Fe(IV) alkylidenes via protonation of Fe(II) vinyl chelates and a comparative Mössbauer spectroscopic study
Treatment of cis-Me2Fe(PMe3)4 with di-1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H2), a tetradentate ligand precursor, afforded (bdvp)Fe(PMe3)2 (1-PMe3) and 2 equiv. CH4, via C–H bond activation. Similar treatments with tridentate ligand precursors PhCHNCH2(E-CHCHPh) ((pipp)H2) and PhCHN(2-CCMe-Ph) ((pipa)H) under dinitrogen provided trans-(pipp)Fe(PMe3)2N2 (2) and trans-(pipvd)Fe(PMe3)2N2 (3), respectively; the latter via one C–H bond activation, and a subsequent insertion of the alkyne into the remaining Fe–Me bond. All three Fe(II) vinyl species were protonated with H[BArF4] to form the corresponding Fe(IV) alkylidene cations, [(bavp)Fe(PMe3)2][BArF4] (4-PMe3), [(piap)Fe(PMe3)3][BArF4] (5), and [(pipad)Fe(PMe3)3][BArF4] (6). Mössbauer spectroscopic measurements on the formally Fe(II) and Fe(IV) derivatives revealed isomer shifts within 0.1 mm s−1, reflecting the similarity in their bond distances.