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Studies into the constitution of aluminium isopropoxide
J. inorg, nucl. Chem., 1969, Vol. 31, pp. 1609 to 1613. Pergamon Press.
Printed in Great Britain
STUDIES INTO THE CONSTITUTION ALUMINIUM ISOPROPOXIDE
OF
J. G. O L I V E R , P. K. P H I L L I P S and 1. ,l. W O R R A L L Department of Chemistry, The University, Lancaster
(Received 6 August 1968) Abstract- Freshly prepared crystalline aluminium isopropoxide contains a mixture of trimeric and tetrameric molecules. Pure tetramer is readily obtained from this mixture by removing the trimer with pyridine. The p.m.r, spectra of pyridine solutions of the trimer show that strong solvent interactions occur, and that another form, possibly the dimer, is produced at high pyridine concentrations, The structures of the various forms are discussed. INTRODUCTION
THE FACTORSwhich determine the structures of aluminium alkoxy compounds are complex. The tendency for the metal to achieve its maximum coordination number, which results in polymerised stystems, is opposed by steric factors, which increase as the size o f the alkoxy group increase. It appears that for aluminium isopropoxide these two effects are finely balanced. Molecular weight measurements[l] have shown that solid isopropoxide in benzene solution is tetrameric and p.m.r, spectroscopy of solutions of the solid in various solvents supports structure I. RO
OR
R \A~ / \/t O
R
].A,
i Ai
fIR
i
RO/ ~6~-0 ~b R RO/
~OR
(I)
If the solid is melted, heated at 200 ° and then rapidly supercooled the molecular weight in benzene solution corresponds to the trimer which is thought to have structure I 1 [ 1].
RO I/OR
I
R 1. V . J . Shiner, D. Whittaker and V. P. Fernandez, J. Am. chem. Soc.85, 2318 (1963). 1609
1610
J.G.
OLIVER, P. K. P H I L L I P S and I. J. W O R R A L L
On standing the trimer slowly changes to the tetramer. Shiner [ 1] has demonstrated the effectiveness of n.m.r, in alkoxide structure determinations and has investigated aluminium isopropoxide in some detail. The work described in this paper is the first part of an investigation into the structures of group 3 alkoxides. RESULTS AND DISCUSSION
Freshly distilled aluminium isopropoxide was crystallised a t - 2 0 °. Crystallisation was complete after about 48 hr. The p.m.r, spectrum of the benzene solution of this solid was determined and the methyl group part of the spectrum is shown in Fig. 1. (The H C O part of all spectra is complex and is not shown).
80179,78 84i
I
Fig. I. The p.m.r, spectrum of freshly crystallised aluminium isopropoxide in benzene (C.P.S. from internal T.M.S.).
Excess pyridine was added to the solid which partly dissolved. The undissolved solid was filtered, dissolved in benzene and the p.m.r, spectra of this and the pyridine filtrate were recorded.
(a) Benzene solution of the solid The methyl part of the spectrum is shown in Fig. 2. It consists of three doublets with overlap between components of the high field and middle doublet. This is essentially the spectrum observed by Shiner[l] and corresponds to that of pure tetramer-structure I. Shiner has interpreted the high field doublet in terms of the terminal isopropoxy methyl groups and the two lower field doublets to nonequivalent methyls on fridging isopropoxy groups. The nonequivalence is ascribed to steric effects. We have recently suggested the alternative explanation of this phenomenon in terms of asymmetry of the bridging oxygen atoms[2]. The preparative method readily enables the isolation of pure tetramer from freshly crystallised solid. (b) Pyridine solutions of the extract The p.m.r, spectrum is shown in Fig. 3. It consists principally of two sharp peaks and a broad shoulder. The two very small low field peaks at 102 and 96 2. T. N. Huckerby, J. G. Oliver and 1. J. Worrall, Chem. Comm. 918 (1968).
Studies into the constitution of aluminium isopropoxide
1611
80
74
Fig. 2. The p.m.r, spectrum of tetrameric aluminium isopropoxide.
