Towards a Better Understanding of the Magnetic Interactions withinm-Phenyleneα-Nitronyl Imino Nitroxide Based Biradicals

Abstract

An extensive investigation of the magnetic properties of three series of biradicals (bis-nitronyl nitroxides diNN-R, bis-imino nitroxides diIN-R and mixed INNN-R, where R is either hydrogen, a triple bond or trimethylsilylacetylenic group) has been carried out to give clear values of the intramolecular interactions through the m-phenylene coupling unit with alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN). An EPR study of the molecules in the isolated state is validated by ab initio calculations, which show the respective influence of spin polarisation and molecular conformation on the singlet-triplet gaps. All these results indicate that the triplet state is the ground state for such biradicals, except when the imidazolyl cycles are orthogonal to the phenyl ring. The magnetic properties of the biradicals in the solid state can be rationalised by examination of the short contacts produced between the ONCNO and ONCN groups. EPR studies on single crystals of the H-substituted series have confirmed the presence of a structural distortion for diNN-H whereas diIN-H and INNN-H do not exhibit such a peculiarity. The magnetic behaviour of diIN-H is described well by a four-spins model, with a strong intermolecular antiferromagnetic interaction of -90 K, whereas in the case of the two other compounds, a supplementary contact involves more complex interactions between the dimers. The compound diNN-tmsa exhibits a ferromagnetic intermolecular interaction of +11 K within the dimers, and this could be attributed to the relative disposition of the imidazolyl rings. Compound diNN-tr reveals a chain-like behaviour, whereas diIN-tr shows a predominant antiferromagnetic interaction within the dimers. The values for the intramolecular interactions in the solid state are in good agreement with those found for the isolated molecules.

1,3-Alternate calix[4]arene nitronyl nitroxide tetraradical and diradical: synthesis, X-ray crystallography, paramagnetic NMR spectroscopy, EPR spectroscopy, and magnetic studies

“…This estimate is supported by the experimental values of the singlettriplet gaps in m-phenylene-based diradicals that are 2J/k ≈ +600 K and +20 K for bis(nitroxide) and bis(nitronyl nitroxide), respectively. [30], [35], & [36] Therefore, the through-bond exchange coupling mediated by the m-phenylene-CH 2 -mphenylene unit in nitronyl nitroxide tetraradical 1 is expected to be weaker by a factor of 25-30, compared to |J/k| ≈ 1 K in the corresponding nitroxide tetraradical.…”

First coordination compounds based on a bis(imino nitroxide) biradical and 4f metal ions: synthesis, crystal structures and magnetic properties

“…23 This is important, because the IPhIN ligand adopts a coordination mode where each oxygen atom from nitroxide groups is coordinated to (at most) one lanthanide ion. This situation is different from some previously reported lanthanide-based compounds coordinated by bis(nitronyl nitroxides), where oxygen atoms of both the radical units are coordinated to the same ion, adopting a chelating mode.…”

“…In addition, with respect to exchange between the biradical spins, the dihedral angle between the imidazole ring and the m-phenylene unit plays a key role for exchange coupling J 2 . 23 For 1, the inter-ring dihedral angles are not large, which is expected to give ferromagnetic inter-radical exchange coupling J 2 , consistent with the fitted result. For compound 2, χ M T remains constant down to 90 K then decreases at lower temperatures.…”

“…Indeed, these two phenylene-radical torsion angles are quite different for the dinuclear compounds (∼2°and ∼35.5°) but quite similar for the mononuclear ones (∼28°and ∼30°). Although the dihedral angles in compound 4 are not so large as would be expected to induce antiferromagnetic interaction between radicals in an uncomplexed biradical, 23 still an antiferromagnetic intra-biradical exchange interaction appears to be present in 4 and it does have more total radical to phenylene torsion than in 1. Possibly there is also some contribution to the overall downturn of the χ M T versus T plot from antiferromagnetic exchange across the O9⋯N2 i short contacts.…”

Diarylethene as a Photoswitching Unit of Intramolecular Magnetic Interaction

“…The magnitude of the interaction becomes weaker exponentially with increase in the spacer length. When molecules have even numbers of sp 2 carbon atoms between the two radicals, as in 1 [7,8], 3 [9], 5 [10] and 6 [10], the interaction is antiferromagnetic (J I 0). 1 shows several examples of bis(nitronyl nitroxide)s and the exchange interactions between two spins.…”