PROTONATION OF FERROCENE BY STRONG ACIDS¹

Abstract

The revised structure I has been demonstrated recently for gibberellic acid by Cross, et al.1 We wish to report work which extends our previous results2 on the stereochemistry of allogibberic acid (II) to that of gibberellic acid itself.

Why is Ferrocene so Exceptional?

“…Other names for ferricinium include ferrocenium and ferricenium; they have also been used since the early days. Because “ferro” is conventionally used for Fe II complexes and “ferri” for Fe III complexes, it would be best to use the name ferricenium or ferricinium, especially because, in addition, the name ferrocenium hydride should be reserved for the protonated form of ferrocene [Fe(H)(η 5 ‐C 5 H 5 ) 2 ] + [PF 6 – ], a “bent” metallocene hydride , …”

“…Because "ferro" is conventionally used for Fe II complexes and "ferri" for Fe III complexes, it would be best to use the name ferricenium or ferricinium, especially because, in addi-tion, the name ferrocenium hydride should be reserved for the protonated form of ferrocene [Fe(H)(η 5 -C 5 H 5 ) 2 ] + [PF 6 -], a "bent" metallocene hydride. [70,71] In cyclic voltammetry a chemically and electrochemically reversible single-electron (1e) redox wave is found upon anodic oxidation around 0.4 V versus SCE, [68] but the exact redox-potential value depends on the solvent, because of the variable interactions of the different solvents with the positively charged central iron. Although ferrocene has long been considered a reference for the measure of redox potentials, decamethylferrocene and other permethylated metallocenes are therefore much more reliable references, because in these cases the central metal atom is protected from these interactions by the cage of ring methyl groups, and the redox potential is then independent of the nature of the solvent.…”

Protonation of Ferrocene: A Low‐Temperature X‐ray Diffraction Study of [Cp2FeH](PF6) Reveals an Iron‐Bound Hydrido Ligand

“…In general, [Cp 2 FeH] + is only stable in media of high acidity and under rigorous exclusion of oxygen (to avoid decomposition to Cp 2 Fe + ), which has hindered its structural characterization thus far. As early as 1959, Rosenblum and Santer reported the formation and isolation of the salt (Cp 2 Fe⋅HAlCl 4 ) . In a follow‐up report one year later, they concluded (based on NMR spectroscopy) that the protonation of ferrocene occurs exclusively at the iron atom and that the Cp rings remain unaltered in the process .…”

Protonation of Ferrocene: A Low‐Temperature X‐ray Diffraction Study of [Cp2FeH](PF6) Reveals an Iron‐Bound Hydrido Ligand

“…In general, [Cp 2 FeH] + is only stable in media of high acidity [3] and under rigorous exclusion of oxygen (to avoid decomposition to Cp 2 Fe + ), [4] which has hindered its structural characterization thus far.A se arly as 1959, Rosenblum and Santer reported the formation and isolation of the salt (Cp 2 Fe·HAlCl 4 ). [5] In af ollow-up report one year later, they concluded (based on NMR spectroscopy) that the protonation of ferrocene occurs exclusively at the iron atom and that the Cp rings remain unaltered in the process. [6] More than 15 years later,Siebert and co-workers used acombination of HI and BI 3 to prepare and isolate [Cp 2 FeH](BI 4 ).…”