Verdazyl Radical Building Blocks: Synthesis, Structure, and Sonogashira Cross‐Coupling Reactions

Abstract

A general and effective method for the synthesis of 3‐phenylveradzyl radicals bearing a variety of iodophenyl substituents has been developed. The synthesized radicals have been characterized by ESR, UV/Vis spectroscopy, and cyclic voltammetry. Structures of biphenyl‐substituted radicals have been solved by X‐ray crystal structure analysis. The synthesized iodoverdazyls are applicable in the Sonogashira coupling reaction for the preparation of a wide range of ethynyl derivatives. Both N‐2 and C‐6 substituents were functionalized through Sonogashira coupling.

A Weakly Antiferromagnetically Coupled Biradical Combining Verdazyl with Nitronylnitroxide Units

“…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. [9,11] Therefore we chose this substrate as an iodine-containing moiety for further coupling. [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.…”

“…[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. [11] The reaction proceeded with the formation of a target product in the mixture of NNR by-products and due to this reason, good yield of the desired biradical 1 was achieved only in case of addition of 1.5 eq. [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..…”

“…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] The most intriguing issue in the synthesis of the desired biradical was the reaction between the two building blocks. [11] The reaction proceeded with the formation of a target product in the mixture of NNR by-products and due to this reason, good yield of the desired biradical 1 was achieved only in case of addition of 1.5 eq. [9,11] Therefore we chose this substrate as an iodine-containing moiety for further coupling.…”

“…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). [9,11] Therefore we chose this substrate as an iodine-containing moiety for further coupling. The hetero-biradical 1 was prepared by using a convergent approach that included the reaction between ethynyl-and iodo-substituted radicals (Scheme 1).…”

“…While it is difficult to achieve such a hetero-biradical upon two different kinds of condensation of dialdehyde precursors, Sonogashira coupling between the two different radical fragments is considered as a prospective pathway. [11] The reaction proceeded with the formation of a target product in the mixture of NNR by-products and due to this reason, good yield of the desired biradical 1 was achieved only in case of addition of 1.5 eq. 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] The most intriguing issue in the synthesis of the desired biradical was the reaction between the two building blocks.…”