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

Abstract

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 2,5-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 2,5-diferrocenyl-3,4-bis(N,N-dimethylamino)-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 (KC) in the order 1 < 2 ≈ 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 π 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 ≈ 3 < 4.