L. Catala et al., Towards a better understanding of the magnetic interactions within m-phenylene alpha-nitronyl imino nitroxide based biradicals, CHEM-EUR J, 7(11), 2001, pp. 2466-2480
An extensive investigation of the magnetic properties of three series of bi
radicals (bis-nitronyl nitroxides diNN-R, bis-imino nitroxides diIN-R and m
ixed INNN-R. where R is either hydrogen, a triple bond or trimethylsilylace
tylenic group) has been carried out to give clear values of the intramolecu
lar interactions through the m-phenylene coupling unit with alpha -nitronyl
nitroxides (NN) or alpha -imino nitroxides (IN). An EPR study of the molec
ules in the isolated state is validated by ab initio calculations, which sh
ow the respective influence of spin polarisation and molecular conformation
on the singlet-triplet gaps. All these results indicate that the triplet s
tate is the ground state for such biradicals. except when the imidazolyl cy
cles are orthogonal to the phenyl ring. The magnetic properties of the bira
dicals in the solid state can be rationalised by examination of the short c
ontacts produced between the ONC-NO and ONCN groups. EPR studies on single
crystals of the H-substituted series have confirmed the presence of a struc
tural distortion for diNN-H whereas diIN-H and INNN-H do not exhibit such a
peculiarity. The magnetic behav lour 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 con
tact involves more complex interactions between the dimers. The compound di
NN-tmsa exhibits a ferromagnetic intermolecular interaction of +11 K within
the dimers, and this could be attributed to the relative disposition of th
e imidazolyl rings. Compound diNN-tr reveals a chain-like behaviour, wherea
s diIN-tr shows a predominant antiferromagnetic interaction within the dime
rs. The values for the intramolecular interactions in the solid state are i
n good agreement with those found for the isolated molecules.