been applied. The 2D-ESTN technique gives straightforward information on the spin multiplicity which cannot be derived in terms of cw-ESR spectroscopy. [7] We have prepared both tetraradical 1 and mesityl nitroxide 2 (Scheme 1), which corresponds to a monoradical component of 1. Compound 2 is used to check any subtle effect of the calixarene structure on the spin structu...

Covalent Inhibition of the Lymphoid Tyrosine Phosphatase

“…70 Finally, spin‐labelled calixarenes were found to have accessible high‐spin states, this being an important property for the applications of these macrocycles as molecular magnetic materials 32. 71–74…”

Pulsed electron spin nutation spectroscopy of weakly exchange-coupled biradicals: a general theoretical approach and determination of the spin dipolar interaction

“…33,34 For determination of spin dipolar interactions from the experimental side, continuous-wave (CW) fine structure ESR spectroscopy has been well established. [35][36][37][38] In addition, twodimensional (2D) pulse-based electron spin transient nutation (2D-ESN or 2D-ESTN) spectroscopy can differentiate different spin multiplicities in a mixture straightforwardly, to yield zerofield splitting parameters by reliable spectral simulations. 32,34,[39][40][41][42][43][44][45][46][47] The nutation experiments in non-oriented media can be carried out with conventional X-band pulsed ESR spectrometers equipped with 1 kW TWT microwave amplifiers.…”

Coordination Compounds Based on Metacyclophane Derivatives

“…Synthesis of nitroxide tetrasubstituted compound presenting some interesting magnetic properties has also been described. [17] In addition, [1.1.1.1]metacyclophane organometallic host for anions has also been previously obtained. [18] Some tetrasubstituted [1.1.1.1]metacyclophanes for specific organic anions recognition, like aminopyridinium [19] imidazolium derivatives have been reported.…”