Nitronylnitroxides

Fig. 17 Potential exchange pathways for phenolic nitronylnitroxide radicals.

Fig. 18 Different geometries of NO -HO contacts in phenolic nitronylnitroxides lead to different exchange behaviors.

Shiomi et al., shortly after the Ur6IN work, reported the synthesis and magnetostructural characterization of cytosine-functionalized nitronylnitroxide... 

At about the time that the work on BABI appeared, Yoshioka et al. reported magnetostructural studies of other variations of BIm having a nitronylnitroxide group in the 2-position, such as BImNN.194 Nitronylonitroxides are much less delocalizable than the aminoxyl radicals used in BABI and PhBABI. Computed... 

Fig. 29 Hydrogen-bonded chains that organize radicals into FM exchange-coupled piled stack motif among nitronylnitroxide radicals in BlmNN. Inset (right) shows the piled stack spin orbit N-O/C contact that gives rise to FM exchange.

Tetramethylimidazolidines 658 are oxidized to the corresponding nitronylnitroxides 659 (1%%, R = Ph), via a 1,3-dihydroxy intermediate (Scheme 159) <2001CEJ2007>. KMn04 effects C-H oxidation of the derivative 660 to give the imidazolidinone 661 <2004SC1617>. 

The phenalenyl radical 4 represents the most fundamental and widely explored member in this family. Unlike typical stable neutral radicals such as TEMPO and a-nitronylnitroxide derivatives with a spin-localized nature, 4 is characterized by a planar, rigid structure with the spin spread over the whole molecular skeleton (Fig. 3). The resonance structures of 4 show that the spin density is predominantly at its oc positions, while the spin at the peripheral positions (jS positions) is much smaller, which can be explained by the spin polarization effect. In addition, 4 exhibits a high amphoteric redox ability with thermodynamically stable cation, neutral radical and anion species. All of these features of 4 lead to new insights in the field of physical chemistry. 

The ESR studies of labeled dendrimers intended to clarify their role as MRI contrast agents have been described in several papers. Spin labeling of several different dendrimers (DAB and PAMAM, at different generations) let the authors calculate the rates of nitroxide reduction due to superoxide, by observing the change of the low-field spectral peaks in the ESR spectra as a function of time (Table 3). Dendrimers labeled with nitronylnitroxides work well as contrast agents in MRI, and therefore their characterization by means of ESR is of interest. ... 

An interesting medical application of the nitronylnitroxide-labeled dendrimers is spin trapping of nitric oxide. Nitronyl nitroxides can spin trap NO, but are unstable... 

Otsuka T, Okuno T, Awaga K, Inabe T (1998) Crystal structures and magnetic properties of acid-base molecular complexes, (p-pyridyl nitronylnitroxide)2X (X = hydroquinone, fumaiic acid and squaric acid). J Mater Chem 8 1157-1163... 

Fig. 13 (a, b) MAS NMR spectra of two nitronylnitroxide radicals (spinning rates of 10 kHz and 15 kHz, respectively), together with the corresponding sign patterns of the spm densities experimentally derived from the Fermi contact shifts up-spins indicate positive spin), (e) Side view of an unsubstituted nitronylnitroxide down from the center of the bond C4-C5 to C2. The spin-carrying MO is located on the NO groups. Reprinted with permission from [51], (C) 1999 American Chemical Society... 

Heise H, Kohler EH, Mota E, Novoa JJ, Veciana J (1999) Determination of the spin distribution in nitronylnitroxides by solid-state H-1, H-2, and C-13 NMR spectroscopy. J Am Chem Soc 121 9659-9667... 

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