2 Homometallic Alkoxides

R
R
X X
M′
O
RO
RO
M
O
R
O
O
R
OR
M
O
O
Figure 3.5 Schematic representation
of the structural
pattern of M 0 fM2⊲OR⊳9g
derivatives (where M is a
tetravalent metal and M 0 is
a mono- or divalent metal).
Heterometallic Alkoxides 217
In view of the availability of actual X-ray structure elucidations (Chapter 4), this
brief description will be focussed mainly on the information available from other
physico-chemical characteristics. For example, the 1 H and 13 C NMR spectra of
KZr2⊲OPr i ⊳9 (Fig. 3.5, with R D Pr i ;MD Zr and M 0 D K) in C6D6 at 25 Ž Carein
complete accord 64 with the C2v symmetry of the suggested structure: 1 H: υ 1.2–1.6
(doublets, intensity ratio D 1:2:2:2:2⊳; 13 Cf 1 Hg: υ 68–71 (CH, intensity ratio D 4:1:2:2)
and υ 27–28.5 (CH3, intensity ratio D 2:2:2:1:2).
In fact, this type of structure for KZr2⊲OPr i ⊳9 was first 41 arrived at by the replaceability
of a maximum of 5 out of 9 OPr i groups by bulkier OBu t groups when
KZr2⊲OPr i ⊳9 was treated with excess of tertiary butanol, fractionating out the liberated
isopropanol azeotropically with benzene:
KZr2⊲OPr i ⊳9 C 5Bu t OH ! KZr2⊲OPr i ⊳4⊲OBu t ⊳5
(A)
R
R
C 5Pr i OH " ⊲3.122⊳
The formation of the final product (A) (characterized by careful analysis of metal,
isopropoxide, and tertiary butoxide contents separately) was explained on the basis
that the replacement of four terminal isopropoxide groups on two zirconium atoms
and a bridging OBu t group between them sterically hindered further coordination with
another tertiary butyl alcohol molecule. In view of the repeated confirmation of the
plausible structure(s) in a large number of such cases (Section 1), a brief account of the
revealing parameters will be included particularly in cases for which X-ray structural
data are not yet available.
In fact, the formation of the final product KZr2⊲OPr i ⊳4⊲OBu t ⊳5 has been further
confirmed by the instability of KZr2⊲OBu t ⊳9, which could be isolated 41,182 by the
interaction of KOBu t and Zr⊲OBu t ⊳4 in a 1:2 molar reaction; the reaction did result
in a quantitative yield of recrystallizable white powder corresponding in analysis to
KZr2⊲OBu t ⊳9, which was, however, found to disproportionate (Eq. 3.123) into the more
volatile Zr⊲OBu t ⊳4 (confirmed 182 by thermogravimetric analysis) leaving behind crystalline
[KZr⊲OBu t ⊳5]n:
KZr2⊲OBu t ⊳9
1
!
vacuum
1
n [KZr⊲OBut ⊳5]n C Zr⊲OBu t ⊳4 " ⊲3.123⊳