2 Homometallic Alkoxides

90 Alkoxo and Aryloxo Derivatives of Metals
1 H NMR studies on solution of tetrameric [Al(OPr i )3]4 by Shiner et al. 489 and
Mehrotra and Mehrotra 578 show three doublets for the CH3 protons in a 1:1:2 ratio
(Fig. 2.7), as the methyl groups in the bridging isopropoxo ligands are nonequivalent.
These observations are consistent with structure (2-IX) which has been confirmed by
X-ray crystallography 579,580 (when R D Pr i ).
In order to study the structure of aluminium isopropoxide in detail, Worrall
et al. 490,492,581 measured the 1 H NMR spectra of aluminium isopropoxide under
different experimental conditions. The freshly distilled aluminium isopropoxide on
crystallization at 20 Ž C was dissolved in benzene and the 1 H NMR spectrum of this
solution was almost identical to that obtained by Shiner et al. 489 consisting of three sets
of doublets, thus confirming yet again the proposed tetrameric structure for aluminium
isopropoxide. However, these workers assumed the nonequivalence of methyl protons
to be due to the asymmetric nature of the molecule which has D3 symmetry and in
principle is optically active.
The results of Shiner et al. 489 and those of Mehrotra and Mehrotra 578 are presented in
Table 2.21. In the case of the 100 MHz spectrum, two distinct sets of septets with equal
intensity ratio were observed; these peaks could be assigned to the methine protons
of the terminal and bridging isopropoxo groups. Following Worrall et al. 490,492 the
explanation of the splitting of methyl protons in the spectra of tetrameric aluminium
isopropoxide could be sought in terms of magnetic nonequivalence of the two methyls
on the same isopropoxide group due to asymmetry, but then the corresponding methine
protons on the isopropoxide groups in similar environment should be identical. This
general conclusion was confirmed by the appearance of only two sets of methine proton
Figure 2.7 1 H NMR
spectrum of aluminium
isopropoxide in CDCl3 at
60 MHz.