Alkoxide

An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as RO⁻, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands. Alkoxides, although generally not stable in protic solvents such as water, occur widely as intermediates in various reactions, including the Williamson ether synthesis.[1][2] Transition metal alkoxides are widely used for coatings and as catalysts.[3][4]

Structure of the methoxide anion. Although alkali metal alkoxides are not salts and adopt complex structures, they behave chemically as sources of RO⁻.

Enolates are unsaturated alkoxides derived by deprotonation of a C-H bond adjacent to a ketone or aldehyde. The nucleophilic center for simple alkoxides is located on the oxygen, whereas the nucleophilic site on enolates is delocalized onto both carbon and oxygen sites. Ynolates are also unsaturated alkoxides derived from acetylenic alcohols.

Phenoxides are close relatives of the alkoxides, in which the alkyl group is replaced by a derivative of benzene. Phenol is more acidic than a typical alcohol; thus, phenoxides are correspondingly less basic and less nucleophilic than alkoxides. They are, however, often easier to handle, and yield derivatives that are more crystalline than those of the alkoxides.

Structure

Alkali metal alkoxides are often oligomeric or polymeric compounds, especially when the R group is small (Me, Et).[3] The alkoxide anion is a good bridging ligand, thus many alkoxides feature M₂O or M₃O linkages. In solution, the alkali metal derivatives exhibit strong ion-pairing, as expected for the alkali metal derivative of a strongly basic anion.

Structure of the Li₄(OBu-t)₄(thf)₃ cluster, highlighting the tendency of alkoxides to aggregate and bind ether ligands.[5] Color code: dark gray = C, violet = Li, red = O, light gray = H.

Preparation

From reducing metals

Alkoxides can be produced by several routes starting from an alcohol. Highly reducing metals react directly with alcohols to give the corresponding metal alkoxide. The alcohol serves as an acid, and hydrogen is produced as a by-product. A classic case is sodium methoxide produced by the addition of sodium metal to methanol:

2 CH₃OH   +   2 Na   →   2 CH₃ONa   +   H₂

Other alkali metals can be used in place of sodium, and most alcohols can be used in place of methanol. Another similar reaction occurs when an alcohol is reacted with a metal hydride such as NaH. The metal hydride removes the hydrogen atom from the hydroxyl group and forms a negatively charged alkoxide ion.

From electrophilic chlorides

Titanium tetrachloride reacts with alcohols to give the corresponding tetraalkoxides, concomitant with the evolution of hydrogen chloride:

TiCl₄   +   4 (CH₃)₂CHOH   →   Ti(OCH(CH₃)₂)₄   +   4 HCl

The reaction can be accelerated by the addition of a base, such as a tertiary amine. Many other metal and main group halides can be used instead of titanium, for example SiCl₄, ZrCl₄, and PCl₃.

By metathesis reactions

Many alkoxides are prepared by salt-forming reactions from a metal chloride and sodium alkoxide:

n NaOR   +   MCl_n   →   M(OR)_n   +   n NaCl

Such reactions are favored by the lattice energy of the NaCl, and purification of the product alkoxide is simplified by the fact that NaCl is insoluble in common organic solvents.

Copper(I) t-butoxide adopts a square structure, a consequence of the preference of Cu(I) for linear coordination geometry.

By electrochemical processes

Many alkoxides can be prepared by anodic dissolution of the corresponding metals in water-free alcohols in the presence of electroconductive additive. The metals may be Co, Ga, Ge, Hf, Fe, Ni, Nb, Mo, La, Re, Sc, Si, Ti, Ta, W, Y, Zr, etc. The conductive additive may be lithium chloride, quaternary ammonium halide, or other. Some examples of metal alkoxides obtained by this technique: Ti(OCH(CH₃)₂)₄, Nb₂(OCH₃)₁₀, Ta₂(OCH₃)₁₀, [MoO(OCH₃)₄]₂, Re₂O₃(OCH₃)₆, Re₄O₆(OCH₃)₁₂, and Re₄O₆(OCH(CH₃)₂)₁₀.

Properties

Reactions with alkyl halides

The alkoxide ion can react with a primary alkyl halide in an SN₂ reaction to form an ether via the Williamson Ether Synthesis.

Hydrolysis and transesterification

Metal alkoxides hydrolyse with water according to the following equation:[6]

2 L_nMOR   +   H₂O   →   [L_nM]₂O   +   2 ROH

where R is an organic substituent and L is an unspecified ligand (often an alkoxide) A well-studied case is the irreversible hydrolysis of titanium ethoxide:

1/n [Ti(OCH₂CH₃)₄]_n   +   2 H₂O   →   TiO₂   +   4 HOCH₂CH₃

By controlling the stoichiometry and steric properties of the alkoxide, such reactions can be arrested leading to metal-oxy-alkoxides, which usually are oligonuclear complexes. Other alcohols can be employed in place of water. In this way one alkoxide can be converted to another, and the process is properly referred to as alcoholysis (unfortunately, there is an issue of terminology confusion with transesterification, a different process - see below). The position of the equilibrium can be controlled by the acidity of the alcohol; for example phenols typically react with alkoxides to release alcohols, giving the corresponding phenoxide. More simply, the alcoholysis can be controlled by selectively evaporating the more volatile component. In this way, ethoxides can be converted to butoxides, since ethanol (b.p. 78 °C) is more volatile than butanol (b.p. 118 °C).

