2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 271
an unsymmetrical bridge giving each metal a distorted octahedral coordination
(data in Table 4.9, pp. 276–279). 161 Use of the bulky Ph3SiO ligand in place of
the isopropoxide gave rise to the monomeric five-coordinated (TBP) “titanatrane”
[Ph3SiOTi⊲OC2H4⊳3N], whereas the acetato derivative [Ti2f -OC2H4O⊳N⊲C2H4O⊳2g2
⊲OAc⊳2] was dimeric through the bridging chelate ligand but the bidentate acetato
group gave the titanium the coordination number of seven. 163 The dimethylamido
derivative is also dimeric with distorted octahedral Ti, with one arm of the chelate
ligand bridging [Ti2f⊲ -OC2H4⊳N⊲C2H4O⊳2g2⊲NMe2⊳2]. This compound differs from
the isopropoxo derivative in having the dimethylamido-nitrogen trans to one of the
bridging oxygens whereas the isopropoxo ligand is trans to a nitrogen atom. 164 The
structure of the isopropylthiolate [Ti2f⊲ -OC2H4⊳N⊲C2H4O⊳2g2⊲SPr i ⊳2] corresponds
to that of the dimethylamido derivative. 165 Interestingly the dimer formed with the
pinacolato ligand [Tif⊲ -OC2H4⊳N⊲C2H4O⊳2g]2 involves a diolate bridge linking the
two “titanatrane” moieties giving rise to five-coordinated (TBP) Ti with the diolate
oxygens trans to the nitrogen of the chelating ligand. 165 Homochiral trialkanolamines
N⊲CH2CHROH⊳3 have been used in metalatrane synthesis. Thus with titanium the
monomeric chlorotitanatrane [ClTi⊲OCHPr i CH2⊳2N] has been characterized as a fivecoordinate
(TBP) complex with chloride and chelate ligand nitrogen in the axial
positions. 166 Special interest was generated by the structure of the dinuclear tertbutylperoxo
titanatrane [Ti2f⊲ -OC2H4⊳N⊲C2H4O⊳2g2⊲O2Bu t ⊳2]inwhichthe 2 -peroxo
ligand gave rise to seven-coordinated titanium. 167 This molecule has significance
as a model for an intermediate in the oxidation of various organic substrates by
tert-butyl hydroperoxide. Sharpless et al. 168 had proposed the bonding of an 2 -tertbutylperoxo
group to the dinuclear titanium tartrate catalysts involved in the asymmetric
epoxidation of allylic alcohols. The structure of the diisopropoxo titanium-N,N 0 -
dibenzyltartramide dimer [⊲Pr i O⊳4Ti2f -OCH⊲CONHCH2Ph⊳CH⊲CONHCH2Ph⊳Og2]
revealed distorted octahedral titanium atoms each coordinated facially by a tartramide
ligand through two diolate oxygens and one of the carbonyl groups with one diolate
oxygen bridging to the adjacent metal atom (Fig. 4.36). One isopropoxo group is
trans to the bridging diolate oxygen and the other is trans to the carbonyl oxygen.
Exchange of the two isopropoxo groups and dissociation of the carbonyl oxygen
exposes a meridional set of coordination sites to accommodate the allylic alkoxo
group and the 2 -butylperoxo ligand. 168 Later work using the diisopropyltartrato ligand
produced dinuclear, trinuclear, and tetranuclear complexes which were structurally
characterized. 169 A centrosymmetrical dimer was formed by the mixed ligand complex
[Ti2⊲ -OPr i ⊳2⊲OPr i ⊳2⊲OPr i ⊳4fHC⊲SO2CF3⊳2g2] which contains titanium in distorted
octahedral coordination (Table 4.9). 170 In the bis-chelated complex [Ti⊲OEt⊳2⊲diket⊳2]
(diket D 4,4,4-trifluoro-1-phenyl-1,3-butanedionate) the terminal ethoxo groups, as
expected, are in cis positions in the octahedral structure. 171
Although titanium dichloride dialkoxide alcoholates have been known for many
years 9 it was only recently that an X-ray structure was reported. 172 The dimeric
complex [Ti2⊲ -OC2H4Cl⊳2⊲OC2H4Cl⊳2Cl4⊲ClC2H4OH⊳2] forms an edge-shared bioctahedral
molecule with bridging alkoxo groups. The terminal alkoxo group is trans
to the bridging alkoxide and the alcohol ligand is trans to a chloride on one Ti atom
and appears to be hydrogen bonded to the chloride on the adjacent Ti (Table 4.9).
The 3,3 0 -disubstituted-1,1 0 -bi-2-naphtholates R2BINO (R D Me, Me2Bu t Si) gave rise
to the mononuclear [Ti⊲OPr i ⊳2f⊲Bu t Me2Si⊳2binog] four-coordinated complex and