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Table 4. EAA/AlI ratio, share of butoxide and share of aluminium atoms for samples EAO.5—EA3.0 calculated using data in Table 3 and assumed compounds compositions DOI: 10.5562/cca3426

Table 4 EAA/AlI ratio, share of butoxide and share of aluminium atoms for samples EAO.5—EA3.0 calculated using data in Table 3 and assumed compounds compositions DOI: 10.5562/cca3426

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Figure 1. Keto-enol tautomerism and the formation of chelate. Denomination is introduced for the later interpretation of +H and 13C NMR signals. and keto from, which is known as keto-enol tautomerism (Figure 1).!2° In the enolic tautomer a reactive hydroxy group, capable of becoming a bidentate monocharged enolate ligand is present and can substitute an alkoxy group in the metal alkoxide. As a consequence, a six membered chelate ring may be formed, consequently increasing the hydrolytic stability of the alkoxide (Figure 1).!°! Normally, the keto-enol equilibrium heavily favours the keto form, yet the chelate formation will shift the keto-enol equilibrium towards the enolic form,914) was iso-propyl, tert-butyl, SiMe3 and SiPh, while L was acetylacetone or ethyl acetoacetate. They concluded that [Al(OR)2L]2 complexes are dimers containing tetrahedral and octahedral sites on a twofold axis. Two OR groups bridge the two Al sites, while the octahedral Al is additionally bound to two L groups and tetrahedral Al is additionally bound to two OR groups. Only for large OR groups the molecule was a monomer. Aging of [Al(O-i- Pr)2acac]2 over a period of several weeks yielded a linear trimer having two octahedral Al, one bound to two acetylacetone groups, other bound to one acetylacetone group, and one tetrahedral Al bound to two terminal isopropoxide groups, with four bridging isopropoxide groups joining all metal centres. For the [AI(OR)Ls]2 complexes they proposed centrosymmetric dimer structure having two possible isomers for symmetric L, meso and d,/, distinguishable by NMR only at low temperatures. They also noted that compounds of the [AI(OR)L2]2 type decompose in a solution via ligand disproportionation to AlL3 and [Al(OR)2L]2 and that when L=ethyl acetoacetate, the [AIl(OR)2L], complex is more stable. Finally, Wengrovious et a/.!13] noticed that in the prepared molecules, aluminium prefers the coordination numbers of 6 and 4. On the contrary, the occurrence of coordination 5 is scarce.'*3] Complexes of the [Al(O'Pr) aL]. and AlL3 type with Las methyl, ethyl, iso-propyl, tert-butyl, allyl and 2-(methacryloyloxy)ethyl acetoacetate, were investigated by Lichtenberger et al.!27] They managed to
Figure 1. Keto-enol tautomerism and the formation of chelate. Denomination is introduced for the later interpretation of +H and 13C NMR signals. and keto from, which is known as keto-enol tautomerism (Figure 1).!2° In the enolic tautomer a reactive hydroxy group, capable of becoming a bidentate monocharged enolate ligand is present and can substitute an alkoxy group in the metal alkoxide. As a consequence, a six membered chelate ring may be formed, consequently increasing the hydrolytic stability of the alkoxide (Figure 1).!°! Normally, the keto-enol equilibrium heavily favours the keto form, yet the chelate formation will shift the keto-enol equilibrium towards the enolic form,914) was iso-propyl, tert-butyl, SiMe3 and SiPh, while L was acetylacetone or ethyl acetoacetate. They concluded that [Al(OR)2L]2 complexes are dimers containing tetrahedral and octahedral sites on a twofold axis. Two OR groups bridge the two Al sites, while the octahedral Al is additionally bound to two L groups and tetrahedral Al is additionally bound to two OR groups. Only for large OR groups the molecule was a monomer. Aging of [Al(O-i- Pr)2acac]2 over a period of several weeks yielded a linear trimer having two octahedral Al, one bound to two acetylacetone groups, other bound to one acetylacetone group, and one tetrahedral Al bound to two terminal isopropoxide groups, with four bridging isopropoxide groups joining all metal centres. For the [AI(OR)Ls]2 complexes they proposed centrosymmetric dimer structure having two possible isomers for symmetric L, meso and d,/, distinguishable by NMR only at low temperatures. They also noted that compounds of the [AI(OR)L2]2 type decompose in a solution via ligand disproportionation to AlL3 and [Al(OR)2L]2 and that when L=ethyl acetoacetate, the [AIl(OR)2L], complex is more stable. Finally, Wengrovious et a/.!13] noticed that in the prepared molecules, aluminium prefers the coordination numbers of 6 and 4. On the contrary, the occurrence of coordination 5 is scarce.'*3] Complexes of the [Al(O'Pr) aL]. and AlL3 type with Las methyl, ethyl, iso-propyl, tert-butyl, allyl and 2-(methacryloyloxy)ethyl acetoacetate, were investigated by Lichtenberger et al.!27] They managed to
