Nitronyl nitroxides: magnetic

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>. 

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. 

Diradical species 35a and 35b, in which two imino nitroxide 35a or nitronyl nitroxide 35b radical centers are attached to thieno[2,3-. ]thiophene, were prepared and their intramolecular exchange interactions were investigated in frozen solutions by means of electron spin resonance (ESR) spectroscopy and magnetic susceptibility measurements at cryogenic temperature <1996T6893>. 

In the above examples, a transition-metal atom is invariably present. As such, one might believe that the unpaired d electrons of the transition metal are the main cause of ferro/ferrimagnetic ordering, the organic/molecular species only playing a secondary role. There are a few examples of molecular magnets without the transition-metal atoms, albeit showing low T s. These are p-nitrophenyl nitronyl nitroxide (T = 0.6 K)... 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

We shall consider, for purposes of illustration, the system p-(methylthio)-phenyl-nitronyl-nitroxide (MTPNN,7 in Figure 2.1) in toluene solution [38], Principal values and orientations of magnetic and diffusion tensors have been taken from QM calculations,... 

As described earlier, when two nitronyl nitroxides are magnetically coupled via an exchange interaction, the biradical gives a nine-line ESR spectrum. If the exchange interaction is weaker than the hyperfine coupling, the two nitroxide radicals are magnetically independent and give a five-line spectrum. In intermediate situations, the spectrum becomes complex. 

Table 9.1 Magnetic interaction between two nitronyl nitroxide connected by diarylethene photoswitches.

Fig. 23 A spontaneously resolved chiral complex between Mn(hfac)2 and a phenyl nitronyl nitroxide which is a magnet...

Figure 9.7 The structure of NITR radical and the schematic view of the crystal structure of RE(hfac)3NITEt [76], (Reprinted from A. Caneschi, D. Gatteschi, and R. Sessoli, Magnetic properties of a layered molecular material comprising manganese hexafluoroacetylacetonate and nitronyl nitroxide radicals, Inorganic Chemistry 32, 4612 616, 1993. 1993 American Chemical Society.)...

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