Extremely Electron‐Poor Bis(diarylmethylium)‐Substituted Ferrocenes and the First Peroxoferrocenophane

Larissa A Casper,Steffen Oßwald,Lisa‐Catherine Rosenbaum,Rainer F Winter,Patrick Anders

doi:10.1002/zaac.201900347

Larissa A Casper, Steffen Oßwald + Show 3 more

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https://doi.org/10.1002/zaac.201900347

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Abstract

We have prepared and studied extremely electron‐poor, deeply colored dicationic 1,1'‐bis(diarylmethylium)‐substituted ferrocenes [(η5‐C5H4‐CAr2)2Fe]2+ with various aryl substituents as their [B{C6H3(CF3)2‐3,5}4]– salts. Due to the strong acceptor substitution, the redox potential for the ferrocene‐based oxidation of the anisyl‐ or 2‐methylanisyl‐substituted congeners 1b2+ and 1c2+ is close to or even surpasses that of the second oxidation of parent ferrocene, i.e. the Cp2Fe+/2+ couple. The strongly Lewis‐acidic character of these complexes is manifest through strong interactions with donor solvents, which lead to a significant reduction of the intensities of the charge‐transfer bands in their electronic spectra and to solvatochromism. The reduced forms of the complexes tend to dimerize or oligomerize as revealed by EPR spectroscopy. Direduced 1b selectively reacts with molecular oxygen to form a peroxo‐bis(diarylmethyl)[4]ferrocenophane, which was also characterized by X‐ray crystallography.

Highlights

Triarylmethylium substituted ferrocenes constitute the most prominent examples of metal-organic tritylium dyes and were studied early on for their electronic structures.Particular focus was on the ability of the iron nucleus of the ferrocene scaffold to stabilize the positively charged, adjacent methylium center by σ- or π-interactions as expressed by the resonance forms IV and V in Scheme 1.[1]
Such kind of interactions were initially inferred by the 20.7° bending of the CPh2+ plane toward the Fe2+ ion in the crystallographically determined structure of Fc-CPh2+[2] (Fc = ferrocenyl, (η5C5H5)Fe(η5-C5H4)) and by quantum chemical calculations,[3] but later refuted by the notion of a small energy barrier for rotation around the Cp–C+Ar2 bond[4] and the generally large dipole moments of ferrocenyl carbenium ions.[1f]
The electron-donating ferrocenyl substituent and the strong dipolar character of such compounds usually lead to intense charge-transfer absorptions in the visible regime of the electronic spectrum (Vis), which are typical of donor-substituted tritylium dyes

Summary

Introduction

Triarylmethylium (tritylium) substituted ferrocenes constitute the most prominent examples of metal-organic tritylium dyes and were studied early on for their electronic structures.Particular focus was on the ability of the iron nucleus of the ferrocene scaffold to stabilize the positively charged, adjacent methylium center by σ- or π-interactions as expressed by the resonance forms IV and V in Scheme 1.[1]. Vinylogous or alkynylogous expansion of the ferrocenyl arm of such dyes shifts the corresponding chargetransfer band further red and increases its oscillator strength.[5] In ferrocenyl-tritylium dyes Fc-C+Ar2, the CT transitions from the carbocyclic substituents to the methylium center are augmented by an additional, weaker CT band (the HOMOǞ LUMO transition) concomitant with the direct transfer of electron density from the Fe2+ ion to the methylium acceptor. In agreement with this assignment, this band is bleached on oxidation, whereby the ferrocene donor is changed into an electronpoor ferrocenium ion. Neutral ferrocenyl(diaryl)methyl radicals FcCAr2 are prone to dimerize, and an authentic hexaarylethane structure was proposed for these dimers.[6]

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