Synthesis of Siloxyalumoxanes and Alumosiloxanes Based on Organosilicon Diols

We have drawn a few interesting conclusions while studying reaction products of Ph2Si(OH)2 with Al(iBu)3 and tetraisobutylalumoxane. In the first place, this is the production (at a Ph2Si(OH)2 and Al(iBu)3 equimolar ratio) of an oligomer siloxyalumoxane structure with alternating four- and six-member rings. In addition, it shows isobutyl and phenyl group migration between aluminum and silicon due to the formation of an intramolecular four-member cyclic complex [Ph2(OH)SiO]Al(iBu)2 → [(iBu)Ph(OH)SiO]Al(iBu)Ph. Ph2Si(OH)2 interaction with Al(iBu)3 not only starts from intramolecular complex production, but the chain is terminated for the same reason, which in the case of the Ph2Si(OH)2 reaction with tetraisobutylalumoxane results in failure of to obtain high-polymer siloxyalumoxane compounds. When Al(iBu)3 interacts with α- and γ-diols, no oligomer compounds are produced. In the Al(iBu)3 reaction with α, γ-diols are created in monomer compounds that are likely to have a cyclic structure. Notably, when Al(iBu)3 interacts with only α-diol, a double excess of Al(iBu)3 allows for full replacement of hydrogen in the α-diol hydroxyl groups by aluminum alkyl residue with 1,3-bis(diisobutylalumoxymethyl)-1,1,3,3-tetramethyldisiloxane production. At an equimolar ratio of initial reagents, the second isobutyl radical at Al does not interact with the second hydroxyl group of α-diol, apparently due to the steric hindrance, and 1-(diisobutylalumoxymethyl)-3-(hydroxymethyl)-1,1,3,3-tetramethyl-disiloxane is produced. Al(iBu)3 reactions with γ-diol also result in monomer compounds, but the presence of a chain consisting of three CH2-groups between Si and the hydroxyl group facilitates interaction between the second hydroxyl group of γ-diol and the second isobutyl radical Al(iBu)3. Tetraisobutylalumoxane reactions with α- and γ-diols result in oligomer compounds.


Introduction
Organosiloxyalumoxane and organoalumoxanesiloxane oligomers were synthesized 30 years ago [1]. Their probable structure from a conventional viewpoint looked like three-coordinated Al atom. These compounds are amorphous, therefore, it did not seem possible to prove their structure by means of X-ray diffraction. However, in the middle of the 1980s, papers dealing with the non-classical structure of alumoxane and alumosiloxane compounds with four-coordinated Al atoms and three-coordinated oxygen atoms were published [2][3][4]. The coordination number of Al atoms in bicyclic and oligomer alumoxanes and alumosiloxanes was shown to be four and may increase to five (or even to six) [2]. The crystalline structure of alumosiloxane of C 8 H 24 Al 3 Br 5 O 6 Si 4 composition was proved [5]. It was found that the molecule of crystalline alumosiloxane consisted of four condensed nuclei: two planar four-membered rings, built from two aluminum atoms and two oxygen atoms, and two "saddle-shaped" six-membered rings composed of alternating silicon, oxygen, and aluminum

Results and Discussion
To investigate the mechanism of the Al( i Bu) 3 reaction with Ph 2 Si(OH) 2 , we studied (by means of NMR spectroscopy) their interaction in hexadeuterobenzene (C 6 D 6 ). In contrast to the reactions in benzene, deuterobenzene solvent makes it possible to determine not only the isobutyl groups, but also benzene's presence in the 1 H-and 13 C-NMR spectra. In the NMR spectra of the reaction mixture after the Al( i Bu) 3 interaction with Ph 2 Si(OH) 2 was complete, a proton signal was observed at 7.23 ppm, and a signal of 13 C nuclei of unlabeled benzene was observed at 128 ppm [26]. When the siloxyalumoxane oligomer reaction product was released, the produced benzene was distilled along with a solvent (C 6 D 6 ) and was recorded in the NMR spectra of the distillate. Benzene may be produced as the result of phenyl and isobutyl groups exchanging between silicon and aluminum. Earlier, a few authors [27] observed phenyl and alkyl group migration in triphenylsiloxyaluminumalkyl at 250-300 • C ( Figure 1). oxygen, and aluminum atoms. The Al atom, which belongs to all four rings, has a coordination number of five. The remaining atoms of aluminum and silicon have tetrahedral coordination.
