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

Citation
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
Citations number
88
Categorie Soggetti
Chemistry
Journal title
CHEMISTRY-A EUROPEAN JOURNAL
ISSN journal
09476539 → ACNP
Volume
7
Issue
11
Year of publication
2001
Pages
2466 - 2480
Database
ISI
SICI code
0947-6539(20010601)7:11<2466:TABUOT>2.0.ZU;2-Q
Abstract
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.