The ferrocenium/ferrocene couple: a versatile redox switch
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The ferrocenium/ferrocene couple: a versatile redox switch Luigi Fabbrizzi1 Received: 9 July 2020 / Accepted: 12 September 2020 © The Author(s) 2020
Abstract Woodward and co-workers in 1952 characterised the unique structural features of ferrocene (the first sandwich compound), demonstrated its aromatic nature and observed that on treatment with mild oxidising agents (aqueous A g2SO4, p-benzoquinone in organic solvents) the orange solution of ferrocene (Fc) turned blue due to the formation of ferrocenium ( Fc+). A few months later, the one-electron Fc/Fc+ redox change was characterised polarographically by Page and Wilkinson with E1/2 = 0.31 V vs SCE (0.56 V vs NHE) in ethanol/water 9:1. Since then ferrocene has become an icon of organometallic electrochemistry. Owing to the stability of its molecular framework, to the ease of functionalisation at the cyclopentadienyl rings and to the fast, reversible and kinetically uncomplicated Fc/Fc+ redox change, ferrocene has been used as a building block for the design of switchable functional systems. In this review, we will consider (1) electrochemical sensors for metal ions, anions and metal–anion pairs operating through the Fc/Fc+ redox change, (2) ferrocene-based redox switches of fluorescence and (3) cross-transport of electrons and anions through a liquid membrane mediated by lipophilic ferrocene derivatives. Graphic abstract
Keywords Ferrocene · Sandwich compounds · Redox switches · Electrochemical sensors · Liquid membranes
The adventurous discovery of ferrocene * Luigi Fabbrizzi [email protected] 1
Dipartimento di Chimica, Università di Pavia, 27100 Pavia, Italy
The synthesis of ferrocene ([Fe(η5-C5H5)2]) and the elucidation of its structural and bonding properties in the early 1950s changed the way of thinking about coordination chemistry (the chemistry of the interaction of metal ions with non-metal donor atoms) and promoted the tumultuous
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development of organometallic chemistry (the chemistry of the metal–carbon interaction). As it often happens in chemistry, the synthesis of ferrocene was serendipitous. Peter Pauson (1925–2013), a British postdoctoral fellow at the Duquesne University in Pittsburgh, aged 26, was interested in verifying the possible aromatic nature of fulvalene (1) and attempted its synthesis according to the two following steps: (1) coupling of two molecules of cyclopentadienylmagnesium bromide to give the dihydro compound 2, (2) dehydrogenation (oxidation) of 2 with F eCl3 to give 1. To the great surprise of Pauson and of his student Tom Kealy, working for an MS degree, a yellow-orange crystalline precipitate formed, whose C and H microanalysis fitted the formula C10H10Fe. The compound was extremely stable, melting at 173 °C, sublimating under vacuum and being unaffected by boiling strong acids and strong alkalis. Pauson proposed the structural formula 3 (Fig. 1), which implied the formation of two intrinsically weak Fe–C σ bonds. However, in order to account fo
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