The Synthesis, Structure, and FTIR Spectroelectrochemistry of W(CO)5 Complexes of 4‐Oxo‐4‐(2,5‐dimethylazaferrocen‐1′‐yl)butanoic and 5‐Oxo‐5‐(2,5‐dimethylazaferrocen‐1′‐yl)pentanoic Acids

With the aim of developing new IR‐detectable metal–carbonyl tracers for the amino function, we have synthesized W(CO)5 complexes of 4‐oxo‐4‐(2,5‐dimethylazaferrocen‐1′‐yl)butanoic acid (2) and 5‐oxo‐5‐(2,5‐dimethylazaferrocen‐1′‐yl)pentanoic acid (3) by AlCl3‐catalyzed Friedel–Crafts reaction of W(CO)5–2,5‐dimethylazaferrocene (1) with succinic or glutaric anhydride. Complexes 2 and 3 are thermally stable and display sharp, intense absorption bands of tungsten‐coordinated CO ligands at ca. 1923 cm–1. In the crystalline state, molecules of 2 and 3 are stabilized by a network of intra‐ and intermolecular hydrogen bonds, as shown by single‐crystal X‐ray structure analysis. Complex 2 was transformed into the corresponding N‐succinimidyl ester 4. Its utility toward labeling of amino acids was tested in its reaction with glycine methyl ester. Corresponding glycine amide 5 was obtained in 82 % yield and is an air/thermally stable bioconjugate exhibiting intense sharp absorption bands of the W–CO reporting group at ca. 1923 cm–1. Cyclic voltammetry of 1, 2, 3, and acetyl derivative 6 shows the presence of two redox events in each case. The first redox couple is ascribed as an azaferrocene‐centered oxidation–reduction, whereas the second, irreversible process at higher potential originates from a W(CO)5‐centered oxidation. FTIR spectroelectrochemistry allowed us to monitor the spectroscopic changes accompanying the 1/1·+, 2/2·+, and 6/6·+ redox transformations. Significant W–CO absorption band shifts were recorded in the course of these experiments.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) The synthesis and structure of 2,5‐dimethylazferrocene carboxylic acids having IR‐active W(CO)5 markers is reported. Their FTIR spectroelectrochemical studies in OTTLE cell show that IR band shifts of the W(CO)5 moiety are highly sensitive toward the 2,5‐dimethylazaferrocene oxidation state.