3,4-Ferrocenyl-Functionalized Pyrroles: Synthesis, Structure, and (Spectro)Electrochemical Studies

The synthesis of 3,4-diferrocenyl-substituted pyrroles of the type 3,4-Fc(2)-cC(4)H(2)NR [Fc = Fe((5)-C5H4)((5)-C5H5); R = Ph (3a), SO2-4-MeC6H4 (Ts) (3b), SiiPr(3) (3c)], 3,4-(FcCC)(2)-cC(4)H(2)NR [R = Ph (4a), Ts (4b)], and 3-Br-4-FcCC-cC(4)H(2)NR [R = Ph (7a), Ts (7b)] from 3,4-Br-2-cC(4)H(2)NR [R = Ph (2a), Ts (2b), SiiPr(3) (2c)] is discussed. The molecular structures of 3a,b, 5, 4b, and 7b in the solid state are reported and show that the formal double bonds in the heterocyclic core are rather localized relative to pyrrole itself. The investigations with (spectro)electrochemical methods reveal the different capabilities for the formation of mixed-valent species and allows the classification of 3a,b as class II systems, whereas compounds that feature electron-withdrawing -CC- units (4a,b) can be assigned to class I systems according to Robin and Day.