Dibromido[bis(η5-cyclopentadienyl)dimethylsilane]zirconium(IV)

The title molecule, [ZrBr2(C12H14Si)], possesses a crystallographically imposed twofold rotational symmetry with the rotation axis passing through the Zr and Si atoms. The ZrIV centre is in a distorted tetrahedral environment defined by two Cp rings of chelating organic ligands and two Br anions. Two five-membered rings form a dihedral angle of 59.7 (2)°. Unequal Zr—C bonds [2.471 (3)–2.556 (3) Å] in the molecule indicate that the interaction of the central metal with the [(C5H4)2SiMe2]2− ligand contains noticeable η3-allyl and η2-olefin contributions.

The title molecule, [ZrBr 2 (C 12 H 14 Si)], possesses a crystallographically imposed twofold rotational symmetry with the rotation axis passing through the Zr and Si atoms. The Zr IV centre is in a distorted tetrahedral environment defined by two Cp rings of chelating organic ligands and two Br anions. Two five-membered rings form a dihedral angle of 59.7 (2) . Unequal Zr-C bonds [2.471 (3)-2.556 (3) Å ] in the molecule indicate that the interaction of the central metal with the [(C 5 H 4 ) 2 SiMe 2 ] 2À ligand contains noticeable 3 -allyl and 2olefin contributions.

Experimental
Crystal data [ZrBr 2 (C 12 (Borrelli et al., 2002). The most used strategy for the modification of catalytic properties of these metallocenes is the substitution at the cyclopentadienyl rings. Incorporation of an interannular bridge connecting both cyclopentadienyl rings in the molecule of metallocene leads to a broad class of complexes known as ansa-metallocenes.
As part of an investigation of the influence of ring substitution on the properties of cyclopentadienyl complexes, the title compound, (I), was prepared, spectroscopically characterized and its structure determined A perspective view of molecular structure of (I) is shown in Figure 1 with appropriate atom labeling scheme, selected bond distances and angles are summarized in Table 1. In the molecule of (I) the Zr atom is pseudotetrahedrally coordinated by two η 5 -bonded Cp rings and two Br atoms with geometry constrained by crystallographic twofold rotation axis, which bisects Br1-Zr-Br1a angle and passes through the metal and Si atom. The molecular parameters are comparable to those reported for [ZrCl 2 (η 5 -C 5 H 4 ) 2 SiMe 2 ] (Bajgur et al., 1985). On the inspection of parameters associated with Cp rings, the deviation from the ideal η 5 -bonding fashion could be observed. The C3-C4 bond distance of 1.400 (6) Å is shorter than the remaining C-C bonds (average of 1.42 Å). Similarly Zr1-C3 and Zr1-C4 bonds are longer [2.556 (3) Å and 2.552 (4) Å, respectively] than remaining metal-carbon bonds (average of 2.475 Å). These facts indicate the presence of η 3 -allyl and η 2 -olefin bonding fashion of Cp ring rather than η 5 -cyclopentadienyl bonding pattern.

Experimental
Compound (I) was prepared by bromination of analogous chloride derivative using boron tribromide. To the starting complex [ZrCl 2 (η 5 -C 5 H 4 ) 2 SiMe 2 ] (0.2 g; 0.57 mol) in 20 ml of dichloromethane 0.04 ml (0.42 mmol) of BBr 3 was added. The color of the reaction mixture immediatelly turned to green and it was stirred for additional 2 h at 293 K. The solvent was evaporated in vacuum, solid residue was washed with hexane (2x5 ml) and vacuum-dried. Sublimation of crude product at 10 -3 Pa and 475 K gave 0.105 g (42%) of (I). Crystals of (I) suitable for X-ray diffraction measurements were grown during slow evaporation of chloroform solution at 273 K.

Refinement
All H atoms were positioned geometrically and refined as riding on their parent C atoms, with C-H = 0.93 Å, U iso (H) = 1.2U e.g (C) and C-H = 0.96 Å, U iso (H) = 1.5U e.g (C) for cyclopentadienyl and methyl H atoms, respectively. Fig. 1