A Nitronyl Nitroxide‐Substituted Benzotriazinyl Tetraradical**

Abstract

High‐spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium‐catalyzed cross‐coupling reaction of a triiodo‐derivative of the 1,2,4‐benzotriazinyl radical with gold(I) nitronyl nitroxide‐2‐ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X‐ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at −0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high‐level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl‐nitronylnitroxide tetraradical is a new milestone in the field of creating high‐spin systems.

Toward New Horizons in Verdazyl-Nitroxide High-Spin Systems: Thermally Robust Tetraradical with Quintet Ground State

“…Figure a shows the CW EPR spectrum of the Vrd-(NN) 3 tetraradical measured at room temperature in deoxygenated toluene. The spectrum contains a single unresolved line, the width and shape of which are very similar to those of another tetraradical Blt-(NN) 3 that we recently reported (also shown for comparison) . As discussed previously, the absence of resolved hyperfine structure results from several factors, including (i) a decrease of hyperfine coupling constants with increasing number of exchange-coupled electron spins, (ii) relatively large dipolar interactions between spins, and (iii) insufficient averaging of these interactions by molecular rotation.…”

“…The spectrum contains a single unresolved line, the width and shape of which are very similar to those of another tetraradical Blt-(NN) 3 that we recently reported (also shown for comparison) . As discussed previously, the absence of resolved hyperfine structure results from several factors, including (i) a decrease of hyperfine coupling constants with increasing number of exchange-coupled electron spins, (ii) relatively large dipolar interactions between spins, and (iii) insufficient averaging of these interactions by molecular rotation. Altogether, the similarity of our EPR spectrum with the spectrum of the already reported tetraradical Blt-(NN) 3 , which has a fundamentally similar structure, confirms the four-spin nature of Vrd-(NN) 3 .…”

“…If hyperfine splitting is unresolved, comparison with other polyradicals composed of similar building blocks still provides useful information about the electronic structure. Low-temperature CW or pulsed EPR data in glassy frozen solutions can reveal resolved zero-field splitting (ZFS), which for the polyradicals studied is mainly due to dipole–dipole interactions between electron spins …”

“…(a) CW EPR spectra of tetraradicals Vrd-(NN) 3 (blue) and Blt-(NN) 3 (green) as a reference recorded at room temperature in toluene solutions; (b) ED-detected EPR spectrum of Vrd-(NN) 3 in toluene glass at 40 K (blue) and the corresponding simulation (dashed red curve; see SI Section S4 for parameters). A similar spectrum of Blt-(NN) 3 is displayed for comparison (green) …”

Pathway to Polyradicals: A Planar and Fully π-Conjugated Organic Tetraradical(oid)

“…The synthesis of stable polyradicals with large spin angular momentum is still a major challenge due to their high chemical reactivity. The stability of the radical centers can be enhanced by surrounding bulky groups such as in the sterically hindered triphenylmethyl radical or nitronyl/nitroxide polyradicals. − However, this strategy usually leads to difficulties in connecting radical centers close enough to form small polyradicals. Another strategy for the stabilization of unpaired electrons is through delocalization via π-conjugation in planar molecular structures. ,,− This approach is effective to design systems with any number of unpaired electrons and can lead to singlet or doublet ground states with thermally accessible high-spin states (polyradicals) or singlet or doublet ground states with a little higher-lying high-spin states (polyradicaloids).…”