[Al(OiPr)2(β-ketoesterate)]2 and Al(β-ketoesterate)3 (β-ketoesterate = methyl, ethyl, iso-propyl, tert-butyl, allyl and 2-(methacryloyloxy)ethyl acetoacetate) were prepared by reaction of [Al(OiPr)3]4 with the corresponding β-ketoesters. Al(β-ketoesterate)3 derivatives were exclusively formed at room temperature, whereas elevated reaction temperatures, causing thermal de-oligomerization of [Al(OiPr)3]4, were necessary for the formation of [Al(OiPr)2(β-ketoesterate)]2. All compounds were characterized by NMR spectroscopy, and [Al(OiPr)2(tert-butyl acetoacetate)]2 by a single crystal structure analysis. The [Al(OiPr)2(β-ketoesterate)]2 derivatives are asymmetrically substituted dimers with one octahedrally and one tetrahedrally substituted aluminum atom, bridged by two iso-propoxo groups, whereas the Al(β-ketoesterate)3 derivatives are monomers with octahedrally coordinated aluminum. Transesterification as a possible side reaction was only observed at elevated temperatures for Al(tert-butyl acetoacetate)3 in the presence of liberated iso-propanol.
Hinweise
Dedicated to David Avnir on the occasion of his 60th birthday.
1 Introduction
One problem in the sol–gel chemistry of metal alkoxides is their high reaction rates during hydrolysis and condensation, compared to silicon alkoxides, and the resulting tendency to form precipitates instead of gels. Lowering of their reactivity can be achieved by substitution of one or more alkoxo groups by bi- or even multidentate ligands which are bound more strongly to the metal as a consequence of the chelate effect. Since the number of hydrolysable alkoxo groups is thus reduced and additional coordination sites at the metal centers are blocked, the rate of hydrolysis and the degree of cross-linking in the formed network are lowered [1].
Another consequence of the higher Lewis acidity of metal alkoxides is their tendency to form oligomers [M(OR)x]n via alkoxo bridges. The chemical and physical properties of the alkoxides strongly depend on the degree of oligomerization, which in turn is a result of the electronic and steric properties of the metal center and the alkoxo group. For example, aluminum iso-propoxide is tetrameric [Al(OiPr)3]4 [2, 3], whereas the tert-butoxide [Al(OtBu)3]2 is dimeric [4] in the solid state as well as in solution at room temperature. This example shows the inverse correlation between steric demand of the alkoxo group and degree of oligomerization. This tendency of metal alkoxides to oligomerize is retained after modification with bidentate organic ligands. Thus, aluminum alkoxides and siloxides are known to form mono-, di-, tri-, or oligomeric species upon modification with β-diketones [5, 6].
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Modification of metal alkoxides with bidentate organic ligands also opens the possibility to introduce additional functionalities by using ligands bearing a functional organic group, e.g. polymerizable groups for the formation of class II inorganic–organic hybrid polymers. As a matter of fact, allyl (aaa-H) and 2-(methacryloyloxy)ethyl acetoacetate (meaa-H) (see Fig. 1) have often been used in sol–gel chemistry for this purpose for various metal alkoxides. Own previous work was mainly focused on the modification and functionalization of titanium and zirconium alkoxides by nitrogen-containing ligands [7]. In this work, we expand this work on aluminum alkoxides modified by reaction with β-ketoesters. Whereas the influence of the alkoxo or siloxo group on the oligomerization and structure of acac-substituted (acac = acetylacetonate) aluminum alkoxides or siloxides was studied in detail [5], this is not the case for variations of the ester OR groups in β-ketoesterate derivatives (β-ketoesterate = anion of β-ketoester). Since the bonding of β-ketoesterate ligands to the metal center is inherently asymmetrical (contrary to β-diketonate derivatives), because of the different electronic influence of the alkyl and OR substituents at the carbonyl carbon atoms, differences can be anticipated. In this article we report the results of our investigations on the formation and stability of aluminum iso-propoxide derivatives modified by methyl (meac-H), ethyl (etac-H), iso-propyl (iprac-H) and tert-butyl (tbuac-H) acetoacetate as model ligands, as well as allyl (aaa-H) and 2-(methacryloyloxy)ethyl (meaa-H) acetoacetate as ligands with polymerizable groups (Fig. 1), including investigation of transesterification as possible side reaction. The influence of variation the ester group is reported as well as differences in coordination behavior of β-ketoesters compared to β-diketones.
