(μ-Di-tert-butylsilanediolato)bis[bis(η5-cyclopentadienyl)methylzirconium]

The reaction of t-Bu2Si(OH)2 with two equivalents of Cp2Zr(CH3)2 produces the t-Bu2SiO2-siloxide bridged dimer where one methyl group is retained per zirconium atom. The retention of one methyl group per metal center affords a site for further reactivity.


Chemical context
Zirconocene siloxides have been investigated for their ability to bond reactive metal centers to solid glass supports (Samuel et al., 1994) and as potential precursors to novel inorganic polymers by cyclic siloxane ring-opening polymerization (Thieme et al., 2002). In both of these examples, two diorganosilicon dioxide (-R 2 SiO 2 2À ) ligands span two zirconocene units in a cyclic dimer. In contrast, the structure of the title compound 1, shows only one bridging di-tert-butylsilicon dioxide ligand and each zirconocene unit retains one reactive methyl group. The same product is obtained regardless of whether one or two equivalents of Cp 2 Zr(CH 3 ) 2 are used per equivalent of silanediol at room temperature. At higher temperatures, the NMR of the reaction mixture becomes more complicated but we were unable to cleanly obtain the cyclic equivalent of the compounds mentioned above, [Cp 2 Zr] 2 [-t-Bu 2 SiO 2 ] 2 . This compound could potentially serve as an olefin polymerization pre-catalyst by methyl abstraction with [Ph 3 C] + [B(C 6 F 5 ) 4 ] À or similar activators (see for e.g., Babushkin et al., 2014). Initial attempts to thermally eliminate methane and form a bridging methylene complex, [Cp 2 Zr] 2 [t-Bu 2 SiO 2 ][-CH 2 ], led to decomposition.

Figure 1
The molecular structure of 1 with displacement ellipsoids drawn at the 50% probability level; hydrogen atoms omitted for clarity.

Synthesis and crystallization
General. All solvents were purchased from Sigma-Aldrich Chemicals and dried by distillation from sodium under nitrogen. Cp 2 Zr(CH 3 ) 2 was purchased from Sigma-Aldrich Chemicals and used as received. Di-t-butylsilanediol was prepared by the oxidation of t-Bu 2 Si(H)Cl (Sigma-Aldrich) with aqueous KMnO 4 following the procedure of Lickiss & Lucas (1996). NMR spectra were recorded on a Bruker AVIII 300 MHz Spectrometer in sealable Teflon-valved tube and were referenced to residual solvent resonances. Elemental analyses were performed by Canadian Microanalytical Ltd.
Synthesis. The title compound was prepared (Fig. 3) by adding a toluene solution (5 ml) of di-t-butylsilanediol (0.080 g, 0.45 mmol) to a stirred solution of dimethylzirconocene, Cp 2 Zr(CH 3 ) 2 (0.228 g, 0.907 mmol), in toluene (5 ml) in a 50 ml Erlenmyer flask in an inert atmosphere glovebox. After stirring overnight, the solution was concentrated under vacuum, layered with hexane and stored in a 243 K freezer. Large, colourless crystals of 1 deposited within a few days. Yield: 0.196 g (67% Reaction scheme.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Both Cp rings were found to be disordered and modelled over two sets of sites with 50% occupancy with restraints (SIMU cards). H atoms were positioned geometrically and refined as riding, with C-H = 0.95-0.98 Å and U iso (H = 1.2U eq (C) or 1.5U eq (C-methyl). program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Special details
Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 5 sets of ω scans each set at different φ and/or 2θ angles and each scan (5 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 5.035 cm. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Each Cp was disordered and modelled over two positions with 50% occupancy with restraints (SIMU).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (