Symmetrical alkyl-substituted oligothiophenes as ligands: complexation of the [(η-C₅H₅)Ru]⁺ moiety by hexyl-substituted ter-, quater- and quinque-thiophenes

Abstract

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 [(η-C₅H₅)Ru(CH₃CN)₃]PF₆ and an excess of the oligothiophene in CH₂Cl₂, and characterising the product(s) by FAB mass spectrometry and ¹H and ¹³C{¹H} NMR spectroscopy. Upon complexation of a thiophene ring (η⁵), the thienyl proton resonances of that ring shift upfield, as do the ¹³C resonances. With terthiophene 7, in which the outer thiophene rings bear alkyl groups, the product is [(η-C₅H₅)Ru(η-7)]PF₆13, 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 [(η-C₅H₅)Ru(η-6)]PF₆12 is obtained. Similarly, with the symmetrical quinquethiophene 9, two regioisomers of the complex [(η-C₅H₅)Ru(η-9)]PF₆14 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 [(η-C₅H₅)Ru(η-11)]PF₆15 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(II) 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.

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