2 Homometallic Alkoxides

390 Alkoxo and Aryloxo Derivatives of Metals
OH, NO2) (Eq. 5.29): 56
2H3V10O 3 28 C 6RC⊲CH2OH⊳3 ! 3V6O13f⊲OCH2⊳3CRg 2 C 12H2O C V2O5
⊲5.29⊳
The methoxo polyvanadate [V6O12⊲OMe⊳7] has been prepared by a similar method. 57
Using organic-soluble tetrabutyl ammonium salts of [Mo2O7] 2 and [Mo8O26] 4
58 2 the mixed alkoxo polyoxomolybdate species [Mo8O20⊲OMe⊳4f⊲OCH2⊳3CMeg2] and
[Mo3O6⊲OMe⊳f⊲OCH2⊳3CMeg2] 59
were obtained. The interesting heteropolyalkoxo
metallate anion [V2Mo2O8⊲OMe⊳2f⊲OCH2⊳3C⊲CH2OH⊳g2] 2 was prepared by the reaction
of [V2O2Cl2f⊲OCH2⊳2C⊲CH2OH⊳2g2] with [Mo2O7] 2 . 60 The oxo vanadatrane
[OV⊲OC2H4⊳3N] was prepared by treating ammonium metavanadate with triethanolamine.
61
3 CHEMICAL REACTIVITY OF OXO-ALKOXIDES
The further hydrolysis of metal oxo-alkoxides to produce metal oxides is of considerable
importance in the sol–gel process.
Klemperer and co-workers 9–12 have shown that with increasing oxo content titanium
oxo-alkoxides become increasingly resistant to hydrolysis in alcoholic solutions,
although gel formation occurs readily by hydrolysis in nonalcoholic solvents. 11
Alcohol exchange of metal oxo-alkoxides has also been studied. Thus the molybdenum
oxo-alkoxides [MoO2⊲OR⊳2] (RD Pr i ,CH2Bu t ) were obtained from the tertiary
butoxo compound (Eq. 5.30). 34
MoO2⊲OBu t ⊳2 C 2ROH ! MoO2⊲OR⊳2 C 2Bu t OH ⊲5.30⊳
Of considerable interest is the selective alkoxo exchange shown by the more hydrolysed
titanium oxo-alkoxides. The two isomeric forms of [Ti12O16⊲OPr i ⊳16] both reacted with
ethanol (Eq. 5.31) to form isomers of [Ti12O16⊲OEt⊳6⊲OPr i ⊳10] by exchange of terminal
isopropoxo groups bonded to the 6 five-coordinated Ti atoms in each molecule. 11
Ti12O16⊲OPr i ⊳16 C 6EtOH ! Ti12O16⊲OEt⊳6⊲OPr i ⊳10 C 6Pr i OH ⊲5.31⊳
Similarly [Ti11O13⊲OPr i ⊳18] was converted to [Ti11O13⊲OEt⊳5⊲OPr i ⊳13] 11 and the giant
oxo-tert-butoxide [Ti18O28H⊲OBu t ⊳17] also exchanged the 5 terminal Bu t O groups
bonded to five-coordinated titanium with tertiary amyl alcohol. 12 These reactions
demonstrated the robust nature of the metal–oxo core structure in these compounds.
Alcohol exchange of the pentanuclear oxo-alkoxides [M5O⊲OPr i ⊳13] (M D Gd,
Pr) led to reorganization of the M5O14 framework and formation of hexanuclear
[Gd6⊲ 4-O⊳⊲OC2H4OMe⊳16] 62 and octanuclear [Pr8⊲ 4-O⊳4⊲OC2H4OMe⊳16⊲Me3PO⊳2] 63
species. Some surprising results were obtained in alcohol exchange reactions involving
the sparingly soluble [UO2⊲OMe⊳2.⊲MeOH⊳]. 43 With isopropanol a disproportionation
occurred with formation of insoluble [U2O5⊲OPr i ⊳2⊲Pr i OH⊳2] and soluble
[UO⊲OPr i ⊳4⊲Pr i OH⊳]. The latter compound disproportionated on heating in vacuo
producing the volatile hexa-alkoxide U⊲OPr i ⊳6. Similar results were obtained using tertbutanol
and the hexa-alkoxide U⊲OBu t ⊳6 was remarkably air stable. Other work 64 showed
that in the reaction of UO2Cl2 with KOBu t in THF the soluble trinuclear complex