Introduction1-Amino-1 0 -carboxyferrocene H-Fca-OH [1-4] (Scheme 21.1) is a versatile starting material for the preparation of 1,n 0 disubstituted ferrocenes with distinguished and unique properties. By placing the hydrogen atom donor and hydrogen atom acceptor sites in the different strands of 1,n 0 disubstituted ferrocene derivatives, interstrand hydrogen bonds are feasible giving ansa-ferrocenes. For example, an interstrand hydrogen bond has been observed in Me-CO-Fca-OBt (Bt ¼ 1-benzotriazole) with the triazole heterocycle acting as hydrogen atom acceptor and the NH group acting as hydrogen atom donor, giving an eight-membered ring, excluding the iron center ( Figure 21.1a) [4]. In contrast, the N-acyl urea derivative Me-CO-Fca-NCy-CO-NHCy (Cy ¼ cyclohexyl) features a more strained six-membered ring with the Fca carbonyl unit CO Fca being the acceptor, although larger ring motifs are also possible -for example, an eight-membered ring with the ureylene carbonyl CO ureylene as acceptor and a 10-membered ring with the acetyl carbonyl CO Ac as acceptor and the NH ureylene as donor (Figure 21.1b) [4].In cases where there are several acceptor and donor sites in the two arms, many ring motifs become possible. This holds especially true for peptidic Fca conjugates with organic amino acids or with further Fca units as arms, with each additional amide group providing both a potential hydrogen acceptor and donor site. In natural peptides built from a-amino acids, both intramolecular and intermolecular hydrogen bonds allow the formation of a-helices, b-sheets, or turns. In conjugates incorporating 1,n 0 ferrocene units, a very flexible hinge is introduced into the backbone which allows for small and large hydrogen-bonded ring systems. The two strands can be oriented approximately in the same direction (v ¼ 0 AE 36 ; 1,1 0 rotational isomer), rotated clockwise by 360 /5 ¼ 72 (v ¼ 72 AE 36 ; 1,2 0 rotational isomer), rotated clockwise by 2  360 /5 ¼ 144 (v ¼ 144 AE 36 ; 1,3 0 rotational isomer), rotated anticlockwise by À2  360 /5 ¼À 144 (v ¼À 144 AE 36 ; 1,4 0 rotational isomer), or rotated anticlockwise by À360 /5 ¼À 72 (v ¼À 72 AE 36 ; 1,5 0 rotational isomer).