2,2-Diphenylbenzo[c]quinoline-1-oxyl

In the title compound, C25H18NO, a stable phenanthridinic nitroxide, the ring containing the nitroxide function assumes a twist-boat conformation and the dihedral angle formed by adjacent benzene rings is 21.78 (5)°. The phenyl substituents at position 2 are approximately orthogonal to each other, forming a dihedral angle of 81.04 (4)°. The crystal structure is stabilized by an intramolecular C—H⋯O hydrogen bond and by C—H⋯π interactions.

In the title compound, C 25 H 18 NO, a stable phenanthridinic nitroxide, the ring containing the nitroxide function assumes a twist-boat conformation and the dihedral angle formed by adjacent benzene rings is 21.78 (5) . The phenyl substituents at position 2 are approximately orthogonal to each other, forming a dihedral angle of 81.04 (4) . The crystal structure is stabilized by an intramolecular C-HÁ Á ÁO hydrogen bond and by C-HÁ Á Á interactions.

Comment
Most nitroxides (aminoxyls) are stable radicals that have received a great attention since the second half of the last century for the variety of their applications. In fact, in biology they have been used as relatively stable spin-adducts for studying short-lived radicals such as superoxide (Carloni et al., 1996), hydroxy and alkylperoxy radicals (Greci, 1982) that are typical for peroxidation processes. In this field, nitroxides have also been used as spin probes/spin labels for studying membranes and proteins (Likhtenshtein et al., 2008). In medicine, they have been studied as mimics of superoxide dismutase (Damiani et al., 2008), catalase (Krishna et al., 1996) and as contrast agents of NMR-imaging. In pharmacology, they have been used to study the metabolism of drugs (Setjurc et al., 1995). As antioxidants they have been used in polymers, in stabilising monomers for polyaddition during controlled radical polymerization, for the synthesis of living polymers and in large hydrocarbon distilleries for preventing polymerization and incrustation of pipes (Guillaneuf et al., 2007). As antioxidants, they have also been studied in the medical (ischemia-reperfusion) and cosmetic field for protecting against free radical damage . In chemistry, they have been used as inhibitors of radical processes, in radical synthesis and as mediators of controlled oxidations of primary alcohols and aldehydes (Arends et al., 2006). Recently, nitroxides have found applications in supramolecular chemistry (Franchi et al., 2008), in nanomaterials (Bailly et al., 2006) and in other technologies such as the construction of free radical batteries (Bugnon et al., 2007). In view of its potential application in cosmetics and as a precursor of alkoxyamines used in the controlled radical polymerization, the title compound has been synthesized and its crystal structure is reported here.
In the molecule of the title compound ( Fig. 1), geometric parameters are usual. The value of the N1-O1 bond length (1.2811 (13) Å) corresponds well to the mean value of 1.286 (1) Å found from 891 observations yielded by the Cambridge Crystallographic Database (version 5.30;Allen, 2002) for the N-O single bond, and is in agreement with the radical character of the oxygen atom evidenced by an ESR study (Fig. 3). The benzoquinoline ring system is not planar, the dihedral angle between the C2-C7 and C8-C13 benzene rings being 21.78 (5)° as a result of the sp 3 character of the C1 carbon atom.

Refinement
Though all the H atoms were discernible in the difference electron density maps, the H atoms were positioned into idealized positions with C-H = 0.93 Å, and refined using a riding model approximation with U iso (H) = 1.2 U eq (C). Fig. 1. The molecular structure of the title compound. The displacement ellipsoids are drawn at the 50% probability level. The intramolecular C-H···O hydrogen bond is shown as a dashed line.

Special details
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.