Fig. 1
β-Ketoesters used for modification of Al(OiPr)3
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2 Experimental
All operations were carried out in a moisture and oxygen free atmosphere of dry argon using standard Schlenk or glove box techniques. Al(OiPr)3 (Aldrich, 98 + %), methyl acetoacetate (Aldrich, 99%) ethyl acetoacetate (Fluka, p.a.), iso-propyl acetoacetate (Alfa Aesar, 98%), tert-butyl acetoacetate (Aldrich, 98%), allyl acetoacetate (Aldrich, 98%) and 2-(methacryloyloxy)ethyl acetoacetate (Aldrich, 95%) were used as received. All solvents were dried and purified by standard techniques. C6D6 (99.5%, euriso-top) and d8-toluene (99.6%, euriso-top) used for NMR experiments were dried over 3 Å molecular sieve and degassed. 1D 1H and 13C NMR spectra were recorded on a Bruker AVANCE 250 (250.13 MHz {1H}, 62.86 MHz {13C}) spectrometer. 27Al and 2D NMR spectra were recorded on a Bruker AVANCE 300 (300.13 MHz {1H}, 75.47 MHz {13C}, 78.21 MHz {27Al}) spectrometer. Both spectrometers were equipped with a 5 mm broadband probe head and a z-gradient unit. COSY (Correlated Spectroscopy), HSQC (Heteronuclear Single Quantum Correlation), HMBC (Heteronuclear Multiple-Bond Correlation, evolution delay for long range coupling 100 ms), and EXSY (Exchange Spectroscopy, tmix = 1.2 s) were measured with Bruker standard pulse sequences. The 27Al NMR signals were referenced externally against a 2 M solution of AlCl3 in water (0 ppm).
2.1 [Al(OiPr)2(meac)]2
An amount of 1.003 g (4.91 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature followed by slow addition of 0.570 g (4.91 mmol) of methyl acetoacetate under stirring. The clear reaction solution was stirred at 120 °C for 18 h. The volatiles were then removed in vacuo. The crude product was washed with 2 mL of n-pentane and a colorless oil obtained. Yield 1.217 g (95%).
An amount of 0.999 g (4.89 mmol) of Al(OiPr)3 was dissolved in 20 mL of toluene at room temperature followed by slow addition of 0.638 g (4.90 mmol) of ethyl acetoacetate under stirring. The clear reaction solution was stirred at room temperature for 12 h and then at 120 °C for additional 18 h. The volatiles were removed in vacuo and a colorless oil was obtained. Yield 1.297 g (97%).
An amount of 1.006 g (4.93 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature followed by slow addition of 0.708 g (4.91 mmol) of iso-propyl acetoacetate under stirring. The clear reaction solution was stirred at 120 °C for 18 h. The volatiles were removed in vacuo and a colorless oil was obtained. Yield 1.388 g (98%).
An amount of 1.006 g (4.93 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature followed by slow addition of 0.709 g (4.92 mmol) tert-butyl acetoacetate under stirring. The clear reaction solution was stirred at 120 °C for 18 h. The volatiles were then removed in vacuo and a white microcrystalline precipitate was obtained. Crystals suitable for single crystal XRD analysis were obtained upon crystallization from toluene at 4 °C. Yield 1.331 g (89%).
An amount of 0.998 g (4.89 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature followed by slow addition of 0.695 g (4.89 mmol) of allyl acetoacetate under stirring. The slightly turbid reaction solution was stirred at 120 °C for 18 h. The volatiles were removed in vacuo, the crude product washed with 5 mL of dichloromethane and a pale yellow, slightly turbid oil was obtained. Yield 1.300 g (93%).
A solution of 1.009 g (4.94 mmol) of Al(OiPr)3 dissolved in 10 mL of toluene was stirred at 120 °C for 18 h. After cooling to room temperature 1.055 g (4.92 mmol) of 2-(methacryloyloxy)ethyl acetoacetate was slowly added under stirring. The clear reaction solution was stirred at room temperature for additional 18 h. The volatiles were then removed in vacuo and a colorless oil was obtained. Yield 1.696 g (96%).
