2 Homometallic Alkoxides

272 Alkoxo and Aryloxo Derivatives of Metals
Figure 4.36 Structure of
[Ti2⊲dipt⊳3fON⊲CH2Ph⊳2g2]
(dipt D diisopropyltartrate;
H atoms omitted).
the binuclear [Ti2⊲ -Me2bino⊳2⊲OPr i ⊳4] five-coordinated complex. Another fourcoordinated
complex, [⊲Pr i O⊳3Tif⊲Bu t Me2Si⊳2binogTi⊲OPr i ⊳3], was also characterized
(Table 4.9). 173
The hexadentate ligand N,N,N,N 0 ,N 0 -tetrakis-(2-hydroxy propyl) ethylene diamine
(H4THPED) gave rise to the dimeric complex [Ti⊲thped⊳]2 in which the Ti is sevencoordinated
distorted monocapped trigonal prism or distorted pentagonal biprism. 174
The tetraneopentyloxide was found to be a dimer [Ti2⊲ -OCH2Bu t ⊳2⊲OCH2But⊳6] with
the titanium atoms exhibiting distorted TBP five-coordination. 201
It is interesting to note that the mixed alkoxide [Ti2⊲ -OPr i ⊳2⊲OPr i ⊳2(hfip) 4 ] is
dimeric in contrast to the monomeric Ti⊲OPr i ⊳4. 175 Doubtless the electronegative
hfip groups enhance the electrophilicity of the titanium thus strengthening the
isopropoxo bridges and countering the steric effects of the ligands. Nevertheless
the bridge bonding is very unsymmetrical (1.900 ˚A, 2.121 ˚A) and the compound is
monomeric in solution and in the vapour phase. The Ti atoms are five-coordinated
with the TBP configuration. Another mixed ligand dimer [Ti2⊲ -OPr i ⊳2⊲OPr i ⊳2Cl4]also
exhibits five-coordinated titanium. 176 The cis-octahedral configuration for the dialkoxobis-acetylacetonates
Ti(OR) 2⊲acac⊳2 proposed on 1 H NMR evidence in 1965 177
was confirmed by the X-ray crystal structure of [Ti⊲OPr i ⊳2⊲acac⊳2]. 176 However, a
series of trialkoxo-monoacetylacetonates [Ti2⊲ -OR⊳2⊲OR⊳4⊲acac⊳2] (RD Me, Et, Pr i )
and mono-2,2,6,6-tetramethylheptane-3,5-dionates (tmhd) [Ti2⊲ -OR⊳2⊲OR⊳4⊲thmd⊳2]
(R D Me, Pr n , Pr i ) were shown to be dimeric with octahedral Ti. 178 The terminal alkoxo
groups adopted the cis configuration in all of these compounds. Some interesting
tetranuclear complexes [Ti4⊲OPr i ⊳10L2] were obtained using the tridentate ligands L D
tris⊲hydroxymethylethane⊳ (THME) and tris(hydroxymethylpropane) (THMP). In both
compounds the Ti4O16 framework contains two types of distorted octahedral titanium
atoms, one type having three terminal isopropoxo groups in the fac configuration and