Liliane G. Hubert-Pfalzgraf
IRC, Université de Lyon1 - 69626 Villeurbanne Cedex (France)

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-xCl_x 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 aminoalcohols^11b 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 O^2- 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