Symmetrical alkyl-substituted oligothiophenes as ligands: complexation of the [(η-C5H5)Ru]+ moiety by hexyl-substituted ter-, quater- and quinque-thiophenes
作者:Christopher L. Jones、Simon J. Higgins
DOI:10.1039/a809623f
日期:——
The soluble, symmetrical oligothiophenes 3,3 "-dihexyl-2,2':5',2 "-terthiophene 7 and 4',3'''-dihexyl-2,2':5',2 ":5",2"':5''', 2""-quinquethiophene 9 and the unsymmetrical oligothiophenes 3'-hexyl-2,2':5',2 "-terthiophene 6 and 3,3 "-dihexyl-2,2':5',2 ":5",2'''-quaterthiophene 11 have been prepared by Kumada-type coupling reactions, using 2-bromothiophene, 2,5-dibromothiophene, 2-bromo-3-hexylthiophene and 2,5-dibromo-3-hexylthiophene as building blocks. Their behaviour as ligands towards the cyclopentadienylruthenium (II) moiety has been examined, by carrying out ligand exchange reactions between [(eta-C5H5)Ru(CH3CN)(3)]PF6 and an excess of the oligothiophene in CH2Cl2, and characterising the product(s) by FAB mass spectrometry and H-1 and C-13H-1} NMR spectroscopy. Upon complexation of a thiophene ring (eta(5)), the thienyl proton resonances of that ring shift upfield, as do the C-13 resonances. With terthiophene 7, in which the outer thiophene rings bear alkyl groups, the product is [(eta-C5H5)Ru(eta-7)]PF6 13, a single regioisomer in which exclusively an outer thiophene ring is complexed by the cyclopentadienylruthenium (II) moiety. With terthiophene 6, however, in which the inner thiophene ring bears an alkyl substituent, a mixture of all three possible regioisomers of [(eta-C5H5)Ru(eta-6)]PF6 12 is obtained. Similarly, with the symmetrical quinquethiophene 9, two regioisomers of the complex [(eta-C5H5)Ru(eta-9)]PF6 14 are obtained, one in which an outer thiophene ring is complexed, and one in which an inner alkylthiophene ring is complexed. With the unsymmetrical quaterthiophene 11, which includes all four types of thiophene ring (inner and outer alkylthiophene, inner and outer unsubstituted thiophene), 93% of the product [(eta-C5H5)Ru(eta-11)]PF6 15 is the regioisomer in which the outer alkylthiophene ring is complexed. These results show that there are two factors controlling the site of complexation, a tendency for the outermost ring to complex and a tendency for an alkylthiophene ring to complex. Where these conflict, a mixture of regioisomers is obtained. The electrochemistry of both the free oligothiophenes and of their Ru(ll) complexes has been compared. On complexation, there is a substantial positive shift in the oxidation potential of the oligothiophene, and a new, irreversible reduction is observed at ca.-1 V. Although polythiophene films could be deposited by electrooxidation of the free oligothiophenes in acetonitrile, electroactive polymer films could not be deposited by electrooxidation of the complexes in dichloromethane.