An amount of 1.004 g (4.92 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature followed by slow addition of 1.710 g (14.71 mmol) of methyl acetoacetate under stirring. The clear reaction solution was then stirred at room temperature for 18 h. The volatiles were removed in vacuo and the resulting colorless solid washed with n-pentane.
An amount of 1.001 g (4.90 mmol) of Al(OiPr)3 was dissolved in 20 mL of toluene at room temperature and 1.914 g (14.71 mmol) of ethyl acetoacetate was slowly added under stirring. The clear reaction solution was stirred at room temperature for 18 h. The volatiles were removed in vacuo and a white solid was obtained.
An amount of 1.005 g (4.92 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature and 2.125 g (14.74 mmol) of iso-propyl acetoacetate was slowly added under stirring. The clear reaction solution was stirred at room temperature for 18 h. The volatiles were removed in vacuo and a white, partially crystalline solid was obtained.
An amount of 0.999 g (4.89 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature and 2.318 g (14.65 mmol) tert-butyl acetoacetate was slowly added under stirring. The clear reaction solution was stirred at room temperature for 18 h. The volatiles were then removed in vacuo and the resulting white, partially crystalline solid washed with dichloromethane. Yield 2.278 g (93%).
An amount of 0.999 g (4.89 mmol) of Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature and 2.084 g (14.66 mmol) of allyl acetoacetate was slowly added under stirring. The slightly turbid reaction solution was stirred at room temperature for 18 h. The volatiles were then removed in vacuo and the resulting orange oil washed with dichloromethane. Yield 2.167 g (98%).
An amount of 1.001 g (4.90 mmol) Al(OiPr)3 was dissolved in 10 mL of toluene at room temperature and 3.153 g (14.72 mmol) of 2-(methacryloyloxy)ethyl acetoacetate was slowly added under stirring. The clear reaction solution was stirred at room temperature for 18 h. The volatiles were removed in vacuo, the crude product washed with dichloromethane and a colorless rubber-like mass was obtained. Yield 3.114 g (95%).
First attempts to prepare compounds [Al(OiPr)2(β-ketoesterate)]n by reaction of [Al(OiPr)3]4 with one molar equivalent of β-ketoester per Al at room temperature according to literature procedures for the modification of aluminum alkoxides with acetylacetone [5] and of titanium alkoxides with β-ketoesters [8, 9] did not yield the desired products. Even after prolonged reaction times at room temperature, only a mixture of Al(β-ketoesterate)3 and unsubstituted [Al(OiPr)3]4 was obtained for all esters used (Eq. 1).
Coordination of the β-ketoesterate ligand during this reaction was monitored by 1H NMR spectroscopy, showing the disappearance of the signals of the CH2 group between the two carbonyl groups of the non-coordinated esters (2.92–3.02 ppm) and the appearance of the signals for the corresponding CH proton of the deprotonated and coordinated ligand (5.18–5.08 ppm).
The exclusive formation of Al(β-ketoesterate)3 indicates that intermediate [Al(OiPr)x(β-ketoesterate)3−x]n species react more rapidly with β-ketoesters than [Al(OiPr)3]4.
Heating the reaction solution, containing Al(β-ketoesterate)3 and [Al(OiPr)3]4 along with liberated iso-propanol, to 120 °C overnight resulted in the formation of the anticipated [Al(OiPr)2(β-ketoesterate)]n derivatives (Eq. 2). Reaction of isolated Al(β-ketoesterate)3 and [Al(OiPr)3]4 in appropriate stoichiometric ratios in toluene (without free alcohol) gave the same results. After cooling to room temperature no redistribution to Al(β-ketoesterate)3 and [Al(OiPr)3]4 occurred. This proves, that the monosubstituted dimer is the most stable species for this Al/β-ketoester ratio and the formation of Al(β-ketoesterate)3 at room temperature is due to a kinetic effect.
