2 Homometallic Alkoxides

Homometallic Alkoxides 85
1H 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 M2(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. Interestingly
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: M2⊲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
Figure 2.4 Variable temperature 1 H NMR spectra
of [Ta(OMe)5]2.