Based on the data of [2][3][4][5], the results of [1] were analyzed. In addition, the reaction of Al( i Bu)3 with Ph2Si(OH)2 in hexadeuterobenzene was studied. The results obtained suggested a probable scheme for the interaction of Al( i Bu)3 with Ph2Si(OH)2 and the possible structure of the oligomers obtained [6]. However, these results were not published.
Most closely related to our work was the investigation performed by Andrew R. Barron's team, they studied the synthesis, properties, and structure of organosiloxyalumoxanes-precursors of alumosilicate ceramics [16,17].
A considerable number of molecular alumosiloxanes and alumosilicates have been obtained using aluminum halogenides, chalcogenides, hydrides, and organometallic compounds as starting materials and reacting them with the appropriate RnSi(OH)4−n precursor [18].The reaction of Al2Cl6 with an excess of Ph2Si(OH)2 in THF in the presence of pyridine yielded new anionic and cyclic aluminosiloxanes. The structure of the anionic complex comprises separated pyridinium cations and alumosiloxane anions with a tetrahedral arrangement around the Al atom, which is similar to that in natural aluminosilicates; the core of the cyclic aluminosiloxane is a twelve-membered Al2Si4O6 ring in a chair conformation, which contains a Cl group on each of the two Al atoms [19]. Reaction of Al( t Bu)3 with neol-H2 (2,2- may be considered as a bifunctional (two OH groups), tetradentate (4O) ligand as highlighted by its reactivity with Group 13 hydrides and alkyls. Reaction of [Al2( t Bu)4(neol-H)2] with AlH3(NMe3), AlH2Cl(NMe3) and AlMe3 yields the tri-aluminum compounds, [Al3( t Bu)4(X)(neol)2] with X = H, Cl, Me, respectively [20].

Results and Discussion
To investigate the mechanism of the Al( i Bu)3 reaction with Ph2Si(OH)2, we studied (by means of NMR spectroscopy) their interaction in hexadeuterobenzene (C6D6). In contrast to the reactions in benzene, deuterobenzene solvent makes it possible to determine not only the isobutyl groups, but also benzene's presence in the 1 H-and 13 C-NMR spectra. In the NMR spectra of the reaction mixture after the Al( i Bu)3 interaction with Ph2Si(OH)2 was complete, a proton signal was observed at 7.23 ppm, and a signal of 13 C nuclei of unlabeled benzene was observed at 128 ppm [26]. When the siloxyalumoxane oligomer reaction product was released, the produced benzene was distilled along with a solvent (C6D6) and was recorded in the NMR spectra of the distillate. Benzene may be produced as the result of phenyl and isobutyl groups exchanging between silicon and aluminum. Earlier, a few authors [27] observed phenyl and alkyl group migration in triphenylsiloxyaluminumalkyl at 250-300 °C ( Figure 1). In our case, such migration was observed even at 80 °C. In 27 Al-NMR spectra of the products of Al( i Bu)3 reaction with Ph2Si(OH)2 in C6D6 at 25 °C broad signal typical for oligomer structures where aluminum coordination number is four was recorded at 61.0 ppm. In our case, such migration was observed even at 80 • C.
In 27 Al-NMR spectra of the products of Al( i Bu) 3 reaction with Ph 2 Si(OH) 2 in C 6 D 6 at 25 • C broad signal typical for oligomer structures where aluminum coordination number is four was recorded at 61.0 ppm.
