THE REDOX BEHAVIOR OF FERROCENE DERIVATIVES .6. BENZYLFERROCENES - THE CRYSTAL-STRUCTURE OF DECABENZYLFERROCENIUM TETRAFLUOROBORATE

The electrochemical behaviour of the benzyl-substituted ferrocenes [(e ta5-C5Bz5)2Fe] and [(eta5-C5Bz5)Fe(eta5-C5H5)] in non-aqueous solution s has been examined. As expected, they undergo reversible one-electron removal more easily than ferrocene itself, but with significantly mor e 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)2 Fe]+/0 redox change should be accompanied by minor, but detectable geo metrical strains. In order to define more quantitatively the geometric al reorganization accompanying such electron removal processes, the X- ray structure of [(C5Bz5)2Fe][BF4] has been determined. Comparison wit h the previously determined structure of neutral [(C5Bz5)2Fe] shows an elongation of about 0.05 angstrom of the Fe-C(cyclopentadienyl) dista nces, which is a common feature of all ferrocene/ferrocenium couples, but this is also accompanied by reorganization of the peripheral benzy l substituents. In the neutral precursor, the methylene fragments are tilted away from the plane of the cyclopentadienyl rings towards the i ron atom, whereas in the monocation they become coplanar.