2 Homometallic Alkoxides

274 Alkoxo and Aryloxo Derivatives of Metals
bridged dimer [Me4Ti2⊲ -OMe⊳2⊲tritox⊳2] has titanium in the square pyramidal
configuration. 187 The bulky tritox ligands occupy the apical positions with the Ti
atoms located 0.71 ˚A above the basal planes comprising two cis methyl groups and
two bridging methoxides with the two tritox groups being trans across the dimer. The
1,3-bis-(trimethylsilyl)cyclopentadienyl titanium cis-1,3,5-cyclohexan-triolate forms a
monomer [fMe3Si⊳2C5H3gTi⊲O3C6H9⊳]. 188
The bis-cyclopentadienyl titanium(III) alkoxides are dimeric [⊲C5H5⊳4Ti2⊲ -
OR⊳2] (R D Me, Et) and paramagnetic, exhibiting antiferromagnetic behaviour. 189
A novel mononuclear Ti(III) alkoxide was obtained using the functionalized alcohol
Me2PCH2CBu t 2 ⊲OH⊳. Thecomplex[TiCl⊲OCBut 2 CH2PMe2⊳2] involves titanium in a
TBP configuration with the P atoms of the chelating alkoxo ligands occupying the
axial positions and the oxygens and the chlorine in the equatorial positions. 190
2.9.2 Zirconium
Relatively few structures of alkoxo zirconium compounds have been reported.
Although a dimeric structure [Zr2⊲ -OPr i ⊳2⊲OPr i ⊳6⊲Pr i OH⊳2] had been proposed for
the crystalline zirconium isopropoxide isopropanolate many years ago 191 the edgeshared
bi-octahedral structure was not properly determined by X-ray crystallography
until 1990. 192 An interesting feature of this structure is the location of the coordinated
isopropanol molecules which are each linked by a hydrogen bond to the terminal
isopropoxo ligand on the adjacent zirconium atom Pr i O–H ....OPr i . This causes a
marked distortion due to bending of the Zr–OPr i bonds which are drawn together by
the hydrogen bond. A similar situation occurs in the Hf, 170,200 Ce(IV) 122,192 and Sn(IV)
analogues. 88,89 The bulky tris(trimethylsilyl)silyl ligand formed the monomeric fourcoordinated
zirconium tert-butoxo compound [⊲Me3Si⊳3SiZr⊲OBu t ⊳3] which contains
a Zr–Si bond. 193 The bidentate ligand trans-1,2-cyclohexanediolate bridges two
ZrCl3 moieties in the binuclear compound [⊲thf⊳2Cl3Zr⊲O2C6H10⊳ZrCl3⊲thf⊳2]. 194 The
octahedral Zr atoms are each bonded to three chlorines in a meridional configuration
cis to the Zr–O bonds. The remarkable pentanuclear amido imidonitride cluster
Zr5⊲ 5-N⊳⊲ 3-NH⊳4⊲ -NH2⊳4⊲OCBu t 3 ⊳5] is stabilized by the terminal tritox ligands. 195
The zirconiums are all octahedral and the structure bears a striking resemblance to
the pentanuclear oxo isopropoxo clusters [M5⊲ 5-O⊳⊲ 3-OPr i ⊳4⊲ -OPr i ⊳5] (MD Sc,
Y, Ln). The structure of monomeric Zr⊲hftb⊳4 ⊲hftb D OCMe⊲CF3⊳2) revealed an
essentially tetrahedral configuration about the Zr atom. 196 Another mononuclear mixed
ligand complex is [⊲oep⊳Zr⊲OBu t ⊳2] (oepD octaethylporphyrin) where the tert-butoxo
groups occupy cis positions above the plane of the porphyrin ring. 197 The Zr–O–C
bonds are practically linear.
The tridentate ligand tris(hydroxymethyl)ethane forms a tetranuclear complex
[Zr4f -OCH2⊳3CMeg2⊲ -OPr i ⊳2⊲OPr i ⊳8], which exhibits a different structure from the
titanium analogue. 169
2.9.3 Hafnium
Only a few structures of hafnium alkoxo compounds have been reported. As expected,
hafnium isopropoxide [Hf2⊲ -OPr i ⊳2⊲OPr i ⊳6⊲Pr i OH⊳2] has the edge-shared bioctahedral
structure like that of the analogous zirconium compound. 200 The coordinated