Impact of O → S Exchange in Ferrocenyl Amides on the Structure and Redox Chemistry

The conformations and redox chemistry
of ferrocenyl amides have
been investigated in considerable depth in the last few years, while
ferrocenyl thioamides have attracted less interest so far, although
distinctly different conformations and reactivity patterns are expected.
Monoferrocenyl amides Fc-NHC­(O)­CH3 (1) and
1,1′-CH3O­(O)­C-Fn-NHC­(O)­CH3 (2) and diferrocenyl amides Fc-NHC­(O)-Fc (5) and Fc-NHC­(O)-Fn-NHC­(O)­CH3 (6) are easily transformed into the corresponding
thioamides (3, 4, 7, 8) by treatment with Lawesson’s reagent (2,4-bis­(p-methoxyphenyl)-1,3-dithiaphosphetane-2,4-disulfide) (Fc
= Fe­(C5H4)­(C5H5), Fn =
Fe­(C5H4)2). The thioamide conformations
(cis/trans) in 3, 4, 7, and 8 and the hydrogen bond
determined secondary structure of dithioamide 8 have
been elucidated by IR and NMR spectroscopy as well as by DFT calculations
(B3LYP, LANL2DZ, PCM CH2Cl2) and contrasted
with the corresponding amides 1, 2, 5, and 6. The electronic communication via the
thioamide bridge in 7+ and 8+ in comparison to the interaction
in the parent mixed-valence amides 5+ and 6+ has been probed
by cyclic voltammetry, square wave voltammetry, UV/vis spectroelectrochemistry,
EPR spectroscopy, and paramagnetic NMR spectroscopy. Additional chemical
reactivity of the thioamide unit has been detected by electrochemical
analysis.