Chemistry Reference
In-Depth Information
1
H NMR studies show that they are static and remain in the dimeric forms in solution
at low temperatures.
3.4.6
Alkoxides of Group 5 Metals
The
1
H NMR spectrum of dimeric tantalum pentamethoxide in
n
-octane solution at
100
Ž
C gave a sharp signal at
υ
4.22 which indicated the rapid intramolecular exchange
of terminal and bridging methoxo groups.
562
The low-temperature
1
H NMR spectra
of M
2
(OMe)
10
(M D Nb, Ta) show three signals of intensity ratio 2:2:1, consistent
with a dimeric bioctahedral structure with two bridging methoxo groups. Interest-
ingly the temperature dependence of their spectra (see for example, Fig. 2.4) indicates
that terminal exchange of methoxides occurs faster than bridge - terminal exchange.
563
Similar behaviour was also observed for the other normal alkoxides of niobium and
tantalum.
The
1
H NMR spectra of the isopropoxides
562
of niobium and tantalum were found
to be both temperature (Fig. 2.5) and concentration (Fig. 2.6) dependent, consistent
with the equilibrium: M
2
OPr
i
2M
OPr
i
5
. For example the pentaisopropoxide of
tantalum in cyclohexane solution gave three septets in the range
υ
4.7 - 5.0. The intensity
of the high-field signals increased on raising the temperature at the expense of the
+
100
°
C
+
60
°
C
+
50
°
C
+40°C
+27°C
+19°C
+11°C
−11°C
−21°C
−
31
°
C
−
36
°
C
−
58
°
C
1
H NMR spectra
Figure 2.4
Variable temperature
of [Ta(OMe)
5
]
2
.
