Editing Transition metal alkoxide complex

{{short description|Conjugate base of an alcohol}}
[[File:Methanolat-Ion.svg|thumb|144 px|Structure of the methoxide anion.  Although alkali metal alkoxides are not salts and adopt complex structures, they behave chemically as sources of {{chem2|RO-}}.]]

A '''transition metal alkoxide complex''' is a kind of [[coordination complex]] containing one or more [[alkoxide]] ligands, written as {{chem2|RO-}}, where R is the organic [[substituent]].{{citation needed|date=March 2024}} Metal alkoxides are used for coatings and as [[catalyst]]s.<ref name=DCB>{{cite book|last1 = Bradley|first1 = Don C.|author-link1 = Donald Charlton Bradley|last2 = Mehrotra|first2 = Ram C.|author-link2 = Ram Charan Mehrotra|last3 = Rothwell|first3 = Ian P.|last4 = Singh|first4 = A.|title = Alkoxo and Aryloxo Derivatives of Metals|publisher = [[Academic Press]]|location = San Diego|year = 2001|isbn = 978-0-08-048832-5}}</ref><ref>{{cite book|first1 = Nataliya Y.|last1 = Turova|first2 = Evgeniya P.|last2 = Turevskaya|first3 = Vadim G.|last3 = Kessler|first4 = Maria I.|last4 = Yanovskaya|title = The Chemistry of Metal Alkoxides|publisher = [[Kluwer Academic Publishers]]|location = Dordrecht|year = 2002|isbn = 9780792375210}}</ref>

==Preparation==
===By metathesis reactions===
Many alkoxides are prepared by [[salt metathesis reaction|salt-forming reactions]] from a [[metal chloride]]s and [[Sodium methoxide|sodium alkoxide]]:
:{{chem2|MCl_{''n''}  +  ''n'' NaOR  ->  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.
[[Image:Cu4 OBut 4.svg|thumb|220px|[[Copper(I) t-butoxide]] adopts a square structure, a consequence of the preference of Cu(I) for linear coordination geometry.]]

For electrophilic metal halides, conversion to the alkoxide requires no or mild base. [[Titanium tetrachloride]] reacts with alcohols to give the corresponding tetraalkoxides, concomitant with the evolution of [[hydrogen chloride]]:<ref name=Bradley>{{cite book|author1 = Donald Charlton Bradley|author2-link = Ram Charan Mehrotra |author2 = Ram C. Mehrotra |last3 = Rothwell|first3 = Ian P.|last4 = Singh|first4 = A.|title = Alkoxo and Aryloxo Derivatives of Metals|publisher = [[Academic Press]]|location = San Diego|year = 2001|isbn = 978-0-08-048832-5|author1-link = Donald Charlton Bradley }}</ref>

:{{chem2|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]].  Other electrophilic metal halides can be used instead of titanium, for example {{chem2|NbCl5}}.{{citation needed|date=January 2024}}

===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 [[cobalt|Co]], [[gallium|Ga]], [[germanium|Ge]], [[hafnium|Hf]], [[iron|Fe]], [[nickel|Ni]], [[niobium|Nb]], [[molybdenum|Mo]], [[lanthanum|La]], [[rhenium|Re]], [[scandium|Sc]], [[silicon|Si]], [[titanium|Ti]], [[tantalum|Ta]], [[tungsten|W]], [[yttrium|Y]], [[zirconium|Zr]], etc. The conductive additive may be [[lithium chloride]], quaternary ammonium halide, or other. Some examples of metal alkoxides obtained by this technique: {{chem2|Ti(OCH(CH3)2)4}}, {{chem2|Nb2(OCH3)10}}, {{chem2|Ta2(OCH3)10}}, {{chem2|[MoO(OCH3)4]2}}, {{chem2|Re2O3(OCH3)6}}, {{chem2|Re4O6(OCH3)12}}, and {{chem2|Re4O6(OCH(CH3)2)10}}.

==Reactions==
===Hydrolysis and transesterification===
Aliphatic metal alkoxides [[hydrolysis|decompose in water]]:<ref name="hydrolysis">{{cite journal|last1 = Hanaor|first1 = Dorian A. H.|last2 = Chironi|first2 = Ilkay|last3 = Karatchevtseva|first3 = Inna|last4 = Triani|first4 = Gerry|last5 = Sorrell|first5 = Charles C.|title = Single and Mixed Phase TiO2 Powders Prepared by Excess Hydrolysis of Titanium Alkoxide|journal = Advances in Applied Ceramics|year = 2012|volume = 111|issue = 3|pages = 149–158|url = http://handle.unsw.edu.au/1959.4/51780|doi = 10.1179/1743676111Y.0000000059|arxiv = 1410.8255| bibcode=2012AdApC.111..149H |s2cid = 98265180}}</ref> 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 isopropoxide:

:{{chem2|Ti(OR)4  +  2 H2O  ->  TiO2  +  4 HOR}} 

By controlling the [[stoichiometry]] and [[steric hindrance|steric]] properties of the alkoxide, such reactions can be arrested leading to metal-oxy-alkoxides, which usually are oligonuclear.  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 (although there is an issue of terminology confusion with transesterification, a different process - see below). The position of the [[chemical equilibrium|equilibrium]] can be controlled by the [[acidity]] of the alcohol; for example [[phenols]] typically react with alkoxides to release alcohols, giving the corresponding phenoxide.{{citation needed|date=January 2025}} More simply, the alcoholysis can be controlled by selectively [[evaporation|evaporating]] the more volatile component.  In this way, ethoxides can be converted to butoxides, since ethanol (b.p. 78&nbsp;°C) is more volatile than butanol (b.p. 118&nbsp;°C).

