Crystal structures of two ansa-titanocene trifluoromethanesulfonate complexes bearing the Me2Si(C5Me4)2 ligand

The crystal structures of two ansa-titanocene trifluoromethanesulfonate complexes bearing the Me2Si(C5Me4)2 ligand are reported. Both complexes display a bent metallocene unit, the metal centre is coordinated in a distorted tetrahedral geometry.


Chemical context
Titanocene trifluoromethanesulfonate complexes have been investigated by our group as model complexes for overall water splitting (Kessler et al., 2011;Hollmann et al., 2013;Godemann et al., 2015). We have found that the nature of the cyclopentadienyl ligands strongly influences the outcome of the reaction. In case of the unbridged Ti III complex Cp* 2 Ti(OTf) (A, Cp* = 5 -pentamethylcyclopentadienyl), reaction with water gave dihydrogen and the Ti IV complex Cp* 2 Ti(OH)(OTf), which could not be reconverted photochemically into the Ti III starting material (Kessler et al., 2011;Hollmann et al., 2013). In contrast, reaction of the silanediylbridged complex Me 4 Si 2 (C 5 Me 4 ) 2 Ti(OTf) (B) with water was found to yield the Ti III compound [Me 4 Si 2 (C 5 Me 4 )Ti(H 2 O) 2 ]-(OTf), which could be oxidized with TEMPO to give a Ti IV species [Me 4 Si 2 (C 5 Me 4 ) 2 Ti(H 2 O)(OH)](OTf). Photolysis of the latter results in a photoreduction and elimination of the OH ligand to give a Ti III trifluoromethanesulfonate complex. Several cycles of this synthetic model scheme for water splitting can be passed (Godemann et al., 2015).
Variation of the ansa-cyclopentadienyl ligand, i.e. shortening of the bridging unit, should have an influence on the ISSN 2056-9890 reactivity of the corresponding trifluoromethanesulfonate complexes. We therefore aimed at the synthesis of bridged trifluoromethanesulfonate complexes bearing the metallocene [Me 2 Si(C 5 Me 4 )Ti]. (2), respectively. Both metal complexes exhibit distorted tetrahedral coordination geometries and show the typical bent metallocene moiety.

Structural commentary
Complex 1 crystallizes with one molecule of toluene in the asymmetric unit. The crystal structure of 1 confirms the bidentate binding mode of the trifluoromethanesulfonate ligand, which is in contrast to other complexes bearing different metallocene units (Kessler et al., 2011;Godemann et al., 2015).

Supramolecular features
For 1, weakstacking interactions were observed between two neighbouring toluene solvent molecules along the a axis [distance between ring centroids 3.9491 (11) Å and ring slippage of 1.985 Å ].

Synthesis and crystallization
All operations were carried out under argon with standard Schlenk techniques or in a glovebox. The alkyne complex Me 2 Si(C 5 Me 4 ) 2 Ti( 2 -Me 3 SiC 2 SiMe 3 ) was prepared according to a published procedure (Varga et al., 1997). Yb(OTf) 3 was purchased from Sigma Aldrich and used as received. Toluene was purified with the Grubbs-type column system 'Pure Solv MD-5' and dispensed into thick-walled glass Schlenk bombs equipped with Young-type Teflon valve stopcocks.
Synthesis of 1: Me 2 Si(C 5 Me 4 ) 2 Ti( 2 -Me 3 SiC 2 SiMe 3 ) (0.450 g, 0.87 mmol) and Yb(OTf) 3 (0.730 g, 1.17 mmol) were dissolved in 30 ml of toluene and heated at 333 K overnight, resulting in a colour change from dark yellow to green. All volatiles were removed in vacuo and the residue was again dissolved in toluene. The solution was filtered and the solvent was evaporated in vacuum to yield complex 1 as a dark-green powder. Single crystals suitable for an X-ray analysis were obtained from a saturated toluene solution at 195 K.
Synthesis of 2: In an experiment which aimed at the synthesis of the above Ti III trifluoromethanesulfonate complex 1, a batch of the alkyne complex Me 2 Si(C 5 Me 4 ) 2 Ti( 2 -Me 3 SiC 2 SiMe 3 ) was used that contained significant amounts of the monochloride complex Me 2 Si(C 5 Me 4 ) 2 TiCl, which was formed by incomplete reduction of the dichloride complex Me 2 Si(C 5 Me 4 ) 2 TiCl 2 during synthesis of the alkyne complex. Reaction of the monochloride complex with Yb(OTf) 3 yields the ansa-titanocene(IV) chloride trifluoromethanesulfonate complex 2. Single crystals suitable for an X-ray analysis were The structures of the molecular components of compound 1. Displacement ellipsoids correspond to the 30% probability level. H atoms have been omitted for clarity.

Figure 2
The molecular structure of complex 2. Displacement ellipsoids correspond to the 30% probability level. H atoms have been omitted for clarity. obtained from a saturated toluene solution by slow cooling from 353 K to room temperature.

2) Chlorido[dimethylbis(η 5 -tetramethylcyclopentadienyl)silane](trifluoromethanesulfonato-κO)titanium(IV)
Crystal data Special details 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )