Radical ions and germyloxyaminoxyls from nitrospiro[indoline-naphthopyrans]. A combined electrochemical and EPR study

Abstract

The radical ions from 5-nitrospiro[indoline-2,3′-naphthopyran]1, 8′-nitrospiro[indoline-2,3′-naphthopyran]2, and 5,8′-dinitrospiro[indoline-2,3′-naphthopyran]3 have been investigated electrochemically and by means of electron paramagnetic resonance spectroscopy.The first reduction potentials of these compounds reflect the position of the nitro groups and vary between –1.532 V for 1 and –1.273 V for 2vs. SCE, while 3 exhibits two reduction waves at, potentials similar to those of 1 and 2, suggesting no or very little interaction between the two moieties of the molecules. The oxidation potentials are dominated by the indolinic moiety increasing in the order 2 < 1 < 3.The reduction with butoxide in DMF leads to the EPR observation of the expected radical anions of 1 and 2, while the spectrum observed with 3 shows, against expectations, reduction of the indolinic moiety. Photoreduction with Bu4NBH4 in THF affords the radical anions of 2 and 3 but not of 1, the measured hfs constants being similar to those measured in DMF without light. Addition of in situ generated triphenylgermyl radicals to 1–3 affords rather persistent germyloxy-aminoxyls which give identical spectra both under UV irradiation and in the dark.