The same result was also obtained for an alternative preparation route, where a toluene solution of [Al(OiPr)3]4 was thermally pre-treated. This is known to cause de-oligomerization of the tetrameric units [2, 10, 11]. According to literature, the solution consists mainly of dimeric and trimeric species after fast cooling to room temperature, but some tetrameric [Al(OiPr)3]4 is still present or reformed. This was confirmed by 27Al NMR spectroscopy, showing a broad signal for pentacoordinate Al of trimeric species in the range from about 45 to 15 ppm [12], as well as a sharp signal at about 0 ppm for hexacoordinate Al of the tetramer, besides signals from about 80 to 50 ppm for tetracoordinate Al of di-, tri-, and tetrameric species (Fig. 2). 1H NMR and EXSY spectroscopy additionally shows fast exchange between the alkoxo groups of all species present in the solution. After de-oligomerization of tetrameric [Al(OiPr)3]4, the solution was cooled to room temperature, the β-ketoester was added and allowed to react for several hours. The resulting product was identified as [Al(OiPr)2(β-ketoesterate)]2, which means that pre-heating of the parent alkoxide and subsequent addition of ligand results in the same products as the in-situ heating of a mixture of the metal alkoxide and the ligand. This result shows (i) that re-formation of [Al(OiPr)3]4 from the dimer and trimer is slow at room temperature and (ii) that the dimer and trimer react faster with the β-ketoesters than the tetramer, and also faster than intermediate [Al(OiPr)x(β-ketoesterate)3−x]n, giving directly the monosubstituted compounds.
Fig. 2
Tetra-, tri- and dimeric form of [Al(OR)3]n (n = 2–4)
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The alternative synthetic pathway, i.e. thermal pre-treatment of the alkoxide, also opens the possibility to use temperature-sensitive ligands for modification. For example, gelation occurs when 2-(methacryloyloxy)ethyl acetoacetate (meaa-H), with its polymerizable methacrylic double bond, is reacted with Al(OiPr)3 at elevated temperatures, and no defined product could be isolated. In contrast, thermal de-oligomerization of the metal alkoxide tetramer and coordination of the ester under mild conditions (room temperature) yielded a well defined product, useable as precursor for inorganic–organic hybrid materials. Characterization by NMR spectroscopy showed the expected coordination of the β-ketoester giving rise to a signal at 5.05 ppm for the COCHCO proton. Signals for the =CH2 protons of the methacrylate at 6.12 and 5.18 ppm showed preservation of the polymerizable double bond (Fig. 3). The small signal at 6.06 ppm results from minor impurities of Al(meaa)3, which is formed because in the pre-treated Al(OiPr)3 solution still some tetrameric [Al(OiPr)3]4 is present, as mentioned above. Additional signals at about 7.10–7.00 ppm and at 2.10 ppm result from residual toluene.
Fig. 3
1H NMR spectrum of [Al(OiPr)2(meaa)]2 in C6D6
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The allyl acetoacetate derivative as an alternative compound bearing a polymerizable group can be synthesized by the “conventional” route because the ligand has a sufficient thermal stability.
All Al(OR)2(β-ketoesterate) derivatives show two 1H NMR signals for the methine protons of the iso-propoxo groups, indicating the existence of two chemically different ligands. According to the X-ray analysis reported below, the derivatives are asymmetrically substituted, alkoxo-bridged dimers (Fig. 4). The structure type is the same as that of [Al(OSiMe3)2(acac)]2 [5]. In this structure, one aluminum center is tetrahedrally coordinated by two bridging and two terminal alkoxo groups, whereas the other aluminum center is octahedrally coordinated by two chelating β-ketoesterate ligands and the two bridging alkoxo groups. Given the asymmetric nature of the β-ketoesterate ligands, this results in three possible isomers, one C1 and two C2 symmetric (Fig. 4), each forming a pair of enantiomers, giving 6 stereoisomers overall. In the NMR spectrum, the C2 symmetric complexes should show one set of signals for the ester ligands and two sets for the iso-propoxo groups (one for the bridging and one for the terminal) and splitting of these signals in two sets each for the C1 symmetric complex. Since, in some cases, more than two signals were observed for the β-ketoesterate ligands and only two signals for the methine protons of the iso-propoxo groups, different isomers coexist in solution for which the iso-propoxo signals are not distinguishable. All signals were assigned by COSY, HSQC and HMBC NMR spectroscopy, showing the coexistence of more than one species in solution, but with one clearly dominating form. The methine signals of the iso-propoxo are clearly assigned to bridging and terminal groups, but in the CH3 region multi signal overlap was observed and thus a definite assignment is difficult. The [Al(OiPr)2(β-ketoesterate)]2 complexes also show small signals beside a sharp main signal for the COCHCO proton. Only [Al(OiPr)2(tbuac)]2 gave three signals of almost equal intensity.
