Two equivalents of the unsymmetrical Schiff base ligand (L(tBu))(-) (4-tert-butyl phenyl(pyrrolato-2-ylmethylene)amine) and MoCl(2)(NtBu)O(dme) (dme = 1,2-dimethoxyethane) gave a single stereoisomer of a mixed imido/oxido Mo(VI) complex 2(tBu). The stereochemistry of 2(tBu) was elucidated using X-ray diffraction, NMR spectroscopy, and DFT calculations. The complex is active in an oxygen atom transfer (OAT) reaction to trimethyl phosphane. The putative intermediate five-coordinate Mo(IV) imido complex coordinates a PMe(3) ligand, giving the six-coordinate imido phosphane Mo(IV) complex 5(tBu). The stereochemistry of 5(tBu) is different from that of 2(tBu) as shown by NMR spectroscopy, DFT calculations, and X-ray diffraction. Single-electron oxidation of 5(tBu) with ferrocenium hexafluorophosphate gave the stable cationic imido phosphane Mo(V) complex [5(tBu)](+) as the PF(6)(-) salt. EPR spectra of [5(tBu)](PF(6)) confirmed the presence of PMe(3) in the coordination sphere. Single-crystal X-ray diffraction analysis of [5(tBu)](PF(6)) revealed that electron transfer occurred under retention of the stereochemical configuration. The rate of OAT, the outcome of the electron transfer reaction, and the stabilities of the imido complexes presented here differ dramatically from those of analogous oxido complexes.

Diferrocenyl/diferrocenium substituted dioxido molybdenum(VI) complexes [Fe2MoO2] 2(Fc)/[2(FC)]²⁺ mimic the catalytic active site including the redox subunits as well as the catalytic function of bacterial sulphite oxidases.

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).