Zinc−Zinc Bonded Zincocene Structures. Synthesis and Characterization of Zn₂(η⁵-C₅Me₅)₂ and Zn₂(η⁵-C₅Me₄Et)₂

While, in general, decamethylzincocene, Zn(C5Me5)2, and other zincocenes, Zn(C5Me4R)2 (R = H, But, SiMe3), react with dialkyl and diaryl derivatives, ZnR‘2, to give the half-sandwich compounds (η5-C5Me4R)ZnR‘, under certain conditions the reactions of Zn(C5Me5)2 with ZnEt2 or ZnPh2 produce unexpectedly the dizincocene Zn2(η5-C5Me5)2 (1) in low yields, most likely as a result of the coupling of two (η5-C5Me5)Zn• radicals. An improved, large scale (ca. 2 g) synthesis of 1 has been achieved by reduction of equimolar mixtures of Zn(C5Me5)2 and ZnCl2 with KH in tetrahydrofuran. The analogous reduction of Zn(C5Me4R)2 (R = H, SiMe3, But) yields only decomposition products, but the isotopically labeled dimetallocene 68Zn2(η5-C5Me5)2 and the related compound Zn2(η5-C5Me4Et)2 (2) have been obtained by this procedure. Compound 2 has lower thermal stability than 1, but it has been unequivocally characterized by low-temperature X-ray diffraction studies. As for 1 a combination of structural characterization techniques has provided unambiguous evidence for its formulation as the Zn−Zn bonded dimer Zn2(η5-C5Me4Et)2, with a short Zn−Zn bond of 2.295(3) Å indicative of a strong Zn−Zn bonding interaction. The electronic structure and the bonding properties of 1 and those of related dizincocenes Zn2(η5-Cp‘)2 have been studied by DFT methods (B3LYP level), with computed bond distances and angles for dizincocene 1 very similar to the experimental values. The Zn−Zn bond is strong (ca. 62 kcal·mol-1 for 1) and resides in the HOMO-4, that has a contribution of Zn orbitals close to 60%, consisting mostly of the Zn 4s orbitals (more than 96%).总体而言，十甲基二茂锌（decamethylzincocene, Zn(C5Me5)2）与其他二茂锌类衍生物（zincocenes, Zn(C5Me4R)2，其中R=H、叔丁基But、三甲基硅基SiMe3）均可与二烷基、二芳基锌衍生物ZnR‘2发生反应，生成半夹心化合物(η⁵-C5Me4R)ZnR‘。但在特定条件下，Zn(C5Me5)2与二乙基锌ZnEt2或二苯基锌ZnPh2的反应却会意外得到低产率的二锌茂Zn2(η⁵-C5Me5)2（化合物1），其生成极有可能源于两个(η⁵-C5Me5)Zn•自由基的偶联反应。研究人员已开发出化合物1的改进型大规模合成方法：在四氢呋喃（tetrahydrofuran, THF）中，以氢化钾（KH）还原等摩尔比的Zn(C5Me5)2与氯化锌ZnCl2的混合物，产物量可达约2 g。而对Zn(C5Me4R)2（R=H、三甲基硅基SiMe3、叔丁基But）的同类还原反应仅能得到分解产物，但通过该方法可制备得到同位素标记的二金属茂⁶⁸Zn2(η⁵-C5Me5)2，以及相关化合物Zn2(η⁵-C5Me4Et)2（化合物2）。化合物2的热稳定性低于化合物1，且已通过低温X射线衍射（low-temperature X-ray diffraction）研究得到明确表征。针对化合物1，多种结构表征技术联用已确凿证实其为Zn-Zn键合二聚体Zn2(η⁵-C5Me4Et)2，其中2.295(3) Å的短Zn-Zn键表明存在强Zn-Zn成键相互作用。研究人员采用密度泛函理论（DFT, Density Functional Theory）B3LYP泛函级别（B3LYP level）方法，对化合物1及相关二锌茂Zn2(η⁵-Cp‘)2的电子结构与成键性质开展了理论研究，计算得到的二锌茂1的键长与键角均与实验值高度吻合。该Zn-Zn键键能较强（化合物1的键能约为62 kcal·mol⁻¹），且分布于第4个最高占据分子轨道（HOMO-4, Highest Occupied Molecular Orbital）中；该轨道的锌轨道贡献占比接近60%，主要由锌4s轨道构成（占比超96%）。