Fig. 4
Possibly isomers of [Al(OR)2(β-ketoeterate)]2 (only one enantiomer shown for each geometry)
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Intermolecular ligand exchange between the different species was observed in EXSY experiments for all complexes. For the complexes [Al(OiPr)2(meac)]2, [Al(OiPr)2(iprac)]2, and [Al(OiPr)2(tbuac)]2 splitting of the CH3 protons of the methoxo, iso-propoxo and tert-butoxo ester groups, respectively, was also observed in the 1H NMR spectra. Whereas this results in two signals of equal intensity for the iprac and tbuac derivative, one main signal at 3.59 ppm was found for the meac derivative, along with three smaller signals at 3.71, 3.52, and 3.24 ppm. In Fig. 5 the corresponding region of the EXSY spectrum is reproduced, showing exchange between all of these four signals.
Fig. 5
EXSY spectrum of [Al(OiPr)2(meac)]2 in d8-toluene, OCH3-region
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In accordance with the NMR spectra of [Al(OiPr)3]4 (see below), the signals for the bridging groups are shifted to lower field. The shifts of the OiPr methine protons for the non-functional Al(OR)2(β-ketoesterate) derivatives are compared with that of [Al(OiPr)3]4 in Table 1. The methine proton signals for all complexes are upfield shifted, corresponding to the reduced Lewis acidity of the substituted aluminum center. A slight trend to higher ppm values with increasing size of the ester OR groups for the 1H NMR signals of the methine protons is observed. Corresponding 13C NMR signals are not influenced by the ester OR group.
Table 1
1H NMR shifts of iso-propoxo groups of [Al(OiPr)2(β-ketoesterate)]2 (β-ketoesterate = meac, etac, iprac, tbuac) compared to [Al(OiPr)3]4 (in C6D6)
Crystals were obtained for the complex [Al(OiPr)2(tbuac)]2, and an X-ray crystal structure analysis was carried out. Although the quality of the data set was affected by formation of thin platelets and partial decomposition of the crystal during the measurement, the dimeric nature of the compound with one octahedrally and one tetrahedrally coordinated aluminum atom was confirmed (Fig. 6), and the C1 isomer (Fig. 4) with the two ester groups occupying an equatorial and an axial position was found (“equatorial position” refers to the Al2(μ-OR)2 plane). Selected bond distances and angles are given in Table 2.
