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

Abstract

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.