Synthesis, structures and electrochemistry of bis(alkynylferrocene) complexes with fluorene spacers

New bis(alkynylferrocene) complexes with fluorenyl-based spacers [(eta (5)- C5H5)Fe(eta (5)-C5H4)C drop CRC dropC(eta (5)-C5H4)Fe(eta (5)-C5H5)] (R=flu oren-9-one-2,7-diyl, 1; fluorene-2,7-diyl, 2; 9-ferrocenylmethylenefluorene -2,7-diyl, 3) have been prepared in excellent yields by the Sonogashira cou pling reactions of ethynylferrocene with half an equivalent of the correspo nding dibromofluorene derivatives BrRBr. All the compounds have been charac terized by FTIR, H-1-NMR and UV-vis spectroscopies and fast atom bombardmen t mass spectrometry (FABMS). The molecular structures of I and 2 have been elucidated by single-crystal X-ray analyses. The redox chemistry of these f luorene-linked biferrocenes has been investigated by cyclic voltammetry and the interaction between the ferrocene units is rather weak in each case. T he half-wave potential of the terminal ferrocenyl moieties increases progre ssively in the order 3 <2 <1 when the substituent at the 9-position of the central spacer changes from an electron-donating ferrocenyl group (as in 3) to an electron-deficient oxo group (as in 1). This is consistent with the loss of electron density from the ferrocenyl unit to the net electron-accep ting fluorenone through the ethynyl bridge in 1. (C) 2001 Elsevier Science B.V. All rights reserved.