Fig. 6
Molecular structure of [Al(OiPr)2(tbuac)]2 in the crystalline state
Table 2
Selected bond distances [pm] and angles [deg] of [Al(OiPr)2(tbuac)]2
Al(1)–O(1)
171.4(5)
Al(2)–O(6)
190.4(4)
Al(1)–O(2)
171.5(4)
Al(2)–O(8)
185.0(4)
Al(1)–O(3)
180.4(4)
Al(2)–O(9)
192.6(4)
Al(1)–O(4)
180.0(4)
O(5)–C(14)
128.1(6)
Al(2)–O(3)
190.1(4)
O(6)–C(16)
126.9(6)
Al(2)–O(4)
191.7(4)
O(8)–C(22)
129.7(7)
Al(2)–O(5)
187.2(4)
O(9)–C(24)
125.7(7)
O(1)–Al(1)–O(2)
113.5(2)
O(4)–Al(2)–O(6)
93.9(2)
O(3)–Al(1)–O(4)
113.2(2)
O(4)–Al(2)–O(9)
87.4(2)
Al(1)–O(3)–Al(2)
100.4(2)
O(5)–Al(2)–O(6)
91.0(2)
Al(1)–O(4)–Al(2)
99.9(2)
O(5)–Al(2)–O(8)
89.4(2)
O(3)–Al(2)–O(4)
77.1(2)
O(6)–Al(2)–O(8)
92.9(2)
O(3)–Al(2)–O(5)
89.9(2)
O(6)–Al(2)–O(9)
86.0(2)
O(3)–Al(2)–O(8)
96.2(2)
O(8)–Al(2)–O(9)
90.8(2)
O(3)–Al(2)–O(9)
93.1(2)
O(3)–Al(2)–O(6)
170.9(2)
O(4)–Al(2)–O(5)
92.8(2)
O(4)–Al(2)–O(8)
172.9(2)
O(5)–Al(2)–O(9)
177.0(2)
×
The Al–O distances of the chelating tbuac are longer for the “ester” oxygen compared to that of the “keto” oxygen. An additional trans effect from the bridging alkoxo groups causes shorter bond lengths for the equatorial bonds. The Al–O bond distances of the tbuac ligands decrease in the order Al–Oester,ax (192.6(4) pm) > Al–Oester,eq (190.4(4) pm) > Al–Oketo,ax (187.2(4) pm) > Al–Oketo,eq (185.0(4) pm). Vice versa, the different substituents trans to the bridging alkoxo groups result in an asymmetric bridging situation (Al(2)–O(3) 190.1(4) pm and Al(2)–O(4) 191.7(4) pm).
The bridging Al–O distances Al(2)–O(3) and Al(2)–O(4) are significantly longer than Al(1)–O(3) (180.4(4) pm) and Al(1)–O(4) (180.0(4) pm), showing the bond distances from the bridging oxygen atoms to the octahedral aluminum center to be significantly longer than to the tetrahedral center. As expected, the distances Al(1)–O(1) 171.4(5) and Al(1)–O(2) 171.5(4) of the terminal alkoxo groups are distinctly shorter than that of the bridging groups.
For a better understanding of the coordination behavior of the β-ketoesterate ligands and their above mentioned formation at room temperature, the trisubstituted monomeric Al(β-ketoesterate)3 complexes were also studied for all ligands. The complexes were prepared by addition of three molar equivalents of the β-ketoester to the alkoxide. The structure of Al(tbuac)3 was previously determined [13]. The 27Al chemical shift of 4.78 ppm in the NMR spectrum of Al(etac)3 is in line with the octahedral coordination of the aluminum atom. 1H and 13C NMR spectra of the Al(β-ketoesterate)3 compounds showed up to 4 signal sets for the ester groups. This is a result of the coexistence of different isomers in solution, viz. a C3 symmetric isomer with the keto and ester oxygen atoms respectively in fac arrangement and a C1 symmetric isomer with mer arrangement. In the C3 symmetric complex, all ligands are symmetrically equivalent giving rise to one set of signals in the NMR spectra, whereas for the C1 symmetric complex all ligands are non-equivalent, causing one set of signals for each ligand (Fig. 7). As a matter of fact each isomer forms a pair of enantiomers, which in the context of the NMR studies has no further consequences. Although the signals could not be assigned to the C1 or C3 isomer, the observation of four signal sets of equal intensity in the 1H NMR spectra corresponds to a 1:3 ratio of C3 and C1 symmetric species. Variable temperature NMR showed that the isomers can transform into each other. Coalescence was observed above 80 °C, causing an averaged signal set (Fig. 8). EXSY experiments confirmed exchange of the β-ketoesterate signals between the different isomers. The CH3 region of the EXSY spectrum of Al(etac)3 is given in Fig. 9, showing exchange for the CH3CO (1.90–1.80 ppm) and OCH2CH3 (1.12–0.92 ppm) signals.
Fig. 7
Different isomers of Al(β-ketoesterate)3 complexes
Fig. 8
Temperature dependent 1H NMR OCH2CH3 resonances of Al(etac)3 (d8-toluene)
Fig. 9
EXSY spectrum of Al(etac)3 at 20 °C in d8-toluene, CH3-region
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The 1H chemical shifts of the β-keto protons of the coordinated ligands for the mono- and trisubstituted complexes are compared in Table 3. The signals for the tri-substituted species are shifted to somewhat higher ppm values for all complexes (5.18–5.08 ppm compared to 5.10–4.98 ppm for [Al(OR)2(β-ketoesterate)]2). No correlation between the chemical shifts of β-keto protons and carbons and the size of the ester OR groups was observed. The 13C NMR shift differences between the mono- and trisubstituted complexes were not significant.