According to the abovementioned, we can propose the following mechanism of siloxyalumoxane oligomers production through an active intramolecular complex (A*), where it is probable isobutyl and phenyl groups exchange at aluminum and silicon (Scheme 1).
Molecules 2017, 22, 1776 3 of 14 According to the abovementioned, we can propose the following mechanism of siloxyalumoxane oligomers production through an active intramolecular complex (A*), where it is probable isobutyl and phenyl groups exchange at aluminum and silicon (Scheme 1).

Scheme 1. The formation of active intramolecular complex (A*).
At an equimolar ratio of the starting reagents, hydrogen of the hydroxyl group reacts with the i Bu-group of another Al( i Bu)3 molecule, and one of the organic groups at aluminum (preferably the phenyl one as it has more electronegative character than the isobutyl group) interacts with hydrogen of OH group of the next Ph2Si(OH)2 molecule. A six-membered ring is formed; the ring in its turn has active sites able to react with the following Al( i Bu)3 and Ph2Si(OH)2 molecules, which results in a four-membered alumoxane ring, then a six-membered ring is formed, etc. (Scheme 2). The chain is terminated due to the intramolecular A* complex. So, siloxyalumoxane oligomer 1 is formed where one of the phenyl groups is substituted by an isobutyl group at three silicon atoms. Considering the amount of C6D6 taken for synthesis (at equimolar ratio of Al( i Bu)3:Ph2Si(OH)2 = 1:1), the distilled C6D6 must contain about 1.5 wt % of benzene. Mass spectrometric analysis of the distillate showed that along with the used solvent (C6D6) 1.6 wt % of benzene was distilled.
Oligomer 1 production is confirmed by thermo-gravimetric analysis results ( Figure 2). At an equimolar ratio of the starting reagents, hydrogen of the hydroxyl group reacts with the i Bu-group of another Al( i Bu) 3 molecule, and one of the organic groups at aluminum (preferably the phenyl one as it has more electronegative character than the isobutyl group) interacts with hydrogen of OH group of the next Ph 2 Si(OH) 2 molecule. A six-membered ring is formed; the ring in its turn has active sites able to react with the following Al( i Bu) 3 and Ph 2 Si(OH) 2 molecules, which results in a four-membered alumoxane ring, then a six-membered ring is formed, etc. (Scheme 2).
Molecules 2017, 22, 1776 3 of 14 According to the abovementioned, we can propose the following mechanism of siloxyalumoxane oligomers production through an active intramolecular complex (A*), where it is probable isobutyl and phenyl groups exchange at aluminum and silicon (Scheme 1).

Scheme 1. The formation of active intramolecular complex (A*).
At an equimolar ratio of the starting reagents, hydrogen of the hydroxyl group reacts with the i Bu-group of another Al( i Bu)3 molecule, and one of the organic groups at aluminum (preferably the phenyl one as it has more electronegative character than the isobutyl group) interacts with hydrogen of OH group of the next Ph2Si(OH)2 molecule. A six-membered ring is formed; the ring in its turn has active sites able to react with the following Al( i Bu)3 and Ph2Si(OH)2 molecules, which results in a four-membered alumoxane ring, then a six-membered ring is formed, etc. (Scheme 2). The chain is terminated due to the intramolecular A* complex. So, siloxyalumoxane oligomer 1 is formed where one of the phenyl groups is substituted by an isobutyl group at three silicon atoms. Considering the amount of C6D6 taken for synthesis (at equimolar ratio of Al( i Bu)3:Ph2Si(OH)2 = 1:1), the distilled C6D6 must contain about 1.5 wt % of benzene. Mass spectrometric analysis of the distillate showed that along with the used solvent (C6D6) 1.6 wt % of benzene was distilled.
