Radical cations and anions of benzo[c]cinnolines: an electron spin resonance study

Abstract

The radical cations of benzo[c]cinnoline and its four symmetrical dimethyl derivatives have been prepared by exposure of the parent compounds as dilute solutions in CFCI, t o 6oCo y-rays at 77 K.The e s r . spectra were all characterised by large hyperfine coupling t o t w o equivalent nitrogen atoms. Analysis of the data gave ca. 9% 2s and 44% 2p character o n each nitrogen, thereby establishing a o-structure. Clear deviation from axial symmetry shows that the orbital axes are tilted w i t h respect t o each other, an estimate of the angle between the two principle directions being ca. 40". This accords reasonably well with that derived from the p : s ratio of 4.9. The estimated total spin-density on nitrogen is 1.06, showing that the SOMO is strongly localised. Similar treatment of the 2,9-and 3,8-dimethoxy derivatives gave a broad unresolved singlet. This establishes that the SOMO has switched from the o-(N)-orbital to a .n-orbital. Clearly this has very low spin-density o n the t w o nitrogen atoms. We were not able t o detect any of the radical cations in the liquid phase. Exposure of dilute solutions of all six compounds as dilute solutions in CD,OD gave parallel features characteristic of t w o equivalent I4N nuclei. The perpendicular splittings were close t o zero, thus confirming that the SOMO is n, the estimated spin-density on the t w o nitrogen atoms being ca. 34% each. These anions were also prepared in liquid dimethoxyethane and hexamethylphosphoric triamide, and their e.s.r. and ENDOR spectra were recorded. Relative signs of the 'H coupling constants were obtained using general triple resonance. This has led to full assignments in all cases. Our assignment for the anion of the parent compound differs from that previously proposed.

Elucidating the Mechanism of Uranium Mediated Diazene N═N Bond Cleavage

“…Owing to the broadened signals, each displays a poorly resolved hyperfine splitting pattern consistent with previous studies on the spectroscopic investigations of the expected splitting pattern of the benzo­[ c ]­cinnoline radical anion. − Although poorly resolved, the observed splitting is similar to that noted for the product from the room temperature chemical oxidation of UO 2 (salophen t Bu )­(H 2 O) (salophen t Bu = N,N ′-bis­(3,5-di- tert -butylsalicylidene)-1,2-phenylenediamine), which has been attributed to a radical cation localized on the salophen ligand . The g -values of 5 – 7 are significantly shifted from that of in situ generated K­(benzo­[ c ]­cinnoline) (Figure , green trace) (| g | = 2.002) and its derivatives suggesting that the ligand radicals are associated with uranium but not localized there.…”