2 Homometallic Alkoxides

Heterometallic Alkoxides 189
Ti⊲OPr i ⊳4 1:2 ! 1
2 [BaTi2⊲OPr i ⊳10]2 81 (3.18)
Ti⊲OPr i ⊳4 1:3
Ba⊲OPr i ⊳2 ! [Ti⊲OPr i ⊳5BafTi2⊲OPr i ⊳9g] 81 (3.19)
M⊲OBu t ⊳2 C Zr⊲OBu t ⊳4
where M D Sn(II), Pb(II). 82
Ti⊲OPr i ⊳4 1:4 ! [BafTi2⊲OPr i ⊳9g2] 81 (3.20)
ether
! MZr⊲OBu t ⊳6
Ba⊲OBu t ⊳2 C 2Zr⊲OBu t ⊳4 ! BaZr2⊲OBu t ⊳10 83
2Ba⊲OCEt3⊳2 C 4Cu⊲OCEt3⊳ ! Ba2Cu4⊲OCEt3⊳8 84
2.1.2 Reactions of Magnesium and Alkaline Earth Metals with Alcohols in the
Presence of Other Metal (Al, Zr, Nb, etc.) Alkoxides (B)
⊲3.21⊳
⊲3.22⊳
⊲3.23⊳
Reactions of alkaline earth metals and magnesium in alcohols is rather slow even in the
presence of catalysts such as HgCl2 and/or I2; this may be due to the low solubility of
their bisalkoxides, fM⊲OR⊳2gn in alcohols, resulting from their highly associated nature
(large value of n). The dissolution of these bivalent metals appears to be considerably
facilitated 35,36 by the presence of metal alkoxides like Al(OR)3, Zr(OR)4, Nb(OR)5, or
Ta(OR)5 (Eqs 3.24–3.27):
M C 2ROH C 2M 0 ⊲OR⊳3
M C 2ROH C 4M 00 ⊲OR⊳4
M C 2ROH C 2M 000 ⊲OR⊳5
M C 2ROH slow
! 1
n [M⊲OR⊳2]n C H2 "
(low solubility)
facile
! MfM 0 ⊲OR⊳4g2 C H2 " 85
facile
! MfM 00
2 ⊲OR⊳9g2 C H2 " 86
facile
! MfM 000 ⊲OR⊳6g2 C H2 " 87–89
⊲3.24⊳
⊲3.25⊳
⊲3.26⊳
⊲3.27⊳
where M D Mg, Ca, Sr, Ba; R D Et, Pr; M 0 D Al, Ga; M 00 D Zr, Hf; M 000 D Nb, Ta.
It has been suggested 16 that the reactivity of the alcohols may be enhanced by the
formation of adducts with metal alkoxides, R O M′(OR) 3 , as a result of which
H
the alcoholic proton is rendered more labile and hence, more reactive as a result of the
electron drift shown above.
In the case of magnesium, the liquid product MgfAl⊲OPri⊳4g2 showed90 a tendency
to yield an X-ray characterizable crystalline product, Mg2Al3⊲OPri⊳13 on standing for
a few weeks (Eq. 3.28):
[MgfAl⊲OPr i ⊳4g2]n
! n
2 Mg2Al3⊲OPr i ⊳13 C n
2 Al⊲OPri ⊳3
⊲3.28⊳