===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:<ref>{{cite journal|first = Nataliya Y.|last = Turova|title = Metal oxoalkoxides. Synthesis, properties and structures|journal = [[Russian Chemical Reviews]]|year = 2004|volume = 73|issue = 11|pages = 1041–1064|doi = 10.1070/RC2004v073n11ABEH000855|bibcode = 2004RuCRv..73.1041T}}</ref>
:{{chem2|2 L_{n}MOR  ->  (L_{n}M)2O  +  ROR}}
Additionally, low valent metal alkoxides are susceptible to oxidation by air.{{citation needed|date=March 2024}}

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 [[bridging ligand|bridge metals]].{{citation needed|date=March 2024}}

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 {{chem2|Re4O_{6−''y''}(OCH3)_{12+''y''} }} allowed one to obtain complexes {{chem2|Re_{4−''x''}Mo_{''x''}O_{6−''y''}(OCH3)_{12+''y''} }} in the range {{nowrap|0 ≤ ''x'' ≤ 2.82}} and {{chem2|Re_{4−''x''}W_{''x''}O_{6−''y''}(OCH3)_{12+''y''} }} in the range {{nowrap|0 ≤ ''x'' ≤ 2}}.

===Insertion into M-OR bond===
Alkoxide ligands are often nucleophilic.  For example, molybdenum alkoxides undergo insertion reactions with unsaturated substrates such as [[carbon dioxide]] and [[isocyanate]]s:<ref>{{cite journal |doi=10.1021/ja00474a014|title=The Molybdenum-Molybdenum Triple Bond. 4. Insertion Reactions of Hexakis(alkoxy)dimolybdenum Compounds with Carbon Dioxide and Single-Crystal X-ray Structural Characterization of Bis(tert-butylcarbonato)tetrakis(tert-butoxy)dimolybdenum |year=1978 |last1=Chisholm |first1=Malcolm H. |last2=Cotton |first2=F. Albert |last3=Extine |first3=Michael W. |last4=Reichert |first4=William W. |journal=Journal of the American Chemical Society |volume=100 |issue=6 |pages=1727–1734 |bibcode=1978JAChS.100.1727C }}</ref>
:{{chem2|Mo2(O\st\sBu)6  + 2 CO2  ->  Mo2(O2CO\st\sBu)2(O\st\sBu)4}}

===Hydrogenolysis===
The metal-alkoxide bond is susceptible to [[hydrogenolysis]], especially for platinum metal derivatives:<ref>{{cite journal |doi=10.1021/ja205824q|title=Hydrogenolysis of Palladium(II) Hydroxide, Phenoxide, and Alkoxide Complexes |year=2011 |last1=Fulmer |first1=Gregory R. |last2=Herndon |first2=Alexandra N. |last3=Kaminsky |first3=Werner |last4=Kemp |first4=Richard A. |last5=Goldberg |first5=Karen I. |journal=Journal of the American Chemical Society |volume=133 |issue=44 |pages=17713–17726 |pmid=21932859 |bibcode=2011JAChS.13317713F }}</ref><!--could not find a review on this subject but they must exist-->
:{{chem2|L(n)M\sOR  + H2  ->  L(n)MH  +  HOR}}

==Illustrative alkoxides==
[[Image:ReOOMe.jpg|thumb|The structure of tetranuclear rhenium oxomethoxide (hydrogen atoms omitted for the sake of simplicity).<ref>{{cite journal|first1 = P. A.|last1 = Shcheglov|first2 = D. V.|last2 = Drobot|title = Rhenium Alkoxides|journal = Russian Chemical Bulletin|year = 2005|volume = 54|issue = 10|pages = 2247–2258|doi = 10.1007/s11172-006-0106-5|s2cid = 195234048}}</ref>]]
{| class="wikitable" border="1"
|-
! name
! molecular formula
! comment
|-
|[[Titanium isopropoxide]]
| Ti(O<sup>''i''</sup>Pr)<sub>4</sub>
| monomeric because of steric bulk, used in [[organic synthesis]]
|-
| [[Titanium ethoxide]]
| Ti<sub>4</sub>(OEt)<sub>16</sub>
|  for sol-gel processing of Ti oxides
|-
| [[Titanium ethoxide#Zirconium ethoxide|Zirconium ethoxide]]
| Zr<sub>4</sub>(OEt)<sub>16</sub>
| for sol-gel processing of Zr oxides
|-
| [[Vanadyl isopropoxide]]
| VO(O<sup>''i''</sup>Pr)<sub>3</sub>
| precursor to catalysts
|-
| [[Niobium ethoxide]]
| Nb<sub>2</sub>(OEt)<sub>10</sub>
| for sol-gel processing of Nb oxides
|-
| [[Tantalum ethoxide]]
| Ta<sub>2</sub>(OEt)<sub>10</sub>
| for sol-gel processing of Ta oxides
|-
| [[Hexa(tert-butoxy)dimolybdenum(III)]]
| {{chem2|Mo2(OCMe3)6}}
| metal alkoxide with a triple metal-metal bond
|-
| [[Hexa(tert-butoxy)ditungsten(III)]]
| {{chem2|W2(OCMe3)6}}
| metal alkoxide with a triple metal-metal bond
|}

==References==
<references/>

{{Coordination complexes}}
[[Category:Functional groups]]
[[Category:Bases (chemistry)]]
[[Category:Alkoxides| ]]
[[Category:Coordination chemistry]]