Ferrocenyl fluoride

The reaction of ferrocene and formaldehyde in either concentrated sulfuric acid or liquid hydrogen fluoride, followed by reduction, produces a compound containing two ferrocenyl and two methylene groups (57, 98, 123). After several incorrect assignments had been proposed for the structure of this condensation product, Rinehart and coworkers showed by an unequivocal synthesis that the product was 1,2-diferrocenylethane (XIX) (104). The mechanism of the reaction presumably involves the initial formation of ferrocenylcarbinol (XX) followed by ionization in the strongly acidic medium to the ferrocenylmethyl-carbonium ion (XXI). Conversion to radical ion XXII followed by dimerization and subsequent reduction produces the product. 

The violet fluoro derivative WO(F)Fc3 is formed from WO(X)Fc3 (X = Cl, OFc, OnBu) in the presence of complex fluoride salts such as Mc30 BFT in CH2CI2 it is almost insoluble in organic solvents [176]. The H and NMR spectra of the diamagnetic oxotungsten(vi) complexes are temperature-dependent, suggesting a hindered rotation of the ferrocenyl ligands around the W-C(ferrocenyl) bond [175,176] in the case of WO(OFc)Fc3 the free activation enthalpy, AG (7 ), was determined to be 62.5 + 0.5 kJ/mol [175]. 

The functionalized boronic acids (15 and 16) are useful for sensing fluoride ion in aqueous solution. Electrochemical redox is used for the detection of fluoride ion with ferrocenyl-boronic acid (15) [8] and for fluorescence detection with aminoboronic acid (16) [9]. Molecule (16) can effectively detect concentration of fluoride ion in the range of 5-30 mM, where the fluoride adduct is stabilized by the additional hydrogen bonding with protonated amine at pH 5.5 as shown in structure 17 (Scheme 3.35). 

Many reports have focused on the functionalization and derivatization of cyclo-pentadienyl groups bound to iron in ferrocene complexes. As an example, a series of ferrocenyl esters have been prepared via solvent-free reactions of ferrocenoyl fluoride with substituted phenols after microwave heating for just 1 min. " In the case of 4-bromophenol, the YIF was 1.2, but most substrates gave somewhat lower yields than the conventional method. However, the microwave approach did not require the addition of A, A -dimethylaminopyridine that was necessary conventionally. Other reports include a green approach to the acylation of ferrocene using the polymeric sulfonic acid Nation as an acid catalyst " and a one-pot approach to the synthesis of l,5-dioxo-3-substituted[5]ferrocenophanes from l,T-diacetylferrocene and aldehydes via a Claisen-Schmidt reaction. " " ... 

An easily visualised example is the multidentate Lewis-acid host 2.77, a colourimetric fluoride sensor. The receptor contains a non-irmocent ferrocenyl linker between the boronate groups. Reaction of 2.77 with n-Bu4N+F in CHCI3... 

Shinkai futher elaborated his ferrocenyl system and created a colorimetric system capable of visually detecting fluoride binding, thus devising an optical sensor for fluoride. Taking advantage of a redox reaction between the dye molecule methylene blue and ferrocenyl boronic acid, it was possible to visually determine fluoride concentrations. Decolorization of the dye occurs over a 4 x 10- -3 X 10 mM fluoride range (monitored by UV-vis spectroscopy as a decreasing absorbance at 665 nm). 

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