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The synthesis of a series of 2,5-diferrocenyl-1,3,4-thiadiazoles using Negishi and Sonogashira C,C cross-coupling reactions is reported. Electrochemical and spectroelectrochemical studies have been used to investigate the redox properties of these unusual ferrocene derivatives. [Display omitted]
► Synthesis and characterization of ferrocenyl thiadiazoles. ► Single X-ray structure analysis of one sample. ► Electrontransfer studies including CV, LSV, SWV, UV–Vis/NIR spectroscopy. ► Electronic communication between the ferrocenyl moieties via the heterocyclic core.
2,5-Diferrocenyl-1,3,4-thiadiazole, 2,5-Fc2-cC2N2S, (3) has been synthesized by a two-fold Negishi ferrocenylation of dibromothiadiazole (1) with FcZnCl (2) (Fc=Fe(η5-C5H4)(η5-C5H5)) in presence of [Pd(Ph3P)4] as catalyst. Additional spacer units between the ferrocenyls and the cC2N2S core could be introduced by using the Sonogashira C,C cross-coupling protocol. Reaction of 2,5-Br2-cC2N2S (1) or 2,5-(C6H4-4′-I)2-cC2N2S (6) with FcCCH (4) using [PdCl2(Ph3P)2] and [CuI] as catalyst produced the appropriate organometallics 2,5-(FcCC)2-cC2N2S (5) or 2,5-(C6H4-4′-CCFc)2-cC2N2S (7). The electronic and structural properties of 3, 5, and 7 were investigated with UV–Vis spectroscopy and single crystal X-ray diffraction (3). Complex 3 adopts a solid state structure with none of the ferrocenyl substituents being coplanar with the thiadiazole ring. Cyclic, square wave, linear sweep voltammetry and in-situ NIR spectro-electrochemistry highlight the electrochemical properties of 3. In dichloromethane (0.1molL−1 [N(nBu)4][B(C6F5)4]), compound 3 displays two well resolved electrochemical reversible one-electron events with formal reduction potentials of 0.192 and 0.338V versus FcH/FcH+. In contrast, in presence of [N(nBu)4][PF6], the thiadiazoles 3 (E0=0.22V), 5 (E0=0.18V) and 7 (E0=0.09V) show simultaneously oxidation of the two ferrocenyl termini versus FcH/FcH+. Spectro-electrochemical studies, performed in a dichloromethane solution of 0.2molL−1 [N(nBu)4][B(C6F5)4], also show that 3 can successively be oxidized via 3+ to 32+. A weak IVCT absorption (ε ca. 300Lmol−1cm−1) at 1560nm was found and is consistent with appreciable interactions between neutral ferrocenyl and positively charged ferrocenium mixed valent intermediates. Mixed-valent compound 3+ corresponds to a class II molecule according to Robin and Day.