II. HOMOMETALLIC
METAL ALKOXIDES
II.1. Synthesis
Metal alkoxides M(OR)n with usual
OR ligands are available commercially for a large number of metals. They can
be tailored by substitution reactions (see II.3). Their synthesis will not
be extensively covered here, most of them are obtained from metallic
chlorides. If the substitution is incomplete, halogenoalkoxides M(OR)n-xClx
are obtained.2 Anodic dissolution of the metal can provide an one
pot and easy to scale up synthesis of alkoxides for many metals.10
Reactions between oxides or hydroxides and dialkylcarbonates, 11a
polyols or aminoalcohols11b can also be cost effective routes to
metal alkoxides of some elements (M = Ti, Si) . The volatility of metal
alkoxides or their solubility allow purification either by distillation,
sublimation or crystallisation. Halogenoalkoxides are generally less
soluble, less volatile and less moisture sensitive than the homoleptic ones
( with only one type of ligands linked to the metal as for . M(OR)n).
Characterization techniques are mostly based on single crystal X-Ray
diffraction, Nuclear Magnetic Resonance (NMR) in solution and infrared (IR)
spectroscopies.
II.2. Physical properties and structural
aspects
In contrast to silicon alkoxides which are
liquids and monomeric, alkoxides based on metals are often solids. Since
metals have might have coordination numbers higher than 4, their
alkoxides are oligomeric species namely [M(OR)n]m in
the absence of donor molecules L. In those oligomers, the alkoxide ligands
occupy bridging-doubly (µ2-) or triply-bridging (µ3-)positions, and thus can
link two or three metals . 1,2,3 The degree of association m
depends on the bulk of the organic group R. For instance aluminium
terbutoxide is a dimer whereas the isopropoxide is a tetramer. In general,
methoxides are the least soluble due to their higher nuclearity.
Isopropoxides are reasonably soluble for most metals with the exception of
magnesium, lead, zinc and late dn transition metals (Cr, Fe, Cu, Co, …). For
the latter, isopropoxides are polymeric, solubility can be reached by using
functional alcohols (see II.3.1)
Fig 2 collects some common frameworks of
dinuclear (A), trinuclear (B, C) and tetranuclear (D, E) metal alkoxides.
Bond lengths vary in the order M-OR terminal < M-µ-OR < M-µ3-OR. These
structures are generally retained in non-polar organic media. Polynuclear
species can also be obtained via an oxo O2- ligand and metal
alkoxides of large elements are often oxoalkoxides.12 For
instance, the compound long formulated as yttrium trisisopropoxide Y(OPri)3
is actually a pentanuclear oxoisopropoxide Y5(µ5-O)(OPri)13
(fig. 2, F) with a central oxo ligand . The oxo ligand is an assembling and
versatile ligand. It can be linked to more metals (from two to six) than an
OR ligand - and thus increase the metal coordination number (six here) in
the absence of a neutral ligand L. Derivatives of other large trivalent (M =
In, Sc, Yb, Sm. R = iPr; M = La, R = tBu.), or divalent (Ba, Sr)
5 metals display the same structure. Thus, M-O-M linkages can
exist in the precursor prior to hydrolysis. These oxoalkoxides are generally
highly soluble due to a compact (closo) framework and an hydrocarbon
periphery, their structure is retained in solution12
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