The Redox Behavior of Ferrocene Derivatives .6. Benzylferrocenes - the Crystal-structure of Decabenzylferrocenium Tetrafluoroborate

The electrochemical behaviour of the benzyl-substituted ferrocenes [(eta5-C5Bz5)2Fe] and [(eta5-C5Bz5)Fe(eta5-C5H5)] in non-aqueous solutions has been examined. As expected, they undergo reversible one-electron removal more easily than ferrocene itself, but with significantly more difficulty than [(eta5-C5R5)2Fe] (R = Me or Et). In dichloromethane solution, the formal electrode potentials (vs. SCE) are as follows: E-degrees'([(C5Bz5)2Fe]+/0) = +0.38 V, E-degrees'([(C5Me5)2Fe]+/0) = -0.10 V, E-degrees'([(C5Et5)2Fe]+/0) = -0.06 V, E-degrees'([(C5H5)2Fe]+/0) = +0.45 V. The electrochemical properties suggest that the [(C5Bz5)2Fe]+/0 redox change should be accompanied by minor, but detectable geometrical strains. In order to define more quantitatively the geometrical reorganization accompanying such electron removal processes, the X-ray structure of [(C5Bz5)2Fe][BF4] has been determined. Comparison with the previously determined structure of neutral [(C5Bz5)2Fe] shows an elongation of about 0.05 angstrom of the Fe-C(cyclopentadienyl) distances, which is a common feature of all ferrocene/ferrocenium couples, but this is also accompanied by reorganization of the peripheral benzyl substituents. In the neutral precursor, the methylene fragments are tilted away from the plane of the cyclopentadienyl rings towards the iron atom, whereas in the monocation they become coplanar.