Oligomer 1 production is confirmed by thermo-gravimetric analysis results ( Figure 2). The chain is terminated due to the intramolecular A* complex. So, siloxyalumoxane oligomer 1 is formed where one of the phenyl groups is substituted by an isobutyl group at three silicon atoms. Considering the amount of C 6 D 6 taken for synthesis (at equimolar ratio of Al( i Bu) 3 :Ph 2 Si(OH) 2 = 1:1), the distilled C 6 D 6 must contain about 1.5 wt % of benzene. Mass spectrometric analysis of the distillate showed that along with the used solvent (C 6 D 6 ) 1.6 wt % of benzene was distilled.
Oligomer 1 production is confirmed by thermo-gravimetric analysis results ( Figure 2). The TGA curve shows that in the temperature range of 30-230 °C, the weight loss is about 30 wt %, while the SDTA curve shows an endo-effect at 180 °C. This corresponds to the complete removal of isobutyl groups in the form of isobutene (theoretically, the weight loss is 31.33 wt %). In the temperature range of 230-650 °C, the weight loss is about 40 wt %, while the SDTA curve shows an exo-effect at 280 °C. This corresponds to the complete removal of the phenyl groups in the form of benzene (theoretically the weight loss is 38.18 wt %). Further pyrolysis in an inert atmosphere (argon) to 1100 °C leads to the formation of an alumosilicate ceramic residue of 29.89 wt %, which actually coincides with the theoretically calculated value for oligomer 1 (30.42 wt %) ( Figure 2).
With Al( i Bu)3 (Al:Si = 2:1) excess, the complex is stabilized due to the second Al( i Bu)3 molecule attachment and formation of bis(diisobutylalumoxy)diphenylsilane; taking into account isobutyl and phenyl group migrations between aluminum and silicon, the interaction mechanism can be presented as follows (Scheme 3). When the reaction proceeds at Ph2Si(OH)2 (Al:Si = 1:2) excess, organosiloxyalumoxane formation may be presented in two ways (Scheme 4a,b). The TGA curve shows that in the temperature range of 30-230 • C, the weight loss is about 30 wt %, while the SDTA curve shows an endo-effect at 180 • C. This corresponds to the complete removal of isobutyl groups in the form of isobutene (theoretically, the weight loss is 31.33 wt %). In the temperature range of 230-650 • C, the weight loss is about 40 wt %, while the SDTA curve shows an exo-effect at 280 • C. This corresponds to the complete removal of the phenyl groups in the form of benzene (theoretically the weight loss is 38.18 wt %). Further pyrolysis in an inert atmosphere (argon) to 1100 • C leads to the formation of an alumosilicate ceramic residue of 29.89 wt %, which actually coincides with the theoretically calculated value for oligomer 1 (30.42 wt %) ( Figure 2).
With Al( i Bu) 3 (Al:Si = 2:1) excess, the complex is stabilized due to the second Al( i Bu) 3 molecule attachment and formation of bis(diisobutylalumoxy)diphenylsilane; taking into account isobutyl and phenyl group migrations between aluminum and silicon, the interaction mechanism can be presented as follows (Scheme 3). The TGA curve shows that in the temperature range of 30-230 °C, the weight loss is about 30 wt %, while the SDTA curve shows an endo-effect at 180 °C. This corresponds to the complete removal of isobutyl groups in the form of isobutene (theoretically, the weight loss is 31.33 wt %). In the temperature range of 230-650 °C, the weight loss is about 40 wt %, while the SDTA curve shows an exo-effect at 280 °C. This corresponds to the complete removal of the phenyl groups in the form of benzene (theoretically the weight loss is 38.18 wt %). Further pyrolysis in an inert atmosphere (argon) to 1100 °C leads to the formation of an alumosilicate ceramic residue of 29.89 wt %, which actually coincides with the theoretically calculated value for oligomer 1 (30.42 wt %) ( Figure 2).
