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

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Abstract
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%).

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