Crystal structures of titanium–aluminium and –gallium complexes bearing two μ2-CH3 units

The molecular structures of two isotypic titanium(III) complexes bearing an trimethylaluminium or -gallium motif are reported. In both compounds, two methyl groups coordinate to the metal atoms, viz. Ti and Al(Ga), and in a μ 2 manner.


Chemical context
Trimethylaluminium, AlMe 3 , is of great interest because of its use in the synthesis of methylaluminoxane as co-catalyst in olefin polymerization (Wang, 2006;Janiak, 2006). In organometallic chemistry, many reactions involving trimethylaluminium have been investigated, e.g. the Tebbe reagent Cp 2 ZrCl(CH 2 Al(CH 3 ) 2 ) (Cp = cyclopentadienyl), which can be used for methylation reactions (Tebbe et al., 1978;Thompson et al., 2014). Employing multiple C-H activation reactions, the formation of zirconium-or hafnium-containing clusters [(Cp*M) 3 Al 6 Me 8 (CH 2 ) 2 (CH) 5 ] (M = Zr, Hf) have been described (Herzog et al., 1996). In a similar manner, the formation of [TiAl(C)CH 3 ] or [TiAl(CH 2 ) 2 ] metallacycles have been reported (Kickham et al., 2002;Stephan, 2005). It is noteworthy that all these complexes result from C-H activation reactions. Since bond activation reactions employing pentafulvene-substituted metal complexes have been of great interest in our work group (Oswald et al., 2016;Manssen et al., 2015;Ebert et al., 2014), we therefore investigated the reactivity of a dinuclear nitrogen-bridged pentafulvene titanium complex towards AlMe 3 and its heavier analogue GaMe 3 . Here we report on syntheses and crystal structures of the resulting title compounds, 1 and 2.

Structural commentary
Figs. 1 and 2 show the molecular structures of 1 and isotypic 2, respectively. Both complexes show the formation of a titanium trimethylaluminium or -gallium metallacycle, in which the EMe 3 units are still intact and exhibit a 2 -bridging mode of

Synthesis and crystallization
All reactions were carried out under a dry nitrogen atmosphere using Schlenk techniques or in a glove box. The starting titanium complex was prepared according to a published procedure (Scherer et al., 2005). AlMe 3 and GaMe 3 solutions were purchased from Sigma Aldrich and used as received. Solvents were dried according to standard procedures over Na/K alloy with benzophenone as indicator and distilled under a nitrogen atmosphere.

Figure 3
A view along the c axis showing the packing of molecules in the crystal structure of compound 1. No significant supramolecular features can be observed. Colour code: C grey, H white, Al pink, Ti turquoise spheres.

Figure 1
The molecular structure of complex 1. Displacement ellipsoids correspond to the 50% probability level. H atoms have been omitted for clarity except for those of methyl groups C26, C27 and C28. 0.632 mmol) was dissolved in toluene and AlMe 3 (2 M solution in toluene, 0.65 ml, 1.3 mmol) was added. The colour of the solution changed from blue to green, after 48 h the volume had reduced to 5 ml and another 5 ml of n-hexane were added. Crystals suitable for X-ray diffraction separated after 48 h directly from the mother liquor.

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.