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

Abstract

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.

Solution Conformation and Self‐Assembly of Ferrocenyl(thio)ureas

“…The C=S group of thiourea 6 is more strongly electron‐withdrawing than the C=O group in urea homologue 1 , leading to higher half‐wave potentials for the first and the second oxidation of the ferrocenyl moieties. An analogous trend was observed for Fc‐NHC(O)R and Fc‐NHC(S)R pairs . For the sulfur derivative 6 , a third irreversible oxidation wave was found at E p = 800 mV (vs. FcH/FcH + ); this is assigned to a sulfur‐centered oxidation …”

“…A similar structural impact on formal O/S exchange has been observed for related ferrocenyl carboxamides and thioamides FcNHC(X)Me (X = O, S). The ferrocenyl carboxamide forms hydrogen‐bonded tapes (N ··· O 2.73, 2.87 Å) with hydrogen bond lengths comparable to those in 1 – 5 , whereas the sulfur homologue crystallizes as centrosymmetric dimers of cis ‐thioamides (N ··· S 3.393 Å) analogously to 6 and 7 (Table ) , …”

Conformational Switching of Multi‐Responsive Ferrocenyl‐Phenol Conjugates

“…[16,17] On the other hand, [H-4] +· features an absorption band at λ = 771 nm, characteristic of an NHR-monosubstituted ferrocenium ion, fully analogous to [H-A] +· (λ = 770 nm). This electronic coupling is somewhat higher than that obtained for [Ac-Fca-NH-Fc] +· in CH 2 Cl 2 (H AB = 260 ± 10 cm -1 ), [37] but still places [H-4] +· in the class II regime of the Robin-Day classification, [39] similarly to comparable amide-and thioamide-bridged ferrocene/ferrocenium systems. Additionally, [H-4] +· exhibits a broad absorption band at λ = 1074 nm with a full-width-at-half-maximum of 4575 cm -1 (see Figure S11).…”

“…[16,17] As [Fc-NH] +· entities are strong hydrogen-atom donors, [13,16,17] ionic hydrogen bonds [15] involving this positively charged functional group are highly favorable. [13,17,37] For Fca-based contact ion pairs, chelating coordination modes have been suggested as well. Hence, coordination of the [SbF 6 ]counter ion to this NH group to form a contact ion pair is likely.…”

“…[13] This suggests that the [H-4a OHONHFNHF ][SbF 6 ] valence isomer with a terminal NHRsubstituted ferrocenium ion is indeed predominantly present. [16,17,37,40] In addition to the ferrocenium and IVCT bands at λ = 771 and 1074 nm, respectively, a further weak absorption band at λ = 935 nm (ε = 30 M -1 cm -1 ) is required to satisfactorily fit the experimental UV/Vis spectrum (see Figure Such an absorption band is characteristic of amidebridged mixed-valent ferrocene/ferrocenium complexes and has been assigned to an intervalence charge-transfer (IVCT) [38] yielded an electronic coupling constant of H AB = 310 ± 20 cm -1 for [H-4] +· , with the distance between the redox sites assumed as R AB = 7.0 ± 0.5 Å.…”