Spin Trapping of Carbon-Centered Ferrocenyl Radicals with Nitrosobenzene

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.