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^§

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, [FeCC(H)C(R)C(R)C(H)CFe]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.本研究合成了一系列以FeCp*(dppe)（记为Fe）为端帽的3,4-二取代（己-3-烯-1,5-二炔-1,6-二基）二铁配合物，其结构通式为Fe−C⋮CC(R)C(R)C⋮C−Fe（R = H、C⋮C−SiMe3、C⋮C−H、C6H5、p-C6H4CF3），以及相关的（辛-3,5-二烯-1,7-二炔-1,8-二基）二铁配合物Fe−C⋮C−C(H)C(H)C(H)C(H)C⋮C−Fe，并评估了其作为分子导线的性能。该类烯炔配合物经由亚乙烯基中间体[FeCC(H)C(R)C(R)C(H)CFe]2+合成，该中间体衍生自相应的末端炔烃或三甲基硅基（Me3Si）保护的前驱体XC⋮CCC(R)C(R)C⋮C−X（X = H、SiMe3），并通过光谱学与晶体学手段对产物完成了表征。本研究基于电化学测试得到的归中常数（K_C），以及从近红外（near-IR）区域观测到的分离得到的单阳离子自由基物种的价间电荷转移带光谱参数计算得到的Vab值，评估了所得双核烯炔配合物作为分子导线的性能。结果表明，C6-烯二炔配合物属于罗宾-戴（Robin & Day）分类体系中的III类化合物，其K_C值大于10^8，Vab值大于0.35，为性能优异的分子导线，且其性能与相关聚炔二基配合物相当。值得注意的是，在烯炔体系中，通过在烯烃骨架上引入合适的取代基即可便捷调控其性能。因此，含有烯炔间隔基的二铁配合物可被视为可调控型分子导线。