c.p.s, are probably due to small amounts of dissolved tetramer. In an attempt to identify the major components in the filtrate pure trimer was made by heating isopropoxide to 200 °, cooling rapidly, and making separate solutions in benzene and pyridine. The spectrum of the benzene solution corresponded to that previously described[l] and consisted of a single doublet with peaks at 84 and 78 c.p.s. The pyridine solution of this trimer was almost identical to that shown in Fig. 3. The original solid thus consisted of a mixture of trimer and tetramer. Further studies were carried out on the trimer by varying the amounts of pyridine in benzene solutions. The spectra varied from that obtained for the pure pyridine solution (Fig. 3) to that illustrated in Fig. 4 which is the spectrum of a benzene solution of trimer to which a small amount of pyridine has been added. The doublet spectrum of the trimer in benzene has been explained by Shiner in terms of structure It in which rapid exchange occurs between the terminal and bridging isopropoxy groups by the intramolecular process. R
RO-~AI/O~AI/OR
Ro-'[
i-oR "
,, ROYAl/(~ ~ ...OR
I '0/All'oR R
For this mechanism to occur it is necessary for aluminium to 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. In pyridine solutions strong interactions occur. When pyridine alone is the
1612
J. G. OLIVER, P. K. PHILLIPS and I. J. WORRALL 80
74
Fig. 3. The p.m.r, spectrum of trimeric isopropoxide in pyridine. solvent there are two principal peaks at 74 and 80 c.p.s, which on dilution with benzene give two further peaks. T h e trimer doublet is easily identified at 84 and 78 c.p.s. T h e other pair at 80 and 74 c.p.s, we suggest are due to dimer molecules, which are apparently stabilised in the polar solvent. T h e r e are several alternative explanations which involve postulating the formation of alkoxide pyridine complexes. 'It is well k n o w n h o w e v e r that bridging alkoxy groups are not susceptible to attack by ligands such as pyridine[3], so that complete b r e a k d o w n of the cyclic structure is unlikely. It is possible h o w e v e r that w e a k complexing with pyridine m a y occur, in which each aluminium in the cyclic a r r a n g e m e n t would b e c o m e five coordinate. This would effectively slow down the intramolecular exchange rate resulting in the e m e r g e n c e of additional peaks in the spectrum. EXPERIMENTAL Commercial aluminium isopropoxide was purified by vacuum distillation. The solid samples were obtained by crystallisation of the distillate at --20° in a refrigerator. After 48 hr hard white crystals were obtained. Trimer solutions were obtained by heating aluminium isopropoxide in tubes, sealed 3. D. C. Bradley, MetalAlkoxides, In Progress in Inorganic Chemistry Vol. 2, p. 334. Interscience, New York 0960).
Studies into the constitution of aluminium isopropoxide
1613
84, M80'78
Fig. 4. T h e p.m.r, spectrum of a benzene solution containing trimeric isopropoxide and a small a m o u n t of pyridine. under v a c u u m , for 30 min. T h e tubes were cooled rapidly, broken open in a dry box and dissolved in the required solvent. All manipulations were carried out in a conventional v a c u u m line or a nitrogen filled dry box. P.m.r. spectra were recorded at 60 Mc]s. on a Varian A 6 0 spectrometer, All solutions contained the internal standard tetramethylsilane.
Acknowledgement-We thank
S.R.C. for an award to J. G. O. and Dr. T. N. H u c k e r b y for operating the n.m.r, spectrometer throughout this work.
Printed in Great Britain
STUDIES INTO THE CONSTITUTION ALUMINIUM ISOPROPOXIDE
OF
J. G. O L I V E R , P. K. P H I L L I P S and 1. ,l. W O R R A L L Department of Chemistry, The University, Lancaster
(Received 6 August 1968) Abstract- Freshly prepared crystalline aluminium isopropoxide contains a mixture of trimeric and tetrameric molecules. Pure tetramer is readily obtained from this mixture by removing the trimer with pyridine. The p.m.r, spectra of pyridine solutions of the trimer show that strong solvent interactions occur, and that another form, possibly the dimer, is produced at high pyridine concentrations, The structures of the various forms are discussed. INTRODUCTION
THE FACTORSwhich determine the structures of aluminium alkoxy compounds are complex. The tendency for the metal to achieve its maximum coordination number, which results in polymerised stystems, is opposed by steric factors, which increase as the size o f the alkoxy group increase. It appears that for aluminium isopropoxide these two effects are finely balanced. Molecular weight measurements[l] have shown that solid isopropoxide in benzene solution is tetrameric and p.m.r, spectroscopy of solutions of the solid in various solvents supports structure I. RO
OR
R \A~ / \/t O
R
].A,
i Ai
fIR
i
RO/ ~6~-0 ~b R RO/
~OR
(I)
If the solid is melted, heated at 200 ° and then rapidly supercooled the molecular weight in benzene solution corresponds to the trimer which is thought to have structure I 1 [ 1].
RO I/OR
I
R 1. V . J . Shiner, D. Whittaker and V. P. Fernandez, J. Am. chem. Soc.85, 2318 (1963). 1609
1610
J.G.
OLIVER, P. K. P H I L L I P S and I. J. W O R R A L L
On standing the trimer slowly changes to the tetramer. Shiner [ 1] has demonstrated the effectiveness of n.m.r, in alkoxide structure determinations and has investigated aluminium isopropoxide in some detail. The work described in this paper is the first part of an investigation into the structures of group 3 alkoxides. RESULTS AND DISCUSSION
Freshly distilled aluminium isopropoxide was crystallised a t - 2 0 °. Crystallisation was complete after about 48 hr. The p.m.r, spectrum of the benzene solution of this solid was determined and the methyl group part of the spectrum is shown in Fig. 1. (The H C O part of all spectra is complex and is not shown).