With Al( i Bu)3 (Al:Si = 2:1) excess, the complex is stabilized due to the second Al( i Bu)3 molecule attachment and formation of bis(diisobutylalumoxy)diphenylsilane; taking into account isobutyl and phenyl group migrations between aluminum and silicon, the interaction mechanism can be presented as follows (Scheme 3). When the reaction proceeds at Ph2Si(OH)2 (Al:Si = 1:2) excess, organosiloxyalumoxane formation may be presented in two ways (Scheme 4a,b). When the reaction proceeds at Ph 2 Si(OH) 2 (Al:Si = 1:2) excess, organosiloxyalumoxane formation may be presented in two ways (Scheme 4a,b). Scheme 4a is the most probable. This is confirmed by the fact that in the 1 H-NMR spectra of siloxyalumoxane with molar ratio Al:Si = 1:2, low resolution signals of isobutyl group protons at 1.05 ppm of CH3 ( i Bu-Si) and 1.4 ppm of CH2 ( i Bu-Si) were observed. Moreover, if one mole of Al( i Bu)3 interacts with two moles of Ph2Si(OH)2, the amount of gas released is much less as compared to the theory and amounts to less than 70%, which corresponds to two isobutyl groups migrating from aluminum to silicon.

Scheme 5. Disproportionation of tetraisobutylalumoxane.
At a temperature of 80 °C, a mixture of triisobutylaluminum and pentaisobutyltrialumoxane (B) reacts with Ph2Si(OH)2, therefore, a part of Ph2Si(OH)2 interacts with Al( i Bu)3 forming the intramolecular A* complex. The other part of Ph2Si(OH)2 reacts with the double alumoxane chain, the alumoxane chain, therewith, is terminated by the A* complex, which means it is impossibile to obtain high-polymer siloxyalumoxane compounds from tetraisobutylalumoxane and Ph2Si(OH)2 (Scheme 6).  It should be noted that at a temperatures above 20 • C tetraisobutylalumoxane disproportionation takes place with triisobutylaluminum release [3,13] (Scheme 5). It should be noted that at a temperatures above 20 °C tetraisobutylalumoxane disproportionation takes place with triisobutylaluminum release [3,13] (Scheme 5).

Scheme 5. Disproportionation of tetraisobutylalumoxane.
At a temperature of 80 °C, a mixture of triisobutylaluminum and pentaisobutyltrialumoxane (B) reacts with Ph2Si(OH)2, therefore, a part of Ph2Si(OH)2 interacts with Al( i Bu)3 forming the intramolecular A* complex. The other part of Ph2Si(OH)2 reacts with the double alumoxane chain, the alumoxane chain, therewith, is terminated by the A* complex, which means it is impossibile to obtain high-polymer siloxyalumoxane compounds from tetraisobutylalumoxane and Ph2Si(OH)2 (Scheme 6). At a temperature of 80 • C, a mixture of triisobutylaluminum and pentaisobutyltrialumoxane (B) reacts with Ph 2 Si(OH) 2 , therefore, a part of Ph 2 Si(OH) 2 interacts with Al( i Bu) 3 forming the intramolecular A* complex. The other part of Ph 2 Si(OH) 2 reacts with the double alumoxane chain, the alumoxane chain, therewith, is terminated by the A* complex, which means it is impossibile to obtain high-polymer siloxyalumoxane compounds from tetraisobutylalumoxane and Ph 2 Si(OH) 2 (Scheme 6). It should be noted that at a temperatures above 20 °C tetraisobutylalumoxane disproportionation takes place with triisobutylaluminum release [3,13] (Scheme 5).

Scheme 5. Disproportionation of tetraisobutylalumoxane.
