2 Homometallic Alkoxides

Metal Oxo-alkoxides 385
[Ti3⊲ 3-O⊳⊲ -OPr i ⊳4fMe2C⊲O⊳CHC⊲O⊳CH2C⊲O⊳Me2g] 66 in which the tridentate ligand
formed by condensation of acetone spans the three Ti atoms.
2.2 Formation of Oxo-alkoxides by Nonhydrolytic Reactions of Alkoxides
Metal oxo-alkoxides unintentionally obtained during attempts to prepare metal alkoxides
have generally resulted from the presence of adventitious water leading to hydrolysis,
but it appears that the formation of some oxo-alkoxides is due to the instability of
the sought after alkoxide.
An early example was the failure to obtain penta-tertiary alkoxides of niobium
owing to the preferential formation of the oxo compounds Nb2O⊲OR⊳8 and NbO⊲OR⊳3
in contrast to tantalum which formed thermally stable Ta⊲OBu t ⊳5. This behaviour was
explained in terms of the greater tendency of niobium to form the metal-oxo double
bond. 13 In the reactions of W⊲NMe2⊳6 with ROH the hexa-alkoxides W⊲OR⊳6 (R D Me,
Et, Pr i , allyl) were formed but not the tertiary butoxide which gave WO⊲OBu t ⊳4
instead. 14 The tendency of MoO⊲OR⊳4 compounds to form MoO2⊲OR⊳2 by ether elimination
has also been noted (Eq. 5.4). 15
LxM⊲OR⊳2 ! Lx MDO C R2O ⊲5.4⊳
The formation of multiple metal–oxo bonds by penta or hexa valent transition metals
(d 0 ) is not surprising but the more recent discovery that scandium, yttrium, and
the tervalent lanthanides form oxo-isopropoxides rather than triisopropoxides was
unexpected. 16 Considerable efforts have been devoted to ensuring that the formation of
these pentanuclear oxo-alkoxides [M5⊲ 5-O⊳⊲ 3-OPr i ⊳4⊲ -OPr i ⊳4⊲OPr i ⊳5] was not due
to hydrolysis, and there is evidence that elimination of ether is involved. An alternative
mode of decomposition would be alkene elimination with concomitant formation of
alcohol (Eq. 5.5).
LxMfOCH⊲CH3⊳2g2 ! Lx MDO C CH2DCHMe C Me2CHOH ⊲5.5⊳
This process would be expected to be facilitated by tertiary alkoxo groups but it is
significant that the tris-tertiary alkoxides of these metals are readily prepared and
are thermally stable. It is also noteworthy that whereas scandium forms pentanuclear
oxo-alkoxides Sc5O⊲OR⊳13 with ethoxo and isopropoxo ligands, the trifluorethoxo and
hexafluoroisopropoxo ligands form stable tris-alkoxides. 17 The thermal desolvation of
[Ce2⊲OPr i ⊳8⊲Pr i OH⊳2] gave rise to the oxo-species [Ce4O⊲OPr i ⊳14] by elimination of
diisopropyl ether. 67
Heterometallic oxo-alkoxides may also be formed by conversion from heterometallic
alkoxides. Thusthe insoluble [Pb⊲OPr i ⊳2]x reactedwithTi⊲OPr i ⊳4 toform[Pb4Ti4O3⊲OPr i ⊳18]
and [Pb2Ti2⊲ 4-O⊳⊲ 3-OPr i ⊳2⊲ -OPr i ⊳4⊲OPr i ⊳4] presumably by ether elimination. 18
Similarly, from reactions of barium alkoxides and titanium alkoxides the heterometallic
oxo-alkoxides f[Ba4Ti4O4⊲OPr i ⊳16⊲Pr i OH⊳4][Ba4Ti4O4⊲OPr i ⊳10⊲Pr i OH⊳3]g 19 and
[Ba2Ti4O⊲OMe⊳18⊲MeOH⊳7] 20 were obtained as a result of ether elimination, although
similar complexes are readily prepared when the barium alkoxide solution is aerobically
oxidized. 21