alpha,omega-diferrocenyl cumulene molecular wires. Synthesis, spectroscopy, structure, and electrochemistry

Cumulene sp-carbon molecular wires C2 [Fc(Ph)CC(Ph)Fc] up to C7 [Fc(Ph)CCCCCCC(Ph)Fc] endcapped by two electroactive ferrocenyl groups are presented in this report. Synthetically, ferrocenyl cumulenes can be built-up by a modular strategy using C1 synthon ferrocenyl(phenyl)ketone as starting material with various acetylenic/propargylic/homopropargylic C2−C5 reagents, taking into account and exploiting the efficient stabilization of an electron-deficient carbenium center by an adjacent ferrocenyl moiety. With increasing cumulene chain length the reactivity of cumulenes increases considerably, indicating steric protection as the main requirement for bulk stability. Even cumulenes C2, C4, and C6 are conjugated “molecular wires” effecting electronic communication between the terminal ferrocenyl substituents, whereas odd cumulenes C3, C5, and C7 are nonconjugated and electronically decoupled due to their orthogonal terminal π-systems. Electrochemically, separate redox waves can be detected up to a C6 cumulene spacer, but the electronic communication between the endcapping redox-active ferrocenyl substituents decreases with increasing cumulene length.