Carbon-rich organometallics: synthesis and characterization of new ferrocenyl end-capped bis(butadiynyl) fluorene derivatives

A new series of ferrocenyl end-capped bis(butadiynyl) fluorene complexes [(η⁵-C₅H₅)Fe(η⁵-C₅H₄)CCCCRCCCC(η⁵-C₅H₄)Fe(η⁵-C₅H₅)] (R = fluoren-9-one-2,7-diyl, 1; 9,9-dihexylfluorene-2,7-diyl, 2; 9-ferrocenylmethylenefluorene-2,7-diyl, 3; 9-ferrocenylphenylmethylenefluorene-2,7-diyl, 4) have been synthesized in moderate yields by the oxidative coupling reactions of ethynylferrocene with half equivalents of the appropriate diethynylfluorene derivatives. All the new complexes have been characterized by FTIR, NMR and UV-vis spectroscopies and fast atom bombardment mass spectrometry. The molecular structures of selected molecules have been determined by X-ray crystallographic techniques. The electronic absorption and redox properties of these carbon-rich molecules were investigated and the data were compared with those for the corresponding 2,7-bis(ferrocenylethynyl)fluorene counterparts. Cyclic voltammetry indicates that the half-wave potential of the terminal ferrocenyl moieties becomes more anodic when the number of ethynyl units increases and when the 9-substituent of the central fluorene ring changes from an electron-donating group to an electron-deficient group.