Ferrocenes Bridged by Ethylenediamino Thiophene: Varying Charge Transfer Properties in a Series of 3,4-Di-N-substituted 2,5-Diferrocenyl Thiophenes

J. Matthaeus Speck, Marcus Korb, Alexander Schade, Stefan Spange, Heinrich Lang

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A series of 3,4-di-N-substituted 2,5-diferrocenyl thiophenes is reported with the aim to explore the electronic interaction between the organometallic termini over their bridging unit. The syntheses of thiophenes 1-4 were implemented on the basis of 2,S-diferrocenyl-3,4-dinitrothiophene (1), which is accessible within a Pd-promoted Suzuki -Miyaura C,C cross-coupling reaction. Reduction of the nitro functionalities and a subsequent methylation or condensation reaction gives 2,S-diferrocenyl-3,4-bis(N,N-dimethylninino)-thiophene (2) or 5,7-diferrocenylthieno[3,4-b]pyrazine (3). Finally, 5,7-diferrocenyl-1,4-dimethyl-1,2,3,4-tetrahydrothieno-[3,4-b]pyrazine (4) could be synthesized through conversion of 3 in a two-step reduction/methylation reaction. The spectroscopic characterization of 1-4 was complemented by the investigation of their structural properties in the solid state. The latter disclose no significant influence of the electronically different N substituents on the thiophene bond distances and angles in the solid state. UV-vis solvatochromic studies on thiophenes 1 and 3 in 40 different solvents offer a moderate positive solvatochromic shift of the MLCT absorption of 1; however, for pyrazine 3 a slight negative solvatochromism of the appropriate absorption band was observed. The electrochemical Studies on thiophenes 1-4 reveal an increasing thermodynamic stability of the mixed-valent species (K-C) in the order 1 <2 approximate to 3 <4. In situ UV-vis-NIR spectroelectrochemical examinations verify these observations. Thus, the NIR absorptions of the corresponding mixed-valent species become more intense and less solvatochromic as the electron-donating effect and the extent of the electronic pi system of the bridging unit increase. Hence, the electronic coupling between the iron-based redox centers over the thiophene-based core increases in the order 1 <2 approximate to 3 <4.

Original languageEnglish
Pages (from-to)3788-3798
Number of pages11
Issue number15
Publication statusPublished - 10 Aug 2015
Externally publishedYes

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