Mechanisms and dynamics of protonation and lithiation of ferrocene

Abstract

By elucidating the mechanism of the simplest electrophilic substitution reaction of ferrocene, it was found that the verification of the protonation reaction has been a difficulty. In the work reported here, ab initio chemical dynamics simulations were performed at B3LYP/DZVP level of theory to understand the atomic level mechanisms of protonation and lithiation of ferrocene. Protonation of ferrocene resulted in the agostic and metal-protonated forms. Trajectory calculations revealed that protonation of ferrocene occurs by exo and endo mechanisms, with exo being the major path. H(+) was found to be mobile and hopped from the Cp ring to the metal center and vice versa after the initial attack on ferrocene, with the metal-complex having a shorter lifetime. These results remove the ambiguity surrounding the mechanism, as proposed in earlier experimental and computational studies. Lithiation of ferrocene resulted in the formation of cation-π and metal-lithiated complexes. Similar to protonation, trajectory results revealed that both exo and endo paths were followed, with the exo path being the major one. In addition, lithiated-ferrocene exhibited planetary motion. The major path (exo) followed in the protonation and lithiation of ferrocene is consistent with the observations in earlier experimental studies for other hard electrophiles.

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

“…[9] Overall, the literature holds contradicting mechanistic reports on electrophilic proton substitution, purporting either protonation or proton exchange. [13][14][15][16] Our DFT calculations (see the Supporting Information) show that two isomers are distinguishable ( Figure 2): an agostic and ap urely hydridic form, with the latter being about 9kJmol À1 higher in energy (M06L/def2TZVPP). [13][14][15][16] Our DFT calculations (see the Supporting Information) show that two isomers are distinguishable ( Figure 2): an agostic and ap urely hydridic form, with the latter being about 9kJmol À1 higher in energy (M06L/def2TZVPP).…”

“…[13][14][15][16] Our DFT calculations (see the Supporting Information) show that two isomers are distinguishable ( Figure 2): an agostic and ap urely hydridic form, with the latter being about 9kJmol À1 higher in energy (M06L/def2TZVPP). [16,18] Additionally,i tw as stated that the attack of the hard electrophile H + should occur preferably through an attack on the Cp ring (exo mechanism). The Fe À Hd istances vary from 1.491 in the purely hydridic species to 1.646 in the agostic form;the latter possesses an additional CÀHc ontact with ad istance of 1.243 .I nt he agostic form, however, this contact results in as ignificantly broader variation in the CÀCb ond lengths (1.398-1.471 ) compared to the bond lengths calculated for the hydridic species (1.411-1.431 ).…”

“…Theoverall molecular geometry is very similar in both cases.T he Cp planes are not parallel to each other (with calculated tilt angles of 9.928 8 and 12.588 8). [16] Based on NMR experiments (at room temperature), Mueller-Westerhoff and co-workers postulated that exo attack of the proton on the Cp ring occurs. In general, however, it seems that the potential energy surface associated with the Cp ring tilt and FeÀHbond length is very flat.…”