The Unusual Redox Properties of Fluoroferrocenes Revealed through a Comprehensive Study of the Haloferrocenes

Abstract

ABSTRACT:We report the synthesis and full characterization of the entire haloferrocene (FcX) and 1,1'-dihaloferrocene (fcX 2 ) series (X = I, Br, Cl, F; Fc = ferrocenyl, fc = ferrocene-1,1'-diyl). Finalization of this simple, yet intriguing set of compounds has been delayed by synthetic challenges associated with the incorporation of fluorine substituents. Successful preparation of fluoroferrocene (FcF) and 1,1'-difluoroferrocene (fcF 2 ) were ultimately achieved using reactions between the appropriate lithiated ferrocene species and N-fluorobenzenesulfonimide (NFSI). The crude reaction products, in addition to those resulting from analogous preparations of chloroferrocene (FcCl) and 1,1'-dichloroferrocene (fcCl 2 ), were utilized as model systems to probe the limits of a previously reported 'oxidative purification' methodology. From this investigation and careful solution voltammetry studies, we find that the fluorinated derivatives exhibit the lowest redox potentials of each of the FcX and fcX 2 series. This counter-intuitive result is discussed with reference to the spectroscopic, structural and first principles calculations of these and related materials.

Improved Syntheses of 1,2‐Disubstituted Ferrocenes

“…Interestingly, the HOMO stabilization caused by 1,2‐difluoro substitution (0.33 eV) is smaller than that caused by 1,1′‐difluoro substitution (0.46 eV). The stabilizations are more similar for the respective chloro (0.50 vs. 0.56 eV), bromo (0.53 vs. 0.60 eV), and iodo substitutions (0.48 vs. 0.57 eV). As observed for 1,1′‐dihaloferrocenes there is no obvious correlation between the electronegativities of the substituents of 1,2‐dihaloferrocenes and the respective HOMO energies.…”

“…The only known 1,2‐heterodisubstituted fluoroferrocenes are 1‐[(dimethylamino)methyl]‐2‐fluoroferrocene, the synthesis of which caused a violent explosion, and 1‐fluoro‐2‐(2‐pyridyl)ferrocene . 1,2‐Heterodihaloferrocenes with one fluorine substituent have not been reported so far.…”

“…Although the calculated HOMO/LUMO gaps significantly deviate from those experimentally observed for ferrocene, they may serve for comparison within this group of compounds. While the electronic structures of 1,1′‐dihaloferrocenes have been investigated experimentally as well as more recently theoretically, this is not the case for 1,2‐dihaloferrocenes. There may be differences, because 1,1′‐dihaloferrocenes are conformationally flexible with respect to the distance between the halogen atoms.…”

Functionalization of 3‐Iodo‐N,N‐Diisopropylferrocene‐Carboxamide, a Pivotal Substrate to Open the Chemical Space to 1,3‐Disubstituted Ferrocenes

“…By 13 C NMR spectroscopy, the electronwithdrawing nature of the aldehyde, 2-pyridinyl and sulfone substituents was confirmed (effect more pronounced on C β than on C α relative to the substituents). As observed for fluoroferrocene, [39] C α (C2 and C4) and C β (C1 and C5) became more shielded while C ipso (C3) was strongly deshielded. The larger differences between the shifts of C2 and C4 were noticed for the iodo and the two sulfur-containing derivatives although no clear explanation can be currently advanced.…”

“…The aqueous layer was extracted with EtOAc (10 mL) and the combined organic layers were dried over MgSO 4 , filtrated over cotton wool and concentrated under vacuum using a rotary evaporator to give the crude product. This was purified by column chromatography over SiO 2 , using PET/EtOAc (90:10) to give the title product as an orange oil (643 mg, 90% (39). A solution of phenylmagnesium bromide in THF (0.30 M, 4.20 mL, 1.30 mmol, 0.95 equiv) was added dropwise to a solution of compound 35 (453 mg, 1.30 mmol, 1.00 equiv) in THF (13.0 mL) at À 78°C.…”