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

Abstract

While, in general, decamethylzincocene, Zn(C₅Me₅)₂, and other zincocenes, Zn(C₅Me₄R)₂ (R =\nH, Bu<i>^t</i>, 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\nunexpectedly the dizincocene Zn₂(η⁵-C₅Me₅)₂ (<b>1</b>) in low yields, most likely as a result of the coupling of two\n(η⁵-C₅Me₅)Zn^• radicals. An improved, large scale (ca<i>.</i> 2 g) synthesis of <b>1</b> has been achieved by reduction\nof equimolar mixtures of Zn(C₅Me₅)₂ and ZnCl₂ with KH in tetrahydrofuran. The analogous reduction of\nZn(C₅Me₄R)₂ (R = H, SiMe₃, Bu<i>^t</i>) yields only decomposition products, but the isotopically labeled\ndimetallocene ⁶⁸Zn₂(η⁵-C₅Me₅)₂ and the related compound Zn₂(η⁵-C₅Me₄Et)₂ (<b>2</b>) have been obtained by\nthis procedure. Compound <b>2</b> has lower thermal stability than <b>1</b>, but it has been unequivocally characterized\nby low-temperature X-ray diffraction studies. As for <b>1</b> a combination of structural characterization techniques\nhas provided unambiguous evidence for its formulation as the Zn−Zn bonded dimer Zn₂(η⁵-C₅Me₄Et)₂,\nwith a short Zn−Zn bond of 2.295(3) Å indicative of a strong Zn−Zn bonding interaction. The electronic\nstructure and the bonding properties of <b>1</b> and those of related dizincocenes Zn₂(η⁵-Cp‘)₂ have been studied\nby DFT methods (B3LYP level), with computed bond distances and angles for dizincocene <b>1</b> very similar\nto the experimental values. The Zn−Zn bond is strong (ca<i>.</i> 62 kcal·mol^-1 for <b>1</b>) 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\n96%).