Synthesis and Reactivity of Bis(η5:η1-pentafulvene)zirconium Complexes

Abstract

Here we present the first synthetic route to bis(η5:η1-pentafulvene)zirconium complexes as well as investigations of several consecutive reactions. Sodium amalgam reduction of ZrCl4 in the presence of a bulky substituted pentafulvene in THF leads to the formation of [η5:η1-C5H4═CR2]2Zr(THF) (2a, R = p-tol; 2b, R = Ph). The molecular structure of 2a has been determined by single-crystal X-ray diffraction and exhibits an unusually large Zr–Cexo distance (2.708(2) Å) caused by the bulky substituents at the exocyclic carbon, which allows THF coordination. The reaction of 2a with H-acidic compounds such as hydrogen chloride, amines, and water were examined: addition of HCl leads to the dichloride complex [η5-C5H4-CH(p-tol)2]2ZrCl2 (3) and treatment of 2a with p-methylaniline forms the bis-amide complex [η5-C5H4-CH(p-tol)2]2Zr(NH-p-tol)2 (4), whereas addition of the sterically more demanding dicyclohexylamine furnishes the monoamide complex [η5:η1-C5H4═C(p-tol)2][η5-C5H4-CH(p-tol)2]Zr(NCy2) (5). The cyclotrimeric compound {[η5-C5H4-CH(p-tol)2]2Zr(μ-O)}3 (6) is obtained by reaction of 2a with water. Moreover, insertion reactions of CX multiple bonds (X = N, O) into the Zr–Cexo(Fv) bond of 2a result in the formation of the zirconacycles [η5:η1-C5H4-C(p-tol)2-C(C6H4Cl)═N−]2Zr (7), [η5:η1-C5H4-C(p-tol)2-MeC4H6N−]2Zr (8), and [η5:η1-C5H4-C(p-tol)2-C(Ph)2O−]2Zr (9). Finally, 2a is a valuable agent for the activation of molecular hydrogen under ambient conditions. In this regard, only one pentafulvene ligand per zirconium atom is protonated and the dinuclear zirconium hydride complex {[η5:η1-C5H4═C(p-tol)2][η5-C5H4-CH(p-tol)2]Zr(μ-H)}2 (10) is isolated. All compounds were confirmed by single-crystal X-ray diffraction and NMR spectroscopic measurements.