Synthesis and Electrochemical Investigations of [Ru(η⁵‐Ferrocenyl‐Thiophene)(η⁵‐C₅R₅)]⁺ Sandwich Compounds

Abstract

The synthesis of [Ru(η5‐2‐Fc‐3‐R3‐4‐R2‐5‐R1‐cC4S)(η5‐C5R5)]+[PF6]– (R = H, Me; R1–R3 = H; R1 = Me, R2 = R3 = H; R1–R3 = Me; R1 = Fc, R2 = R3 = OCH2CH2O; Fc = Fe(η5‐C5H4)(η5‐C5H5)) is reported with the aim of a better understanding of the influence of π‐thiophene‐bonded transition metal units on the thermodynamic stabilities and charge transfer interactions of the resulting compounds and their oxidized intermediates. Within the electrochemical studies, reversible one‐electron Fe‐based redox events were found for the mono‐Fc Ru species, whereby the Fc/Fc+ redox potential increases as the degree of methylation of the thiophene and the Ru‐cyclopentadienyl moiety decreases. For [Ru(η5‐2,5‐Fc2‐cC4H2S)(η5‐C5R5)]+[PF6]– a hindered 2nd Fc‐based electrode reaction was observed under “weak‐coordinating” conditions. Two separated Fc/Fc+ redox events were detected by the use of stronger solvating solvents or (smaller) anions. With regard to the non complexed 2,5‐Fc2 thiophenes, the thermodynamic stabilities of the appropriate mixed‐valent species decrease, when the thiophene is η5‐coordinated to [Ru(η5‐C5R5)]+. Contrary to the redox potential, the substitution of the [Ru(η5‐C5R5)]+ moiety does not have a major impact on the thermodynamic stabilities of the mixed‐valent species. The related decrease of electronic coupling between the Fc unit(s) via the thiophene in the corresponding oxidized states, could be verified during UV/Vis/NIR spectroelectrochemical studies.