Syntheses, Characterization, and Structural Studies of Half-Open Zirconocenes

The reaction of Zr(C5H5)Cl2Br with 3 equiv of K(2,4-C7H11) and 1 equiv of dmpe was found to lead to the 18-electron half-open zirconocene Zr(C5H5)(2,4-C7H11)(dmpe) (C7H11 = dimethylpentadienyl; dmpe = Me2PC2H4PMe2), which has been fully characterized. A structural study has revealed that the Zr−C bond distances for the open dienyl ligand are significantly shorter than those for the C5H5 ligand. Alternatively, through a similar reaction involving the use of only 2 equiv of a more selective (pentadienyl)magnesium reagent, the two dienyl anions serve as one-electron reducing agents, and the Zr(II) complex Zr(C5H5)(Br)(dmpe)2 is obtained, which has also been fully characterized. In the absence of dmpe, the reaction between Zr(C5H5)BrCl2 and 3 equiv of the magnesium reagent leads to an unusual Zr(C5H5)(C14H21) complex. The C14H21 ligand was formed from the coupling of two 2,4-C7H11 ligands, followed by the loss of one hydrogen atom, presumably abstracted by the third 2,4-C7H11 ligand. The C14H21 ligand bonds to the zirconium center through both η5-dienyl and η4-“diene” coordination, leading to an 18-electron complex. The structural parameters indicate that the diene coordination can be more appropriately described as enediyl coordination, leading to a formal Zr(IV) instead of Zr(II) complex. The presence of a high formal metal oxidation state would seem to explain the fact that in this complex, as opposed to Zr(C5H5)(2,4-C7H11)(dmpe) and many other related complexes, the M−C bond distances for the C5H5 ligand are much shorter than those for the open dienyl ligand.