Nitronyl nitroxides

The preparation of novel triazole-containing 20-22 membered macrocyclic azacrown ether-thioethers was reported <96JCR(S)182> and the first selective synthetic method fra the synthesis of dicyanotriazolehemiporhyrazines was published <96JOC6446>. 1,2,4-Triazole-containing polyimide beads were prepared and employed as Mo(VI) epoxidation catalyst supports, liie 1,2,4-nitronyl nitroxide 29 was also synthesized and found to have remarkable magnetic properties <96AM60>. 

Busch DH (2005) First Considerations Principles, Classification, and History. 249 in press Bussiere G, Beaulac R, Belisle H, Lescop C, Luneau D, Rey P, Reber C (2004) Excited States and Optical Spectroscopy of Nitronyl Nitroxides and Their Lanthanide and Transition Metal Complexes. 241 97-118 Cadierno V, see Majoral J-P (2002) 220 53-77 Camara M, see Chhabra SR (2005) 240 279-315 Caminade A-M, see Majoral J-P (2003) 223 111-159 CantriU SJ, see Arico F (2005) 249 in press... 

Figure 5. Nitroxide radicals and their SOMO (a) nitronyl nitroxide and (b) imino nitroxide.

Bonnet, M., Luneau, D., Ressouche, E. et al. (1995) The experimental spin density of two nitro-phenyl nitroxides a nitronyl nitroxide and an imino nitroxide, Mol. Cryst Liqu. Cryst, 271-272, 35-53. 

Zheludev, A., Bonnet, M., Ressouche, E. et al. (1994) Experimental spin density in the first purely organic ferromagnet the beta para-nitrophenyl nitronyl nitroxide, J. Mag. Mag. Mat, 135, 147-160. 

Ressouche, E., Zheludev, A., Boucherle, J.X. et al. (1993) Spin densities in nitronyl nitroxide free radicals, Mol. Cryst. Liqu. Cryst., 232, 13-25. 

Zheludev, A., Baron, V., Bonnet, M. et al. (1994) Spin density in a nitronyl nitroxide free radical. Polarized neutron diffraction investigation and ab initio calculation, J. Am. Chem. Soc., 116, 2019-2027. 

Spin density in interacting nitronyl nitroxide radicals... 

Since the reported synthesis [1] of stable 2-phenyl-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide, NitPh, considerable effort has been spent in the physical characterization of nitronyl nitroxide compounds. Their general formula is presented in Figure 1 A. These compounds carry a delocalized, S = 1/2, unpaired electron. Among them, some derivatives were found to be paramagnetic at low temperature (NitPh, [1]), others were found to exhibit an antiferromagnetic or a ferromagnetic behavior [2-7]. 

We report herein a single crystal polarized neutron investigation of the spin density of two purely organic nitronyl nitroxide free radicals which present ferromagnetic interactions the 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-... 

Complexes with nitronyl nitroxide and verdazyl radical ligands 479... 

Concerning molecule-based magnets, the first spin-ladder was synthesized (/ -EPYNN)[Ni (dmit)2] (/ -EPYNN = / -7V-ethylpyridinium o-nitronyl nitroxide). Within the crystal lattice, the radical cation /7-EPYNN units are arranged in ID chains with ferromagnetic interactions. The chains of [Ni(dmit)2] moieties in the ladder formation exhibit coexistent antiferromagnetic interactions.1031,1032... 

In the first case, ID materials exhibit a peculiar magnetic behaviour only in a restricted class of compounds comprising nitronyl-nitroxide radicals. Much richer is the literature on anisotropic 4f ions showing slow relaxation of the magnetization due to SCM behaviour. 

At the beginning of this century slow magnetic relaxation in the paramagnetic phase of a ID system was reported for a Co2+ nitronyl-nitroxide chain [58]. The dynamics was under many respects very similar to that of SMMs and these ID systems were later named SCM to underline the analogies [9]. Since then an intense research activity, though not as spread out as for SMMs, has been devoted to SCMs. The interested reader can find extensive literature on the subject, including exhaustive reviews and a book chapter [59-63]. The aim here is to provide some basic concepts of the phenomenon and to introduce some selected examples of SCMs based on lanthanide ions. 

Radicals have been known for many years to form organic paramagnetic materials with numerous magnetic properties (ferro- or ferri-magnetism, spin Peierls transition, spin frustration, spin ladder systems) (see [51-60] for verdazyl radicals, [61-68] for thiazyl radicals, [69] for nitronyl nitroxide and [70-78] for Tempo radicals) (Fig. 6). When they are in their cationic form, they are valuable candidates for an association with the M(dmit)2 systems they will then provide the magnetic properties thanks to their free electron(s), whereas the M(dmit)2 moieties will provide the electrical properties. 

Several anion-biradicals have been recently reported, which are composed of semiquinone and nitroxide functionalities. Such anion-biradicals are obtained by electron transfer to the p-benzoquinone (BQ) substituted by a nitronyl (Shultz and Farmer 1998) or nitronyl-nitroxide (NN) group (Kumai et al. 1994). Scheme 1.33 gives the BQNN example. 

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