80179,78 84i
I
Fig. I. The p.m.r, spectrum of freshly crystallised aluminium isopropoxide in benzene (C.P.S. from internal T.M.S.).
Excess pyridine was added to the solid which partly dissolved. The undissolved solid was filtered, dissolved in benzene and the p.m.r, spectra of this and the pyridine filtrate were recorded.
(a) Benzene solution of the solid The methyl part of the spectrum is shown in Fig. 2. It consists of three doublets with overlap between components of the high field and middle doublet. This is essentially the spectrum observed by Shiner[l] and corresponds to that of pure tetramer-structure I. Shiner has interpreted the high field doublet in terms of the terminal isopropoxy methyl groups and the two lower field doublets to nonequivalent methyls on fridging isopropoxy groups. The nonequivalence is ascribed to steric effects. We have recently suggested the alternative explanation of this phenomenon in terms of asymmetry of the bridging oxygen atoms[2]. The preparative method readily enables the isolation of pure tetramer from freshly crystallised solid. (b) Pyridine solutions of the extract The p.m.r, spectrum is shown in Fig. 3. It consists principally of two sharp peaks and a broad shoulder. The two very small low field peaks at 102 and 96 2. T. N. Huckerby, J. G. Oliver and 1. J. Worrall, Chem. Comm. 918 (1968).
Studies into the constitution of aluminium isopropoxide
1611
80
74
Fig. 2. The p.m.r, spectrum of tetrameric aluminium isopropoxide.
c.p.s, are probably due to small amounts of dissolved tetramer. In an attempt to identify the major components in the filtrate pure trimer was made by heating isopropoxide to 200 °, cooling rapidly, and making separate solutions in benzene and pyridine. The spectrum of the benzene solution corresponded to that previously described[l] and consisted of a single doublet with peaks at 84 and 78 c.p.s. The pyridine solution of this trimer was almost identical to that shown in Fig. 3. The original solid thus consisted of a mixture of trimer and tetramer. Further studies were carried out on the trimer by varying the amounts of pyridine in benzene solutions. The spectra varied from that obtained for the pure pyridine solution (Fig. 3) to that illustrated in Fig. 4 which is the spectrum of a benzene solution of trimer to which a small amount of pyridine has been added. The doublet spectrum of the trimer in benzene has been explained by Shiner in terms of structure It in which rapid exchange occurs between the terminal and bridging isopropoxy groups by the intramolecular process. R
RO-~AI/O~AI/OR
Ro-'[
i-oR "
,, ROYAl/(~ ~ ...OR
I '0/All'oR R
For this mechanism to occur it is necessary for aluminium to 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. In pyridine solutions strong interactions occur. When pyridine alone is the
1612
J. G. OLIVER, P. K. PHILLIPS and I. J. WORRALL 80
74
Fig. 3. The p.m.r, spectrum of trimeric isopropoxide in pyridine. solvent there are two principal peaks at 74 and 80 c.p.s, which on dilution with benzene give two further peaks. T h e trimer doublet is easily identified at 84 and 78 c.p.s. T h e other pair at 80 and 74 c.p.s, we suggest are due to dimer molecules, which are apparently stabilised in the polar solvent. T h e r e are several alternative explanations which involve postulating the formation of alkoxide pyridine complexes. 'It is well k n o w n h o w e v e r that bridging alkoxy groups are not susceptible to attack by ligands such as pyridine[3], so that complete b r e a k d o w n of the cyclic structure is unlikely. It is possible h o w e v e r that w e a k complexing with pyridine m a y occur, in which each aluminium in the cyclic a r r a n g e m e n t would b e c o m e five coordinate. This would effectively slow down the intramolecular exchange rate resulting in the e m e r g e n c e of additional peaks in the spectrum. EXPERIMENTAL Commercial aluminium isopropoxide was purified by vacuum distillation. The solid samples were obtained by crystallisation of the distillate at --20° in a refrigerator. After 48 hr hard white crystals were obtained. Trimer solutions were obtained by heating aluminium isopropoxide in tubes, sealed 3. D. C. Bradley, MetalAlkoxides, In Progress in Inorganic Chemistry Vol. 2, p. 334. Interscience, New York 0960).
Studies into the constitution of aluminium isopropoxide
1613
84, M80'78
Fig. 4. T h e p.m.r, spectrum of a benzene solution containing trimeric isopropoxide and a small a m o u n t of pyridine. under v a c u u m , for 30 min. T h e tubes were cooled rapidly, broken open in a dry box and dissolved in the required solvent. All manipulations were carried out in a conventional v a c u u m line or a nitrogen filled dry box. P.m.r. spectra were recorded at 60 Mc]s. on a Varian A 6 0 spectrometer, All solutions contained the internal standard tetramethylsilane.
Acknowledgement-We thank
S.R.C. for an award to J. G. O. and Dr. T. N. H u c k e r b y for operating the n.m.r, spectrometer throughout this work.