At a temperature of 80 °C, a mixture of triisobutylaluminum and pentaisobutyltrialumoxane (B) reacts with Ph2Si(OH)2, therefore, a part of Ph2Si(OH)2 interacts with Al( i Bu)3 forming the intramolecular A* complex. The other part of Ph2Si(OH)2 reacts with the double alumoxane chain, the alumoxane chain, therewith, is terminated by the A* complex, which means it is impossibile to obtain high-polymer siloxyalumoxane compounds from tetraisobutylalumoxane and Ph2Si(OH)2 (Scheme 6). Siloxyalumoxane oligomers 4 or 5 containing four-and six-membered rings were produced. Siloxyalumoxanes 1-5 were white, glassy, benzene-soluble oligomers with different molecular weights.
X-ray amorphism of siloxyalumoxane oligomers ( Figure 4) did not allow for the determination of their crystalline characteristics, but the presence of reduced structural units in formulas (1)-(5) was confirmed by multinuclear magnetic resonance, IR spectroscopy, TGA, and elemental analysis. In contrast to the starting Al( i Bu)3, tetraisobutylalumoxane, and Ph2Si(OH)2, their interaction products in the IR spectra have highly intense broad absorption bands in the region of 1050-1075 cm −1 , typical for Al-O-Si bonds, as well as an absorption band of medium intensity in the region of 820-840 cm −1 , corresponding to valence vibrations of Al-O-Al bond (bridge). Presence or absence of a wide absorption band in the region of 3400-3500 cm −1 , typical for hydroxyl group valence vibrations, confirms probable oligomer 1-5 structures.
In 1 H-NMR spectra of the reaction mixture after the completion of the Al( i Bu)3 (tetraisobutylalumoxane) reaction with Ph2Si(OH)2 in benzene, isobutyl group proton signals were observed as low resolution multiplets in the region of 0.5 to 2.5 ppm. Siloxyalumoxane oligomers 4 or 5 containing four-and six-membered rings were produced. Siloxyalumoxanes 1-5 were white, glassy, benzene-soluble oligomers with different molecular weights.
X-ray amorphism of siloxyalumoxane oligomers ( Figure 4) did not allow for the determination of their crystalline characteristics, but the presence of reduced structural units in oligomers (1)-(5) was confirmed by multinuclear magnetic resonance, IR spectroscopy, TGA, and elemental analysis. Siloxyalumoxane oligomers 4 or 5 containing four-and six-membered rings were produced. Siloxyalumoxanes 1-5 were white, glassy, benzene-soluble oligomers with different molecular weights.
X-ray amorphism of siloxyalumoxane oligomers ( Figure 4) did not allow for the determination of their crystalline characteristics, but the presence of reduced structural units in formulas (1)-(5) was confirmed by multinuclear magnetic resonance, IR spectroscopy, TGA, and elemental analysis. In contrast to the starting Al( i Bu)3, tetraisobutylalumoxane, and Ph2Si(OH)2, their interaction products in the IR spectra have highly intense broad absorption bands in the region of 1050-1075 cm −1 , typical for Al-O-Si bonds, as well as an absorption band of medium intensity in the region of 820-840 cm −1 , corresponding to valence vibrations of Al-O-Al bond (bridge). Presence or absence of a wide absorption band in the region of 3400-3500 cm −1 , typical for hydroxyl group valence vibrations, confirms probable oligomer 1-5 structures.
In 1 H-NMR spectra of the reaction mixture after the completion of the Al( i Bu)3 (tetraisobutylalumoxane) reaction with Ph2Si(OH)2 in benzene, isobutyl group proton signals were observed as low resolution multiplets in the region of 0.5 to 2.5 ppm. In contrast to the starting Al( i Bu) 3 , tetraisobutylalumoxane, and Ph 2 Si(OH) 2 , their interaction products in the IR spectra have highly intense broad absorption bands in the region of 1050-1075 cm −1 , typical for Al-O-Si bonds, as well as an absorption band of medium intensity in the region of 820-840 cm −1 , corresponding to valence vibrations of Al-O-Al bond (bridge). Presence or absence of a wide absorption band in the region of 3400-3500 cm −1 , typical for hydroxyl group valence vibrations, confirms probable oligomer 1-5 structures.
