Homometallic Alkoxides

1 INTRODUCTION

Metal alkoxides [M(OR)x]n (where M = metal or metalloid of valency x; R = simple alkyl, substituted alkyl, or alkenyl group; and n = degree of molecular association), may be deemed to be formed by the replacement of the hydroxylic hydrogen of an alcohol (ROH) by a metal(loid) atom.

Historically, the first homoleptic alkoxo derivatives of elements such as boron and silicon had been described1, 2 as early as 1846, but later progress in the alkoxide chemistry of only half a dozen metals was rather slow and sporadic till the 1950s; since then the chemistry of alkoxides of almost all the metals in the periodic table has been systematically investigated. With a few exceptions, systematic investigations on the structural aspects of metal alkoxides till the mid-1980s3–10 were limited to studies on molecular association, volatility, chemical reactivity, and spectroscopic (IR, NMR and electronic) as well as magnetic properties. It is only since the early 1980s that definitive X-ray structural elucidation has become feasible and increasingly revealing.

The rapidly advancing applications11–16 of metal alkoxides for synthesis of ceramic materials by sol–gel/MOCVD (metallo-organic chemical vapour deposition) processes (Chapter 7) have more recently given a new impetus to intensive investigations on synthetic, reactivity (including hydrolytic), structural, and mass-spectroscopic aspects of oxo-alkoxide species. 17–21

Some of the exciting developments since 1990 in metal alkoxide chemistry have been focussing on the synthesis and structural characterization of novel derivatives involving special types of alkoxo groups such as (i) sterically demanding monodentate (OBut, CHPr2i, CHBu2t, OCMeEtPri, CBu3t) as well as multidentate (OCR′CH2OPri)2) (R′= But or CF3), CR″2CH2X (R″ = Me or Et, X = OMe, OEt, NMe2) ligands,21–24 (ii) fluorinated tertiary alkoxo (OCMe(CF3)2, OCMe2(CF3), OC(CF3)3, etc.) moieties, 21, 22, 23 and (iii) ligands containing intramolecularly coordinating substituents (CBu2tCH2PMe2, OCH2CH2X (X = OMe, OEt, OBun, NR2, PR2)).21, 22 Compared to simple alkoxo groups, most of these chelating/sterically demanding ligands possess the inherent advantages of enhancing the solubility and volatility of the products by lowering their nuclearities owing to steric factors and intramolecular coordination.

Solubility and volatility are the two key properties of metal alkoxides which provide convenient methods for their purification as well as making them suitable precursors for high-purity metal oxide-based ceramic materials.

It is noteworthy that the homoleptic platinum group metal (Ru, Rh, Pd, Os, Ir, Pt) alkoxides are kinetically more labile possibly owing to β-hydrogen elimination9, 10, 21type reaction(s) (Eq. 2.1):

  (2.1)

These, therefore, are not generally isolable under ambient conditions unless special types of chelating alkoxo ligands21 are used.

Although single crystal X-ray studies presented considerable difficulties in the earlier stages,25 the development of more sophisticated X-ray diffraction techniques has led to the structural elucidation of a number of homo- and heteroleptic alkoxides17–23 and actual identification of many interesting metal oxo-alkoxide systems (Chapter 4).

In this chapter we shall discuss the synthesis,3, 4, 26 chemistry and properties of homometallic alkoxides with more emphasis on homoleptic alkoxides [M(OR)x]n and M(OR)x.Ln with occasional references to metal oxo-alkoxides MOy(OR)(x–2y) and metal halide alkoxides M(OR)x–yXy.Lz (where x = valency of metal, L = neutral donor ligand, X = halide, and n, y and z are integers). The discussion will generally exclude organometallic alkoxides and a considerable range of metal-organic compounds containing alkoxo groups, as in these systems the alkoxo groups play only a subsidiary role in determining the nature of the molecule.

2 METHODS OF SYNTHESIS

Metal alkoxides in general are highly moisture-sensitive. Stringent precautions are, therefore, essential during their synthesis and handling; these involve drying of all reagents, solvents, apparatus, and the environment above the reactants and products. Provided that these precautions are taken, the preparation of metal alkoxides, although sometimes tedious and time consuming, is relatively straightforward.

The method employed for the synthesis3, 4, 8, 17, 21 of any metal/metalloid alkoxide depends generally on the electronegativity of the element concerned. Highly electropositive metals with valencies up to three (alkali metals, alkaline earth metals, and lanthanides) react directly with alcohols liberating hydrogen and forming the corresponding metal alkoxides. The reactions of alcohols with less electropositive metals such as magnesium and aluminium, require a catalyst (I2 or HgCl2) for successful synthesis of their alkoxides. The electrochemical synthesis of metal alkoxides by anodic dissolution of metals (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb) and even metalloids (Si, Ge) in dry alcohols in the presence of a conducting electrolyte (e.g. tetrabutylammonium bromide) appears to offer a promising procedure (Section 2.2) of considerable utility. It may be worthwhile to mention at this stage that the metal atom vapour technique, which has shown exciting results in organometallics, may emerge as one of the potential synthetic routes for metal alkoxides also in future.

For the synthesis of metalloid (B, Si) alkoxides, the method generally employed consists of the reaction of their covalent halides (usually chlorides) with an appropriate alcohol. However, the replacement of chloride by the alkoxo group(s) does not appear to proceed to completion, when the central element is comparatively more electropositive. In such cases (e.g. titanium, niobium, iron, lanthanides, thorium) excluding the strongly electropositive s-block metals, the replacement of halide could in general be pushed to completion by the presence of bases such as ammonia, pyridine, or alkali metal alkoxides.

Another generally applicable method, particularly in the case of electronegative elements, is the esterification of their oxyacids or oxides (acid anhydrides) with alcohols (Section 2.6), and removing the water produced in the reaction continuously.

In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions of metal dialkylamides with alcohols has provided a highly convenient and versatile route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals, particularly in their lower valency states.

The metal-hydrogen and metal-carbon bond cleavage reactions have also been exploited in some instances (Section 2.10.2).

The following pages present a brief summary of the general methods used for the synthesis of metal and metalloid alkoxides applicable to specific systems. Tables 2.1 and 2.2 in Section 2.1 (pp. 6 7 8 9 10 11 12 13 14) list some illustrative compounds along with their preparative routes and characterization techniques.