2 Homometallic Alkoxides

Metal Oxo-alkoxides 389
Lithium methoxide (Eq. 5.26) was used instead of sodium methoxide because uranyl
methoxide which was insoluble in methanol could thus be separated from lithium chloride
which was soluble. The ethoxide [UO2⊲OEt⊳2⊲EtOH⊳2] was also prepared using
lithium ethoxide but the isopropoxide [UO2⊲OPr i ⊳2⊲Pr i OH⊳], being appreciably soluble
in isopropanol, was prepared using sodium isopropoxide. The triphenyl phosphine
oxide adduct of uranyl tert-butoxide [UO2⊲OBu t ⊳2⊲Ph3PO⊳2] was prepared similarly
from [UO2Cl2⊲Ph3PO⊳2] and KOBu t . 44 The vanadium(V) oxo-trialkoxides [VO⊲OR⊳3]n
were readily prepared from VOCl3, 45–47 but the reaction of [Li2O2C2⊲CF3⊳4] with
CrO2Cl2 ledtoreductiontoCr(V) in the form of [LiCr⊲O⊳2fO2C2⊲CF3⊳4g]. 48 Several
W(VI) oxo-tetra-alkoxides have been prepared from WOCl4 by reactions with NaOR
or alcohol and ammonia. 49
2.4 Preparation of Metal Oxo-alkoxides from Metal Oxides
An alternative approach to the synthesis of metal oxo-alkoxides is the reaction of the
metal oxide with an alcohol (Eq. 5.26).
MOx C 2ROH ! MOx 1⊲OR⊳2 C H2O ⊲5.26⊳
This is in effect a reversal of the hydrolysis of a metal alkoxide (Eqs 5.2 and 5.3),
although there is little evidence for the reversibility of the hydrolysis process. Nevertheless,
the reaction of some organometallic oxo-compounds with alcohols to form
alkoxo derivatives is well documented (e.g. Eq. 5.27). 1
R3GeOGeR3 C 2R 0 OH ! 2R3GeOR 0 C H2O ⊲5.27⊳
Several oxo-alkoxides of vanadium and molybdenum have been prepared from
reactions of the metal oxide or oxometallate anion with alcohols. Thus the
reaction of MoO3 with ethyleneglycol gave [MoO2⊲OC2H4OH⊳2] 50 and with 2,2 0 -
oxodiethanol gave [MoO2⊲OC2H4OC2H4O⊳]. 51 Refluxing MoO3.2H2O with methanol
in the presence of molecular sieve (4A) gave mainly [Mo2O5⊲OMe⊳2] with some
[Na4fMo8O24⊲OMe⊳4g].8MeOH. 52
The vanadium(V) oxo-trialkoxides, [VO⊲OR⊳3] (RD Et, Pr n ,Pr i ,Bu s ,Bu t ,C2H4Cl,
C2H4F, CH2CCl3) were prepared by refluxing finely divided V2O5 with alcohol and
benzene and removing the water produced (Eq. 5.28) by azeotropic distillation. 46
V2O5 C 6ROH ! 2VO⊲OR⊳3 C 3H2O ⊲5.28⊳
Zubieta et al. have obtained the interesting polyoxovanadate anions
involving V(IV): [V10O16fEtC⊲CH2O⊳3g4] 4 , [V10O13fEtC⊲CH2O⊳3g5] , 53
[V10O14⊲OH⊳2f⊲OCH2⊳3CCH2OHg4] 2 , 54 [BafV6O7⊲OH3⊳gf⊲OCH2⊳3CMeg3].3H2Oand
[Na2fV6O7gf⊲OCH2⊳3CEtg4] 55 byhydrothermalreactionsof a mixture of vanadiumoxides
[V(III) andV(V)]. Some mixed valency [V(IV), V(V)] species [V10O16f⊲OCH2⊳3CRg4] 2
(R D Et, Me) 54 and ⊲Me3NH⊳[V6O7⊲OH⊳3f⊲OCH2⊳3CMeg3] 55 were similarly obtained.
Other syntheses have utilized the solubility of quaternary ammonium salts of polyvanadates
to obtain the polyoxo-alkoxo vanadate ions [V6O13f⊲OCH2⊳3CRg2] 3 (R D Me,