Synthesis of a Series of Diiron Complexes Based on a Tetraethynylethene Skeleton and Related C6-Enediyne Spacers, (dppe)Cp*Fe−C⋮CC(R)C(R)C⋮C−FeCp*(dppe): Tunable Molecular Wires

Abstract

A series of 3,4-disubstituted (hex-3-ene-1,5-diyne-1,6-diyl)diron complexes with FeCp*(dppe) (Fe) end caps, Fe−C⋮CC(R)C(R)C⋮C−Fe (R = H, C⋮C−SiMe3, C⋮C−H, C6H5, p-C6H4CF3), and the related (octa-3,5-diene-1,7-diyne-1,8-diyl)diron complex, Fe−C⋮C−C(H)C(H)C(H)C(H)C⋮C−Fe, has been prepared, and their performance as molecular wires has been evaluated. The enyne complexes have been synthesized via vinylidene intermediates, [FeCC(H)C(R)C(R)C(H)CFe]2+, derived from the corresponding terminal alkynes or the Me3Si-protected precursors (XC⋮CCC(R)C(R)C⋮C−X; X = H, SiMe3), and the products have been characterized spectroscopically and crystallographically. The performance of the obtained dinuclear enyne complexes as molecular wires has been evaluated on the basis of the comproportionation constants (KC) obtained by electrochemical measurements and the Vab values obtained from the spectral parameters of the intervalence charge transfer bands of the isolated monocationic radical species appearing in the near-IR region. As a result, the C6-enediyne complexes turn out to be excellent molecular wires, with KC values larger than 108 as well as Vab values larger than 0.35, belonging to class III compounds according to the Robin and Day classification and being comparable to the related polyynediyl complexes as well. It is notable that, in the enyne system, the performance can be readily tuned by introduction of appropriate substituents onto the olefinic part. Thus, diiron complexes containing an enyne spacer can be regarded as tunable molecular wires.