Synthesis and chemical oxidation of 3-ferrocenylpyrrole and ferrocenyl-substituted triazoles: Iron versus ligand based oxidation

Copper catalyzed [3+2] cycloaddition reactions between ethynylferrocene and benzylazides yields 1-benzyl-4-ferrocenyl-1,2,3-triazoles (2–5). Reaction between phenylacetylene and azidoferrocene yields 1-ferrocenyl-4-phenyl-1,2,3-triazole (6). Anodic electrochemistry of 2–6 suggests reversible oxidation at potentials more positive than ferrocene. Chemical oxidation of 2 and 3-ferrocenylpyrrole (1) with dichlorodicyanoquinone (DDQ) yields the salts [2⁺] [DDQ⁻] and [1⁺] [DDQ⁻], respectively. ⁵⁷Fe Mössbauer spectroscopy reveals the presence of low-spin Fe^II in [1⁺][DDQ⁻] while Fe^II is oxidized to low-spin Fe^III in [2⁺][DDQ⁻]. Magnetization measurements indicate that [1⁺][DDQ⁻] is paramagnetic and cannot be viewed as a simple neutral charge transfer complex reminiscent of the mixed stack diamagnetic [ferrocene]⁰[TCNE]⁰.