2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 273
the other type having two cis terminal isopropoxo groups analogous to the terminal
ligands in Ti4⊲OR⊳16. The two ligands L each have two -OCH2 groups and one
3-OCH2 group, e.g. ( -OCH2⊳2CMe⊲CH2O- 3). 179
From a reaction involving Ti⊲OPr i ⊳4 and 2-hydroxy-ethylmethacrylate the remarkable
pentanuclear mixed ligand molecule [Ti5( 4-OC2H4O)( 3-OC2H4O)( -
OPri )(OPri )9] containing ethane-1,2-diolate groups was obtained. 180 This asymmetric
structure (Fig. 4.37) contains 1 seven-coordinate, 3 six-coordinate and 1 five-coordinate
titanium atom whilst the chelating–bridging diolate ligands exhibit four types of
bonding mode.
The isolated [Ti⊲OC2H4O⊳3] 2 octahedral tris-chelated diolato complex has
been structurally characterized in the salts Na2[Ti⊲OC2H4O⊳3]⊲HOC2H4OH⊳4 and
K2[Ti⊲OC2H4O⊳3]⊲HOC2H4OH⊳2.5. 181 The titanium tris-glycolate is also present in
the barium salt [Ba⊲HOC2H4OH⊳4⊲H2O⊳] C 2 [Ti⊲OC2H4O⊳3] 2 . The cation and anion
are linked by hydrogen bonds from the coordinated glycol hydroxyls in the cation
interacting with diolate oxygens in the anion. 182 In the mixed ligand complex
[Ti2⊲ -OEt⊳2⊲OEt⊳4⊲O2CCH2NH2⊳2] in which the glycinate ligand is chelating, the
terminal ethoxo groups are in cis positions. The glycinate nitrogen is trans to one of
the terminal ethoxo groups whilst the carboxylate oxygen is trans to a bridging ethoxo
group.
Some interesting titanosilsesquioxanes have been structurally characterized.
The compounds were obtained from the reactions of the cubic trisilanols
[R7Si7O9⊲OH⊳3] (R D c-pentyl, c-hexyl) with Ti⊲OPri⊳4. The crystalline dimer
[Ti2⊲ -OMe⊳2fO12Si7⊲C6H11⊳7g⊲MeOH⊳2] exhibited octahedral titaniums184 whereas
the mononuclear [TifO12Si7⊲C6H11⊳7g⊲OSiMe3⊳] trimethylsilanolate contains tetrahedral
titanium. 185 These compounds are of special interest as soluble titanium-centred
catalysts for the epoxidation of alkenes by tert-butylhydroperoxide (see Chapter 7).
A few organotitanium alkoxides have been structurally determined. In bis-benzyl
titanium diethoxide a centrosymmetric dimer [⊲C6H5CH2⊳4Ti2⊲ -OEt⊳2⊲OEt⊳2] is
formed involving five-coordinated Ti in a distorted TBP configuration. 186 In bis-methyl
titanium tritox methoxide (tritox D tri-tert-butyl carbinolate, But 3CO) the methoxo
Figure 4.37 Structure of [Ti5⊲ 4-
OC2H4O⊳⊲ 3-OC2H4O⊳3⊲ -OC2H4O⊳⊲ -
OPr i ⊳⊲OPr i ⊳9] ( D Ti; D O; H atoms
omitted).