In 1 H-NMR spectra of the reaction mixture after the completion of the Al( i Bu) 3 (tetraisobutylalumoxane) reaction with Ph 2 Si(OH) 2 in benzene, isobutyl group proton signals were observed as low resolution multiplets in the region of 0.5 to 2.5 ppm. 1 H-NMR spectrum of the concentrated isobutylalumoxanephenylsiloxane oligomer in C 6 D 6 had a broadened signal at 7.5 ppm, typical for phenyl group protons, and another signal at 1.5 ppm, corresponding to isobutyl group protons.
Therefore, an organoaluminum compound reaction with diphenylsilanediol results in organosiloxyalumoxane compounds with alternating four-and six-membered rings containing aluminum atoms with the coordination number equal to four. Isobutyl and phenyl groups at aluminum and silicon may migrate in the process of reaction.
The structure of the alumoxanesiloxane oligomers is confirmed by [16,17] where it was shown that the hydrolytically stable alumoxanes of the formula [Al(O) x (OH) y (X) z ] n (X = OSiR 3 , O 2 CR) are neither linear nor ring structures, but are spatial clusters. These alumoxanes have an isostructural center (nucleus) similar to minerals such as boehmite and diaspore {[Al(O)(OH)] n }, wherein central aluminum atoms are six-coordinated. In the case of siloxy derivatives, four-coordinated aluminum atoms are present circumferentially [16,17].
Al( i Bu) 3 reactions with γ-diol also result in monomer compounds (8,9), but the presence of a chain comprising three CH 2 -groups between the Si atom and a hydroxyl group simplifies the interaction between the second hydroxyl group of γ-diol and the second isobutyl radical Al( i Bu) 3 . Probable triisobutylaluminum reactions with α, γ-diols are presented by Scheme 7.
Molecules 2017, 22,1776 8 of 14 1 H-NMR spectrum of the concentrated isobutylalumoxanephenylsiloxane oligomer in C6D6 had a broadened signal at 7.5 ppm, typical for phenyl group protons, and another signal at 1.5 ppm, corresponding to isobutyl group protons.
Therefore, an organoaluminum compound reaction with diphenylsilanediol results in organosiloxyalumoxane compounds with alternating four-and six-membered rings containing aluminum atoms with the coordination number equal to four. Isobutyl and phenyl groups at aluminum and silicon may migrate in the process of reaction.
The structure of the alumoxanesiloxane oligomers is confirmed by [16,17] where it was shown that the hydrolytically stable alumoxanes of the formula [Al(O)x(OH)y(X)z]n (X = OSiR3, O2CR) are neither linear nor ring structures, but are spatial clusters. These alumoxanes have an isostructural center (nucleus) similar to minerals such as boehmite and diaspore {[Al(O)(OH)]n}, wherein central aluminum atoms are six-coordinated. In the case of siloxy derivatives, four-coordinated aluminum atoms are present circumferentially [16,17].
Al( i Bu)3 reactions with γ-diol also result in monomer compounds (8,9), but the presence of a chain comprising three CH2-groups between the Si atom and a hydroxyl group simplifies the interaction between the second hydroxyl group of γ-diol and the second isobutyl radical Al( i Bu)3. Probable triisobutylaluminum reactions with α, γ-diols are presented by Scheme 7. The comparison of the IR spectra of 1-(diisobutylalumoxymethyl)-3-(hydroxymethyl)-1,1,3,3tetramethyldisiloxane (6) and 1,3-bis(diisobutylalumoxymethyl)-1,1,3,3-tetramethyldisiloxane (7) with the IR spectrum of the starting α-diol confirms our assumptions. In the IR spectrum of compound (6), there is a wide absorption band in the region of 660 cm −1 corresponding to valence vibrations ν(Al-C), and the intensity of an absorption band in the region of 3400 cm −1 ν(OH) significantly decreases. In the IR spectrum of compound (7), there is no absorption in the region of 3400 cm −1 , and an intense absorption band in the region of 665 cm −1 ν(Al-C) appears. Moreover, in both cases, the intense absorption band broadens in the region of 1060 cm −1 ν (Si-O-Si) and has a shoulder of 1015 cm −1 ν (Al-O-C).
