Synthesis and Characterization of Cyclopentadienyl and Pentamethylcyclopentadienyl Ruthenium Complexes of Oligothiophenes

We report the synthesis and characterization of 14 complexes of the general form [CpRu(oligothiophene)]PF₆ and [Cp*Ru(oligothiophene)]PF6 (Cp = η⁵-cyclopentadienyl; Cp* = η⁵-pentamethylcyclopentadienyl). The complexes were synthesized from [CpRu(CH₃CN)₃]PF₆ or [Cp*Ru(CH₃CN)₃]PF₆ and the appropriate oligothiophene via procedures previously reported for other [Cp/Cp*Ru(η⁶-arene)]PF₆ sandwich complexes. For the oligothiophenes 2,2′-bithiophene (Bth), 2,2′:5′,2″-terthiophene (Tth), 2,2′:5′,2″:5″,2‴-quaterthiophene (Qth), and 5,5″-dimethyl-2,2′:5′,2″-terthiophene (Me₂Tth), the Ru is bound η⁵ to the outermost thiophene ring while the complexes of the oligothiophene 5,5″-diphenyl-2,2′:5′,2″-terthiophene (Ph₂Tth) have Ru bound η⁶ to a pendant phenyl group. The complexes with ruthenium bound to an arene are very stable with respect to decomplexation; complexes with ruthenium bound to a thiophene ring are stable in the solid state and in dichloromethane solutions; in acetone, rearrangement reactions occur to produce equilibrium mixtures of free oligothiophene and mono- and diruthenated species. Detailed analysis of ¹H and ¹³C NMR spectra, ¹H-¹H decoupling experiments, heteronuclear multiple-bond correlation (HMBC), and heteronuclear multiple-quantum coherence (HMQC) experiments enabled the assignment of all ¹H and ¹³C resonances. These assignments show that binding ruthenium to a thiophene ring of an oligothiophene shifts the bound ring resonances upfield and the unbound thiophene rings downfield. The coordination of ruthenium exerts a larger electronic effect in oligothiophene complexes than in polyaromatic complexes (quaterphenyl). Complexation of a thiophene ring by ruthenium will affect the electronic structure of up to five thiophene rings: The bound ring and the next two unbound thiophene rings on either side.