Chemistry Reference
In-Depth Information
X
X
M
′
RO
OR
RO
O
O
O
R
O
O
M
M
R
R
R
R
R
Figure 3.5
Schematic re-
presentation of the structural
pattern of M
0
fM
2
OR
9
g
derivatives (where M is a
tetravalent metal and M
0
is
a mono- or divalent metal).
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
Hand
13
C NMR spectra of
KZr
2
OPr
i
9
(Fig. 3.5, with R D Pr
i
;MD Zr and M
0
D K) in C
6
D
6
at 25
Ž
Carein
complete accord
64
with the C
2v
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 (CH
3
, intensity ratio D 2:2:2:1:2).
In fact, this type of structure for KZr
2
OPr
i
9
was first
41
arrived at by the replace-
ability of a maximum of 5 out of 9 OPr
i
groups by bulkier OBu
t
groups when
KZr
2
OPr
i
9
was treated with excess of tertiary butanol, fractionating out the liberated
isopropanol azeotropically with benzene:
KZr
2
OPr
i
9
C 5Bu
t
OH ! KZr
2
OPr
i
4
OBu
t
5
(A)
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 KZr
2
OPr
i
4
OBu
t
5
has been further
confirmed by the instability of KZr
2
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
KZr
2
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 crys-
talline [KZr
OBu
t
5
]
n
:
1
n
[KZr
OBu
t
5
]
n
C Zr
OBu
t
4
"
!
vacuum
KZr
2
OBu
t
9
3
.
123
