Chemistry Reference
In-Depth Information
analogous [HfAl
2
OPr
i
10
] shown in 1997
77
to conform in general to the structures
suggested above.
3.3.2
Hexaalkoxoniobate and -tantalate Derivatives
Compared to the tetra-alkoxoaluminates, the corresponding hexa-alkoxoniobate and
-tantalate derivatives of most of the metals are less stable. Of these two, the niobate
products tend to disproportionate more readily; this is reflected in the conductivity
curves obtained
211
in the titrations of niobium and tantalum pentaisopropoxide with
alkali (Na, K) isopropoxides in isopropyl alcohol (see Fig. 3.2).
All the MM
0
OR
6
derivatives (M D Li, Na, K; M
0
D Nb, Ta; R D Me, Et, Pr
i
,
Bu
t
)
35
,
36
,
212
,
213
are white solids soluble in the parent alcohols (solubility decreasing
from Li to K). If attempts are made to purify them by distillation/sublimation under
reduced pressure, the niobium derivatives tend to disproportionate more readily, yield-
ing the corresponding volatile niobium alkoxide and alkali alkoxide residues. By
contrast, MTa
OR
6
derivatives (M D Na, K; R D Me, Et) are stable up to ¾ 320
Ž
C/
0.1 mm (the boiling points of Ta
OMe
5
and Ta
OEt
5
are 180
Ž
C/10 mm and 202
Ž
C/
10 mm, respectively).
Hexaisopropoxotantalates of Li, Na, K, and Cs tend to sublime around 220
Ž
C under
0.1 mm pressure and all the corresponding tertiary butoxy analogues, MfM
0
OBu
t
6
g
(M D Li, Na, K, Cs; M
0
D Nb, Ta) can be volatilized at lower temperatures around
110 - 120
Ž
C/0.1 mm. The thermal stability of the MTa
OR
6
derivatives appears to
follow the orders: Li
>
Na
>
KandOBu
t
>
OPr
i
>
OEt ³ OMe. Molecular weight
studies also appear to indicate the same pattern.
30
IV
20
II
III
I
10
0
0.4
0.8
1.2
1.6
2.0
2.4
M(OPr
i
)
M′(OPr
i
)
5
Molar ratio
Figure 3.2
Titration between
M
0
OPr
i
5
and M
OPr
i
. Curve I:
(ž)NaOP
i
M
/4.77
vs
Ta
OPr
i
5
M
/93.74; Curve II: (ð) KOPr
i
M
/4.96
vs
Ta
OPr
i
5
M
/93.74;
Curve
III:
i
Nb
OPr
i
5
(Ž)NaOP
M
/4.77
vs
KOPr
i
M
/94.24;
Curve
IV:
(
)
M
/4.98
vs
Nb
OPr
i
5
M
/94.91.