Table 3
Comparison of 1H NMR shifts of β-keto protons of [Al(OiPr)2(β-ketoesterate)]2 and Al(β-ketoesterate)3 complexes (in C6D6)
[Al(OiPr)2(β-ketoesterate)]2
Al(β-ketoesterate)3
meac-H
5.07 (5.06)a
5.17
etac-H
5.10 (5.08)a
5.18/5.17
iprac-H
5.06 (5.04)a
5.16
tbuac-H
5.06/5.05/4.98
5.13/5.10/5.08
aaa-H
5.10
5.15
meaa-H
5.05
5.14
aValues in brackets: signals of minor components with different coordination geometry
It is known that transesterification can occur as a possible side reaction during the reaction of metal alkoxides with esters [8] or can be used selectively as a preparative synthetic method [14]. Transesterification would change the structural and chemical properties of the metal alkoxides; for β-ketoesters with a functional OR group this would also result in the loss of the organic functionality. Transesterification as possible side reaction was studied for the reaction of the β-ketoesters with [Al(OiPr)3]4 by NMR spectroscopy. If transesterification would occur, iso-propyl acetoacetate derivatives would be formed. 1H NMR spectroscopy allows easy monitoring of the transesterification because the signal of the methine proton of the formed iso-propylacetoacetate at 5.15–4.85 ppm (in the monosubstituted complex) and 5.35 ppm (in the trisubstituted complex) is considerably shifted to higher ppm values compared with the signals of all the other esters. Additional evidence was obtained from HMBC experiments which show long range coupling between the methine proton and a carbonyl carbon if the iso-propyl ester was formed.
In no case was transesterification observed, neither at room temperature nor at 120 °C, except for the reaction of [Al(OiPr)3]4 with three equivalents of tbuac-H at 120 °C. Al(tbuac)3, formed at room temperature, reacts at 120 °C overnight with the liberated iso-propanol still present in the reaction solution to give Al(tbuac)3−x(iprac)x. This reaction was studied in more detail by NMR spectroscopy, recording spectra in situ at 80 °C in intervals of 2 and 4 h, respectively. Figure 10 shows the time dependence of the ratio between free iso-propanol and tert-butanol, representing the degree of transesterification, since free tert-butanol only could originate from the transesterification reaction. Longer reaction times finally led to complete substitution of all tert-butoxo groups by iso-propanol, giving Al(iprac)3. Interestingly, for the reaction of [Al(OiPr)3]4 with one equivalent of tbuac-H, after heating for 18 h to 120 °C, no transesterification was observed and crystalline [Al(OiPr)2(tbuac)]2 was the only product. From the results it is assumed that the steric demand of the tert-butoxo group in Al(tbuac)3 favors transesterification and formation of sterically less demanding Al(tbuac)3−x(iprac)x species. Since one tbuac ligand in [Al(OiPr)2(tbuac)]2 is replaced by two iso-propoxo groups, the steric constraint might be low enough to not cause transesterification in this case.
Fig. 10
Time dependence of iPrOH/tBuOH ratio at 80 °C in d8-toluene
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4 Conclusions
The substitution of [Al(OiPr)3]4 with one or three equivalents of the β-ketoesters meac-H, etac-H, iprac-H, tbuac-H, aaa-H and meaa-H per Al resulted in derivatives [Al(OiPr)2(β-ketoesterate)]2 and Al(β-ketoesterate)3, respectively, although special synthesis protocols are required to obtain [Al(OiPr)2(β-ketoesterate)]2 derivatives. The monosubstituted derivatives [Al(OiPr)2(β-ketoesterate)]2 are asymmetric dimers with one octahedrally coordinated Al center substituted by two ester ligands and one tetrahedrally coordinated Al atom surrounded by two bridging and two terminal iso-propoxo groups. The trisubstituted compounds Al(β-ketoesterate)3 are mononuclear octahedral complexes, giving rise to C1 and C3 symmetric isomers due to the asymmetric chelating ligand.
