Di-μ-methanolato-bis[(2-tert-butyl-6-methylphenolato-κO)methyltitanium(IV)]

The molecule of the title compound, [Ti2(CH3)2(CH3O)2(C11H15O)4] or {[Ti(Me)(μ-OCH3)(OC6H3CMe3-2-Me-6)]2}, has a centrosymmetric, dimeric structure with a distorted square pyramidal array about each titanium atom. The methoxide ligands form an asymmetric bridge between the two TiIV atoms [Ti—O bond lengths of 1.9794 (12) and 2.0603 (12) Å] with the two phenolato ligands occupying the remaining basal sites [Ti—O 1.8218 (11) and 1.8135 (11) Å]. The Ti—O—C phenolato bond angles are similar at 161.24 (10) and 160.66 (11)°. The methyl ligand attached to the metal atom has a Ti—C bond length of 2.0878 (17) Å.


Comment
Titanium complexes containing phenolato ligands (OAr) or alkoxo ligands (OR) as well as an alkyl ligand have been described (Janas et al. 2005;Janas et al. 2004;Zhang 2007a,b;Kobylka et al. 2007) but complexes containing both oxygen ligand sets and an alkyl ligand are not known. The chemistry of phenolato and alkylato complexes has been known for many years (Bradley et al. 1978) and in particular alkoxy bridged bis-phenolato dititanium complexes have been prepared (Ejfler et al. 2004). Several examples of X-ray crystal structures for titanium complexes containing terminal and bridging phenolato ligands have been reported (Gowda et al. 2009;Nielson et al. 2006;Svetich & Voge, 1972). During attempts to form bis-dimethyl bis-phenolato complexes of titanium for testing as olefin oligomerization and polymerization catalysts, in one case we reacted [TiCl 2 (OC 6 H 3 CMe 3 -2-Me-6) 2 ] (Nielson et al. 2005;Santora et al. 1999) with two equivalents of methylmagnesium iodide and recrystallized the resulting product from petroleum spirit at low temperatures for a period of several months. A nice crystalline red coloured product was formed which was expected to be the bis-dimethyl complex [Ti(Me) 2 (OC 6 H 3 CMe 3 -2-Me-6) 2 ]. However the X-ray crystal structural analysis showed it was the dimeric methoxy bridged complex [{Ti(Me)(µ-OMe)(OC 6 H 3 CMe 3 -2-Me-6) 2 } 2 ] (1). Alkyl complexes of transition metal complexes are usually air sensitive and it was expected that the methoxo ligand had resulted from moist air entering the crystallization flask adventiciously and either water or oxygen inserting into a Ti-CH 3 bond. A reaction where oxygen inserts into a Ti-CH 3 bond has been reported (Zhang et al., 2007a).
The structure of (1) is centrosymmetric and consists of an asymmetric, methoxy bridged dimer in which each Ti centre has two terminal phenolato ligands and a methyl ligand attached (Fig. 1). Each titanium atom has a distorted square pyramidal geometry in which the base of the square pyramid is made up by the oxygen atoms of the two cis-related terminal phenolato ligands and the oxygen atoms of the methoxy bridging system. A similar distorted square pyramidal structure is found in [{(tbop)Ti(Me)} 2 (µ-OMe) 2 ] (tbop = 2,2-thiobis{4-(1,1,3,3-tetramethylbutyl)phenol} (Janas et al. (1) (1), one phenyl ring is rotated so that its face points inwards towards the other but slightly down and the other ring is rotated so that it points away from and slightly down from the former. The rotation is such that the tert-butyl substituents in the 2-position of the phenyl ring lie adjacent to each other, as do the two methyl substituents on the phenyl ring 6-position. For the two adjacent tert-butyl substituents the methyl carbons are related by a geared disposition which apparently allows a further gearing across the two substituents of the attached hydrogen atoms.

Experimental
Methyl magnesium iodide (11.8 ml of a 1.076 mol/l solution, 12.7 mmole) in diethyl ether was added dropwise to a stirred suspension of [TiCl 2 (OC 6 H 3 CMe 3 -2-Me-6) 2 ] (2.41 g, 5.79 mmole) in petroleum spirit (boiling point 40-60°) cooled in a dry-ice bath. The dry-ice bath was removed and the mixture stirred overnight. The solution was filtered, solvent removed and the residue extracted with hot petroleum spirit to give an orange-red solution. Reduction of the solvent volume and standing at -20° C gave a small quantity of the product as orange-red crystals. Found: C, 68.24; H, 9.10. C 48 H 72 O 6 Ti 2 requires C, 68.56; H, 8.63. A crystal was chosen and the X-ray single crystal structure obtained.

Refinement
All H atoms were included in calculated positions and refined using a riding model [U(H) eq = 1.2UC eq for aromatic CH and U(H) = 1.5U(C) for methyl H atoms]. C-H distances of 0.96 Å and 0.93 Å were assumed for aromatic and methyl groups respectively.

Di-µ-methanolato-bis[(2-tert-butyl-6-methylphenolato-κO)methyltitanium(IV)]
Crystal data [Ti 2  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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