Towards a better understanding of the magnetic interactions within m-phenylene alpha-nitronyl imino nitroxide based biradicals.

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.