The alkoxo group of the esters had no influence on the structure of the products and only minor influence on the electronic properties, reflected in slightly different 1H NMR shifts of the bridging and terminal iso-propoxo groups of the monosubstituted dimers.
Only in two cases the type of ligand used had consequences on the preparation and stability of the substituted aluminum alkoxide derivatives: (i) The complex Al(tbuac)3 undergoes transesterification in the presence of iso-propanol, giving Al(iprac)x(tbuac)3−x. This behavior is attributed to the steric bulk of the tert-butoxo groups. (ii) Due to the thermal reactivity of the methacrylate substituent of meaa-H, direct reaction of [Al(OiPr)3]4 with meaa-H is not possible. However, [Al(OiPr)2(meaa)]2 can be prepared by thermal de-oligomerization of [Al(OiPr)3]4 followed by addition of the ligand at room temperature.
Contrary to analogous reactions with acetylacetone [5, 6], the monosubstituted complexes [Al(OiPr)2(β-ketoesterate)]2 were not formed when the compounds are reacted at room temperature. Mixtures of trisubstituted Al(β-ketoesterate)3 and non-reacted [Al(OiPr)3]4 were instead obtained. Formation of the monosubstituted complexes [Al(OiPr)2(β-ketoesterate)]2 was achieved by heating this mixture at 120 °C, and they are stable after cooling to room temperature, indicating [Al(OiPr)2(β-ketoesterate)]2 has a higher thermodynamic stability. Compared to [Al(OiPr)2(acac)]2 [5], no aging to trimeric [Al(OiPr)2(β-diketonate)]3 was observed for the β-ketoesterate derivatives. Thus, significant differences in reactivity of β-ketoesters and β-diketones with [Al(OiPr)3]4 are noticed, since the structures of the formed derivatives may be different depending on the side conditions (temperature, time). This may have important consequences if those ligands are used to modify aluminum alkoxides for sol–gel processing.
5 X-Ray Structure Analysis
A plate-like crystal of [Al(OiPr)2(tbuac)]2 was mounted on a Siemens SMART diffractometer with an area detector and measured in a nitrogen stream. Mo-Kα radiation (λ = 71.069 pm, graphite monochromator) was used for all measurements. The data collection (Table 4) at 100 K covered a hemisphere of the reciprocal space, by a combination of three sets of exposures. Each set had a different ϕ angle for the crystal, and each exposure took 25 sec and covered 0.3° in ω. The crystal-to-detector distance was 5 cm. The crystal partially decomposed during measurement. The data were corrected for polarization and Lorentz effects, and an empirical absorption correction (SADABS) was employed. The cell dimensions were refined with all unique reflections.
Table 4
Crystallographic and structural parameters of [Al(OiPr)2(tbuac)]2
Empirical formula
C28H54Al2O10
Formula weight
604.7
Crystal system
Monoclinic
Space group
P21/n
a [pm]
9.553(4)
b [pm]
18.029(7)
c [pm]
20.475(9)
α [°]
90.00
β [°]
91.825(8)
γ [°]
90.00
Volume [106 pm3]
3525(3)
Z
4
Calcd. density [g cm−3]
1.139
μ [mm−1]
0.129
Crystal size [mm]
1.60 × 1.40 × 0.10
2θ range [°]
2.41–28.45
Reflections collected/unique
12759/6606
Data/parameters
6606/377
GOF on F2
0.991
R [I > 2σ(I)]
0.1165
wR2
0.3253
Largest peak/hole [e Å−3]
0.581/−0.375
The structure was solved by direct methods (SHELXS97). Refinement was performed by the full-matrix least-squares method based on F2 (SHELXL97) with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms were inserted in calculated positions and refined riding with the corresponding atom.
CCDC-694178 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgment
This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung (FWF), Austria.
Open Access
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
Open AccessThis is an open access article distributed under the terms of the Creative Commons Attribution Noncommercial License (https://creativecommons.org/licenses/by-nc/2.0), which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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