In contrast to the 1 H-NMR spectrum of the starting α-diol, wherein proton signals at 0.38 ppm (CH 3 ), 3.5 ppm (CH 2 ), and 4.62 ppm (OH) were recorded in the spectra of compounds (6,7) and proton signals of isobutyl groups were observed at 0.4 ppm (CH 2 ), 1.07 ppm (CH 3 ), and 1.45 ppm (CH), but the proton signal of the CH 2 group of α-diol was shifted to a weaker field and was manifested at 3.65 ppm.
Tetraisobutylalumoxane (B) reactions with αand γ-diols result in oligomer compounds, but here also, as in the case with diphenylsilanediol, molecular weight growth is due to isobutyl radicals replacement with oxyorganosiloxane groupings. The resulting oligomers are white, glassy products, readily soluble in benzene. Compounds synthesized from tetraisobutylalumoxane and α-diol dissolved in heptane as well, but compounds synthesized from tetraisobutylalumoxane and γ-diol did not dissolve in heptane.
The formation of alumoxanes comprising oxyorganosiloxane fragments may be presented by Scheme 8.
Molecules 2017, 22, 1776 9 of 14 The comparison of the IR spectra of 1-(diisobutylalumoxymethyl)-3-(hydroxymethyl)-1,1,3,3tetramethyldisiloxane (6) and 1,3-bis(diisobutylalumoxymethyl)-1,1,3,3-tetramethyldisiloxane (7) with the IR spectrum of the starting α-diol confirms our assumptions. In the IR spectrum of compound (6), there is a wide absorption band in the region of 660 cm −1 corresponding to valence vibrations ν (Al-C), and the intensity of an absorption band in the region of 3400 cm −1 ν(OH) significantly decreases. In the IR spectrum of compound (7), there is no absorption in the region of 3400 cm −1 , and an intense absorption band in the region of 665 cm −1 ν(Al-C) appears. Moreover, in both cases, the intense absorption band broadens in the region of 1060 cm −1 ν (Si-O-Si) and has a shoulder of 1015 cm −1 ν (Al-O-C).
In contrast to the 1 H-NMR spectrum of the starting α-diol, wherein proton signals at 0.38 ppm (CH3), 3.5 ppm (CH2), and 4.62 ppm (OH) were recorded in the spectra of compounds (6,7) and proton signals of isobutyl groups were observed at 0.4 ppm (CH2), 1.07 ppm (CH3), and 1.45 ppm (CH), but the proton signal of the CH2 group of α-diol was shifted to a weaker field and was manifested at 3.65 ppm.
Tetraisobutylalumoxane (B) reactions with α-and γ-diols result in oligomer compounds, but here also, as in the case with diphenylsilanediol, molecular weight growth is due to isobutyl radicals replacement with oxyorganosiloxane groupings. The resulting oligomers are white, glassy products, readily soluble in benzene. Compounds synthesized from tetraisobutylalumoxane and α-diol dissolved in heptane as well, but compounds synthesized from tetraisobutylalumoxane and γ-diol did not dissolve in heptane.
The formation of alumoxanes comprising oxyorganosiloxane fragments may be presented by Scheme 8. The 1 H-NMR and IR spectra of the reaction products of tetraisobutylalumoxane reacted with αand γ-diols were similar to those of the compounds produced by Al( i Bu) 3 reactions with αand γ-diols. They confirmed hydroxyl group absence in the resulting compounds and the presence of isobutyl groups, Al-C, Al-O-C bonds, and oxyorganosiloxane fragments. The molecular weight of the synthesized oxyorganosiloxane-containing alumoxanes depended on the molecular weight of the starting alkylalumoxane.