alpha,omega-Diferrocenyl Cumulene Molecular Wires.Synthesis,Spectroscopy,Structure,and Electrochemistry

摘要

Cumulene sp-carbon molecular wires C_2 [Fc(Ph)C=C(Ph)Fc] up to C_7 [Fc(Ph)C=C=C= C=C=C=C(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 C_1 synthon ferrocenyl(phenyl)ketone as starting material with various acetylenic/propargylic/ homopropargylic C_2-C_5 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 C_2,C_4,and C_6 are conjugated "molecular wires" effecting electronic communication between the terminal ferrocenyl substituents,whereas odd cumulenes C_3,C_5,and C_7 are nonconjugated and electronically decoupled due to their orthogonal terminal pi-systems.Electrochemically,separate redox waves can be detected up to a C_6 cumulene spacer,but the electronic communication between the endcapping redox-active ferrocenyl substituents decreases with increasing cumulene length.