In the transesterification process, metal alkoxides react with esters to bring about an exchange of alkyl groups between metal alkoxide and ester. With the metal alkoxide complex in focus, the result is the same as for alcoholysis, namely the replacement of alkoxide ligands, but at the same time the alkyl groups of the ester are changed, which can also be the primary goal of the reaction. Sodium methoxide, for example, is commonly used for this purpose, a reaction that is relevant to the production of "bio-diesel".

Formation of oxo-alkoxides

Many metal alkoxide compounds also feature oxo-ligands. Oxo-ligands typically arise via the hydrolysis, often accidentally, and via ether elimination:

2 L_nMOR   →   [L_nM]₂O   +   R₂O

Additionally, low valent metal alkoxides are susceptible to oxidation by air

Formation of polynuclear and heterometallic derivatives

Characteristically, transition metal alkoxides are polynuclear, that is they contain more than one metal. Alkoxides are sterically undemanding and highly basic ligands that tend to bridge metals.

Upon the isomorphic substitution of metal atoms close in properties crystalline complexes of variable composition are formed. The metal ratio in such compounds can vary over a broad range. For instance, the substitution of molybdenum and tungsten for rhenium in the complexes Re₄O_6−y(OCH₃)_12+y allowed one to obtain complexes Re_4−xMo_xO_6−y(OCH₃)_12+y in the range 0 ≤ x ≤ 2.82 and Re_4−xW_xO_6−y(OCH₃)_12+y in the range 0 ≤ x ≤ 2.

Thermal stability

Many metal alkoxides thermally decompose in the range ~100–300 °C. Depending on process conditions, this thermolysis can afford nanosized powders of oxide or metallic phases. This approach is a basis of processes of fabrication of functional materials intended for aircraft, space, electronic fields, and chemical industry: individual oxides, their solid solutions, complex oxides, powders of metals and alloys active towards sintering. Decomposition of mixtures of mono- and heterometallic alkoxide derivatives has also been examined. This method represents a prospective approach possessing an advantage of capability of obtaining functional materials with increased phase and chemical homogeneity and controllable grain size (including the preparation of nanosized materials) at relatively low temperature (less than 500−900 °C) as compared with the conventional techniques.

Illustrative alkoxides

The structure of tetranuclear rhenium oxomethoxide (hydrogen atoms omitted for the sake of simplicity).[7]

name	molecular formula	comment
Titanium isopropoxide	Ti(O^iPr)4	monomeric because of steric bulk, used in organic synthesis
Titanium ethoxide	Ti4(OEt)16	for sol-gel processing of Ti oxides
Zirconium ethoxide	Zr4(OEt)16	for sol-gel processing of Zr oxides
Tetraethyl orthosilicate	Si(OEt)4	for sol-gel processing of Si oxides; Si(OMe)4 is avoided for safety reasons
Aluminium isopropoxide	Al4(O^iPr)12	reagent for Meerwein–Ponndorf–Verley reduction
Vanadyl isopropoxide	VO(O^iPr)3	precursor to catalysts
Niobium ethoxide	Nb2(OEt)10	for sol-gel processing of Nb oxides
Tantalum ethoxide	Ta2(OEt)10	for sol-gel processing of Ta oxides
Potassium tert-butoxide,	K4(O^tBu)4	basic reagent for organic elimination reactions

References

1. Williamson, Alexander (1850). "Theory of Ætherification" (PDF). Phil. Mag. 37 (251): 350–356. doi:10.1080/14786445008646627. (Link to excerpt.)
2. Boyd, Robert Neilson; Morrison, Robert Thornton (1992). Organic Chemistry (6th ed.). Englewood Cliffs, N.J.: Prentice Hall. pp. 241–242. ISBN 9780136436690.
3. Bradley, Don C.; Mehrotra, Ram C.; Rothwell, Ian P.; Singh, A. (2001). Alkoxo and Aryloxo Derivatives of Metals. San Diego: Academic Press. ISBN 978-0-08-048832-5.
4. Turova, Nataliya Y.; Turevskaya, Evgeniya P.; Kessler, Vadim G.; Yanovskaya, Maria I. (2002). The Chemistry of Metal Alkoxides. Dordrecht: Kluwer Academic Publishers. ISBN 9780792375210.
5. Unkelbach, Christian; O'Shea, Donal F.; Strohmann, Carsten (2014). "Insights into the Metalation of Benzene and Toluene by Schlosser's Base: A Superbasic Cluster Comprising PhK, PhLi, and tBuOLi". Angew. Chem. Int. Ed. 53 (2): 553–556. doi:10.1002/anie.201306884. PMID 24273149.
6. Hanaor, Dorian A. H.; Chironi, Ilkay; Karatchevtseva, Inna; Triani, Gerry; Sorrell, Charles C. (2012). "Single and Mixed Phase TiO₂ Powders Prepared by Excess Hydrolysis of Titanium Alkoxide". Advances in Applied Ceramics. 111 (3): 149–158. arXiv:1410.8255. doi:10.1179/1743676111Y.0000000059.
7. Shcheglov, P. A.; Drobot, D. V. (2005). "Rhenium Alkoxides". Russian Chemical Bulletin. 54 (10): 2247–2258. doi:10.1007/s11172-006-0106-5.

Further reading

• Turova, Nataliya Y. (2004). "Metal oxoalkoxides. Synthesis, properties and structures". Russian Chemical Reviews. 73 (11): 1041–1064. Bibcode:2004RuCRv..73.1041T. doi:10.1070/RC2004v073n11ABEH000855.