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

New bis(alkynylferrocene) complexes with fluorenyl-based spacers [(η5-C5H5)Fe(η5-C5H4)CCRCC(η5-C5H4)Fe(η5-C5H5)] (R=fluoren-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 coupling reactions of ethynylferrocene with half an equivalent of the corresponding dibromofluorene derivatives BrRBr. All the compounds have been characterized by FTIR, 1H-NMR and UV–vis spectroscopies and fast atom bombardment mass spectrometry (FABMS). The molecular structures of 1 and 2 have been elucidated by single-crystal X-ray analyses. The redox chemistry of these fluorene-linked biferrocenes has been investigated by cyclic voltammetry and the interaction between the ferrocene units is rather weak in each case. The half-wave potential of the terminal ferrocenyl moieties increases progressively 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-accepting fluorenone through the ethynyl bridge in 1.