1. J. Worrall The University of Lancaster Lancaster, England
The TWO Forms of Aluminium lsopropoxide An nmr experiment
I t is well known that aluminium isopropoxide exists in at least two different forms. Molecular weight determinations1 have shown that in benzene solution "aged" solid is tetrameric while freshly distilled molt,en isopropoxide is trimeric. PMR spectroscopy fully supports I and I1 for the structures of tetramer and trimer, respectively.
are shown in Figure 2 and correspond to that of pure tetramer.' Thespectrum consists of three doublets a t 102, 96; 85, 79; and 80, 74 cps from the internal standard. There is some overlap between components of the high field doublet and themiddledoohlet. All six peaks may be clearly seen if the spectrumis repeated on a dioxane solntion. Trziner. The most convenient method of preparation of trimer is to heat molten isopropoxide to 200DCfor about 10 min in a sealed tube, rapidly sopercool, and then make up in s henzene solution. The pmr ~pectromof the solution consists of s. doublet, with peaks a t 84 and 78 cps from the internal stsndard. I t is apparent that bhe original solid contained a mixtnre of trimer and tetramer. This may he confirmed by comparing the pyridine ext,ract with a solution of the trimer in pyridine.' ~
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Discussion of Spectra
It would be anticipated that structure I would give two doublets of equal intensity, since there are equal
Freshly crystallized solid contains large amounts of both trimer and tetramer.? The pmr spectra of solutions of these forms illustrate sevcral fundamental points and the subject forms the basis of an excellent undergraduate experiment. The pure forms are easily prepared. Telranmr. Here use is made of soluhilit,y studies1 which show that the trimer is very soluble in pyridine while the tetramer is only slightly soluble. Commercial aluminiwn isopropoxide is distilled in vneuo a t 140'C using conventional apparatm. The s~~percooled melt is p,lt aside in a refrigerator a t -20'C. Crystallization is complet,e after ahout, 48 hr. Part of the solid is dissolved in benzene. The methyl prot,on peaks in the 60 Mc/sec pmr spectrum of this sohtion are shown in Figure 1. The spectrum is complex and contains peaks a t 102, 96, 84, 80, 79, 78, and 74 eps from the internal standard tetramet,hylzilane. The solid is then washed with pyridine and the spectrum repeated. The methyl proton peaks
'SHINER, V. J., WHITTAXER, D., A N D FICRNANDEZ, V. P., J . Amer. Chem. Soe., 85,2318 (1963). OLIVER.J. G.. AND WORR.LLL. I. J.. J. Inom. Nuclear Chem..
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Figure 1. The pmr spectrum of freshly cryrtdlirod olvminivm iropmpoiide in benzene.
I t is seen that one component of the middle doublet (85 cps) is similar in size to the components of the low field doublet and so it may be deduced that the intensity ratios are 1:1:2. The presence of two lower field doublets suggests that the two methyl groups attached to the bridging isopropoxy groups are nonequivalent. This has been explained in terms of steric hindrance of these methyl groups, which restricts their rotation, resulting in each having a different environment.' Another possible explanation is that the nonequivalence arises from the asymetry of the bridging oxygen atoms.3 The spectrum expected for structure I1 would he two doublets with the lower field doublet half the intensity of the high field doublet. The fact that only one is observed suggests that rapid exchange occurs between terminal and bridging alkoxy groups. I n fact, by lowering the temperature and hence retarding the exchange rate the two predicted doublets are obtained. The interesting question is why exchange occurs in the tetramer and not in the trimer at normal temperatures? This has been satisfactorily answered by Shiner' who showed that the exchange occurred through an intramolecular process and proposed the following mechanism R
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Figure 2.
The pmr spectrum of ietromeric aluminium iropropoxide.
numbers of bridging and terminal isopropoxy groups, with the terminal isopropoxy methyl doublet appearing a t a highcr ficld than the bridging isopropoxy doublet. In fact the spectrum in dioxane clearly shows three doublets with intensity ratios 1: 1:2.' I n benzene solution the spectrum (Fig. 2) cannot be completely i u t o grated due to partial overlap, but they may be easily estimated. The ratio of intensities of the low field doublet (102,96 cps) to the sum of the two higher field doublets (85,79; S0,74 cps) may be measured and is 1: 3.
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For this mechanism to occur it is necessary for aluminium to readily increase its coordination number. Exchange does not occur in the tetramer since the central aluminium is already six coordinate and further bridging would give the energetically unfavourable seven coordinate arrangement. All solutions were prepared in a nitrogen filled dry box and the pmr spectra were recorded on a Varian A60 spectrometer a t GO Mc/sec. All solutions contained the internal standard TMS. IIuc~muw,T.N., OLIVER,J. G.,A N D WORRALL, I . J., Chem. Comm., 918, (1968).
Volume 46, Number 8, August 1969
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