A Weakly Antiferromagnetically Coupled Biradical Combining Verdazyl with Nitronylnitroxide Units

Abstract

An antiferromagnetically (AFM) coupled biradical based on oxoverdazyl and nitronylnitroxide was synthesized in 46 % yield using Sonogashira coupling. The obtained heterobiradical evidenced distinct properties of both radical entities in solution. Depending on the solvent, the prepared biradical crystallized in two different forms. SQUID magnetization measurements on Form II showed coupling constants J intraII /k B = À 2.1 K and zJ interII / k B = À 11.5 K. Consequently, total intermolecular exchange interactions are five times larger than the intramolecular ones. Further, DFT calculations explained this phenomenon and indicated the advantage of Form I for further in-depth investigations.Weakly antiferromagnetically coupled (AFM) crystalline spin-1 = 2 dimers can serve to investigate critical phenomena such as, the formation of triplon excitations and their interactions in a magnetic field strong enough to close the singlet-triplet gap of the biradicals. Depending on the interdimer exchange interaction (J inter ) relative to the intradimer exchange interaction (J intra ) different dimensionalities of spin systems in a crystalline solid can be observed, such as one-dimensional (1D) spin-ladder systems showing Luttinger-liquid behavior, [1] two-dimensional (2D) Berezinskii-Kosterlitz-Thouless networks, [2] or three-dimensional (3D) Bose-Einstein condensation (BEC) of triplon excitations. [3] Therefore a control of J intra as well as J inter is essential and we have shown earlier that this is possible in organic biradicals, where the intramolecular interactions can be fine-tuned through the applied spacer between the radical units. [4] Besides changing the bridge between the two radical units, two different stable radicals can also be applied for varying the exchange interaction with a given spacer as exhibited for a tetramethoxypyrene with mixed nitronylnitroxide and iminonitroxide. [5] Earlier, we tested two nitronylnitroxides (NN) depicted as A [6,7] in Figure 1 and two iminonitroxides (IN) shown as B [8] with a tolane bridge (1,2-bis(phenyl) acetylene) for achieving 2D and 3D ordering of triplon excitations of these spin dimers in the crystal lattice.. Therefore in the present work, we consider a combination of nitronylnitroxide (NN) and oxoverdazyl radicals (VZ), which is derived from the described homobiradicals A and B (Figure 1). While it is difficult to achieve such a hetero-biradical upon two different kinds of condensation of dialdehyde precursors, Sonogashira [a] Dr.Figure 1. Homobiradicals A and B and the newly designed heterobiradical 1 with tolane bridge.coupling between the two different radical fragments is considered as a prospective pathway.The hetero-biradical 1 was prepared by using a convergent approach that included the reaction between ethynyl-and iodo-substituted radicals (Scheme 1). Both building blocks were synthesized via well-established procedures: verdazyl 2 was prepared from 4-iodobenzaldehyde via published method [9] and the NNR 3 was synthesized according to Klyatskaya et al. [10] Th...

Preparation of Multi‐Spin Systems: A Case Study of Tolane‐Bridged Verdazyl‐Based Hetero‐Diradicals

“…As has been reported previously, the position of the iodo‐phenyl substituent in ‘Kuhn'‐verdazyl structure dramatically affects on the reaction rate: ‘Kuhn'‐verdazyl 1a with the 4‐iodophenyl substituent at N1 demonstrates higher reactivity than 1d in Sonogashira coupling[11d] and oxidative addition . In our study, we observed a similar effect in the reactions of 1a with 2c or 2d , yielding hetero‐diradicals 4ac and 4ad , respectively (Table , Entries 6 and 7).…”

“…Our study was focused on the Sonogashira coupling of iodine‐substituted radicals 1a – 1f with ethynyl‐containing building blocks 2a – 2f (Figure ). The reaction was carried out in the previously optimized conditions[11a] for stable radical alkynylation: iodide 1 (1 equiv.) and ethynyl building block 2 (1.5 equiv.)…”

“…Numerous attempts to grow diradical crystals suitable for X‐ray diffraction led to the formation of fine powders. Suitable single crystals were obtained only for diradical 8fe (crystal structure of diradical 5ce was solved previously[11a]), and its molecular and crystal structure was successfully solved (see Supporting Information, Section S2).…”

“…First of all, verdazyl high‐spin systems have not been systematically studied previously: there are only two examples of verdazyl‐containing hetero‐diradicals; namely, compound D developed by Y. Takahashi and tolane‐bridged diradical E (Figure ). [11a] In addition, combination of verdazyls and nitroxides, as predicted and reported,, could be considered promising for the synthesis of multi‐spin molecules and possible materials for two‐dimensional (2D) and 3D ordering of triplon excitations of spin dimers in a crystal lattice.…”

“…For the preparation of the systematic series of high‐spin molecules, we applied the same strategy that was used for the synthesis of diradical E , where Sonogashira coupling of iodophenyl‐ and ethynylphenyl‐substituted radical blocks was employed. [11a] Moreover, the same reaction was successfully applied to obtain nitronyl nitroxide derivatives . Considering the previous efforts, here we describe a convenient and reliable one‐step approach for the coupling of different spin‐carriers via the Sonogashira reaction.…”