2 Homometallic Alkoxides

138 Alkoxo and Aryloxo Derivatives of Metals
insertion into a M–OR bond, rather than the more common attack by RO on a
carbonyl ligand in the formation of metallocarboxylates.
4.13 Formation of Coordination Compounds
Homoleptic metal alkoxides do not generally form stable molecular adducts with
conventional neutral donor ligands, 3 owing to the preferential intermolecular coordination
of alkoxo oxygen leading to coordination expansion of the metal and formation of
coordinatively saturated oligomeric or polymeric molecules. The latter effect is more
pronounced with sterically less demanding alkoxo ligands such as OMe, OEt, or
OPr n . In some cases, however, the coordination expansion of the metal may also
occur alternatively with the lone pair electrons from added donor ligands. Obviously,
the added ligand (L) will have to compete thermodynamically with the oligomerization
process (Eq. 2.299): 3,8
(RO) x−1M
R
O
O
R
M(OR)x−1 + 2L
2M(OR) x(L) (2.299)
The reaction (Eq. 2.299) in the forward direction will be facilitated by the higher
electrophilicity of the metal atom (M) in the metal alkoxides [M⊲OR⊳x ]n with a
weaker M⊲ -OR⊳2 bridging system. This could be brought about by the following
changes: 3,4,8,21,25 (a) the replacement of R by electron-withdrawing groups (such as
CF3, CH2CH2Cl, CH2CCl3, CH⊲CF3⊳2, CMe2⊲CF3⊳, CMe⊲CF3⊳2, etc.) to make the
alkoxo oxygen less nucleophilic and a weaker bridging agent, (b) an increase in the
steric demand of the group R (Bu t ,CHPr i 2 ,CHBut 2 , CBut 3 ,CPh3,CMe⊲CF3⊳2,C⊲CF3⊳3)
which is more likely to produce a coordinatively unsaturated molecule, and thus provide
an opportunity particularly for compact hard oxygen or nitrogen donor ligands to coordinate
the metal, and (c) the attachment of electron-attracting groups (such as chloride)
to the metal.
These tendencies have been successfully exploited in the synthesis of molecular
addition compounds of metal alkoxides. For example, Gilman et al. 898 observed that
U⊲OCH2CF3⊳5 formed addition complexes of moderate stability with ammonia or aliphatic
amines, and Bradley et al. 3,146 found that Zr⊲OCH2CCl3⊳4 formed a stable complex
Zr[OCH2CCl3]4.⊲2Me2CO⊳ with acetone, while derivatives M[OCMe⊲CCl3⊳CH3]4
(M D Ce or Th) give molecular adducts with pyridine. Interestingly, metal alkoxides
of the lower alcohols can be readily recrystallized from the parent alcohol as molecular
adducts, viz.: Ti⊲OBu i ⊳4.Bu i OH; M⊲OPr i ⊳4.Pr i OH (M D Zr,Hf,Ce,Th,Sn). 3 In addition
to these, a few examples of addition compounds of the types M⊲OPr i ⊳4⊲NC5H5⊳ (M D Zr,
Ce); 3 Ti⊲OPr i ⊳4⊲en⊳, Ti2⊲OR⊳8⊲en⊳ (R D Et, Pr i ), 955 TiClx ⊲OCH2CF3⊳4 x ⊲NCR⊳ (R D
Me, Et; x D 1, 2); 956 Ta2⊲OPr i ⊳10⊲en⊳; 955 Al⊲OPr i ⊳3.N2H4 and Al2⊲OPr i ⊳6.N2H4, 957 as
well as Al⊲OCH2CCl3⊳3⊲py⊳ 403 were reported and the possibilities of potential structures
discussed.