Spin Trapping of Carbon-Centered Ferrocenyl Radicals with Nitrosobenzene

Abstract

In contrast to metal centered 17 valence electron radicals, such as [Mn(CO)5]•, ferrocenium ions [Fe(C5H5)2]+ (1+), [Fe(C5Me5)2]+ (2+), [Fe(C5H5)(C5H4Et)]+ (3+), [Fe(C5H5)(C5H4NHC(O)Me)]+ (4+), and [Fe(C5H5)(C5H4NHC(S)Me)]+ (5+) do not add to nitrosobenzene PhNO to give metal-coordinated stable nitroxyl radicals. In the presence of the strong and oxidatively stable phosphazene base tert-butylimino-tris(dimethylamino)phosphorane, the quite acidic ferrocenium ions 1+–5+ are deprotonated to give a pool of transient and persistent radicals with different deprotonation sites [1–Hx]•–[5–Hx]•. One rather persistent iron-centered radical [4–HN]•, deprotonated at the nitrogen atom, has been detected by rapid-freeze EPR spectroscopy at 77 K. This iron-centered radical [4–HN]• is also inert toward PhNO. The transient carbon-centered radicals [1–Hx]•–[5–Hx]• appear to rapidly abstract hydrogen atoms from the adjacent base or the solvent to regenerate the corresponding ferrocenes 1–5. These transient radicals are only present in trace amounts (<1%). However, some of the transient carbon-centered radicals in the radical pool can be trapped by 1–1.2 equiv of PhNO, even at room temperature. The corresponding resulting stable nitroxyl radicals [6]•–[10]• were studied by EPR spectroscopy at room temperature and at 77 K. The hyperfine coupling pattern to protons close to the spin center allows one to assign the site of PhNO attack in radicals [6]•–[10]•, namely, at the C5H5 ring in [6]•, [9Cp]•, and [10Cp]•, at a methyl group in [7]•, and at the methylene group in [81]•. These studies give a deeper insight into the stability and reactivity of radicals derived from ferrocene derivatives which might also be relevant for the biological activity of high-potent antitumor and antimalaria ferrocene-based drugs and prodrugs such as ferrocifen or ferroquine.