Synthesis, crystal structures and electronic properties of imidazoline nitroxide radicals bearing active groups in electropolymerisation

Abstract

Thiophene-, phenylthiophene-, and indole-based nitronyl nitroxide (NN) and imino nitroxide (IN) radicals have been synthesised and their electrochemical and magnetic properties have been studied. Cyclic voltammetry measurements show that NN radicals (1–3) exhibit a one-electron quasi-reversible oxidation process that results in the formation of the corresponding nitrosonium cations. This behaviour pushes the oxidation potential of the other redox-active moiety to very high values. The oxidation of the thiophene subunit occurs at 2.35 V, whereas the oxidation of the phenylthiophene and indole moieties takes place at 1.65 and 1.39 V, respectively. Oxidation of IN radicals (4–5) is irreversible and occurs at higher potentials, as compared to the parent NN compounds. This process has a weaker effect on the redox behaviour of the thiophene functions. Single-electron reductions are observed for all the radicals at around −1.2 V. The thiophene-based NN radical (1) crystallises in the monoclinic C2/c space group. The analysis of the crystal structure shows the formation of dimers of molecules related by a centre of symmetry. These dimers packed to form a honeycomb lattice of radicals. An estimate of the magnetic interaction between radicals in the solid (J = −3.4 K) has been obtained by fitting the magnetic susceptibility of 1 with the corresponding high-temperature series expansion. The indole-based NN radical (3) crystallises in the monoclinic P21/n space group. The crystal packing shows the presence of strong hydrogen bonds between the nitroxide oxygen atom and the hydrogen of the indolic amino function. This leads to zigzag chains that packed in the solid to form a two-dimensional squared lattice. Using the high-temperature series expansion for a 2D squared lattice of isotropic S = 1/2 spins in the fit of the magnetic susceptibility of 3 affords a value of J = −0.11 K for the interradical exchange coupling parameter. The phenylthiophene-based IN compound (5) crystallises in the triclinic P1 space group. Three independent molecules form its crystal structure. The magnetic properties of compounds 2, 4 and 5 are also indicative of weak antiferromagnetic interactions. Curie–Weiss plots for these compounds afford values of the Weiss constant θ of – 1.5, – 2.8 and – 1.9 K for 2, 4 and 5, respectively.