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

A series of 3,4-di-<i>N</i>-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 <b>1</b>–<b>4</b> were implemented on the basis of 2,5-diferrocenyl-3,4-dinitrothiophene (<b>1</b>), 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(<i>N</i>,<i>N</i>-dimethylamino)-thiophene (<b>2</b>) or 5,7-diferrocenylthieno­[3,4-<i>b</i>]­pyrazine (<b>3</b>). Finally, 5,7-diferrocenyl-1,4-dimethyl-1,2,3,4-tetrahydrothieno­[3,4-<i>b</i>]­pyrazine (<b>4</b>) could be synthesized through conversion of <b>3</b> in a two-step reduction/methylation reaction. The spectroscopic characterization of <b>1</b>–<b>4</b> was complemented by the investigation of their structural properties in the solid state. The latter disclose no significant influence of the electronically different <i>N</i> substituents on the thiophene bond distances and angles in the solid state. UV–vis solvatochromic studies on thiophenes <b>1</b> and <b>3</b> in 40 different solvents offer a moderate positive solvatochromic shift of the MLCT absorption of <b>1</b>; however, for pyrazine <b>3</b> a slight negative solvatochromism of the appropriate absorption band was observed. The electrochemical studies on thiophenes <b>1</b>–<b>4</b> reveal an increasing thermodynamic stability of the mixed-valent species (<i>K</i>_C) in the order <b>1</b> < <b>2</b> ≈ <b>3</b> < <b>4</b>. 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 <b>1</b> < <b>2</b> ≈ <b>3</b> < <b>4