Figure 2. Al3(O"Bu)g linear trimer. and AIL3 type with Las methyl, ethyl, iso-propyl, tert-butyl,
Figure 2. Al3(O"Bu)g linear trimer. and AIL3 type with Las methyl, ethyl, iso-propyl, tert-butyl,
Figure 3. ‘'H NMR spectra of samples EAO.5—EA3.0 in CDCls. Resonance signals are labelled according to Figure 1. Sec- butanol peaks are labelled with the letter A, while the number is analogous to butoxide, hydroxyl group protons are labelled AS. The +H NMR spectra of all six samples exhibit signals due to the enolic-EAA: at 4.8-5.1 ppm methyne protons singlet occurs (E3), at 4.0-4.25 ppm methylene protons quartet occurs (E4), at 1.7—2.0 ppm methyl protons singlet occurs (E2) and at 1.2-1.3 ppm methyl protons triplet occurs (E1). Yet, the sample EA3.0 shows spectra where also free keto-EAA occurs in traces: at 3.45 ppm methylene protons singlet occur (K3), at 2.27 ppm methyl protons singlet occur (K2) and at 1.24 ppm methyl protons triplet occurs (K1). The weak quartet of methylene protons (K4) is overlapped with E4. According to these observations, complete chelation occurs in all samples except in the sample EA3.0, where only a minor part of EAA is free. The intensity of resonances due to EAA increases from sample EAO.5 to sample EA3.0 as the amount of EAA in the samples increases. In addition, for each of the EAA group multiple sets of resonance peaks were recorded. Such spectral features point out that the molecules of the chelating agent are bound to inequivalent chelating sites,'4! either in a
Figure 3. ‘'H NMR spectra of samples EAO.5—EA3.0 in CDCls. Resonance signals are labelled according to Figure 1. Sec- butanol peaks are labelled with the letter A, while the number is analogous to butoxide, hydroxyl group protons are labelled AS. The +H NMR spectra of all six samples exhibit signals due to the enolic-EAA: at 4.8-5.1 ppm methyne protons singlet occurs (E3), at 4.0-4.25 ppm methylene protons quartet occurs (E4), at 1.7—2.0 ppm methyl protons singlet occurs (E2) and at 1.2-1.3 ppm methyl protons triplet occurs (E1). Yet, the sample EA3.0 shows spectra where also free keto-EAA occurs in traces: at 3.45 ppm methylene protons singlet occur (K3), at 2.27 ppm methyl protons singlet occur (K2) and at 1.24 ppm methyl protons triplet occurs (K1). The weak quartet of methylene protons (K4) is overlapped with E4. According to these observations, complete chelation occurs in all samples except in the sample EA3.0, where only a minor part of EAA is free. The intensity of resonances due to EAA increases from sample EAO.5 to sample EA3.0 as the amount of EAA in the samples increases. In addition, for each of the EAA group multiple sets of resonance peaks were recorded. Such spectral features point out that the molecules of the chelating agent are bound to inequivalent chelating sites,'4! either in a
Figure 4. °C NMR spectra of samples EAO.5—-EA3.0 in CDCls3. Resonance signals are labelled according to Figure 1. Butanol peaks are labelled with the letter A, while the number is analogous to butoxide.
Figure 4. °C NMR spectra of samples EAO.5—-EA3.0 in CDCls3. Resonance signals are labelled according to Figure 1. Butanol peaks are labelled with the letter A, while the number is analogous to butoxide.
Figure 5. ”7Al NMR spectra of samples EAO.5—EA3.0 in CDCls. Resonance signals are labelled with the appropriate coordination number. Inset: The deconvolution of 77Al NMR resonances for the sample EA1.5.
Figure 5. ”7Al NMR spectra of samples EAO.5—EA3.0 in CDCls. Resonance signals are labelled with the appropriate coordination number. Inset: The deconvolution of 77Al NMR resonances for the sample EA1.5.
Table 1. Share of aluminium atoms in various coordinations for samples EAO.5—EA3.0 calculated through deconvolution of 27A NMR spectra of the tetracoordinated aluminium share and the increase of the hexacoordinated aluminium share, clearly occur on behalf of the replacement of alkoxide groups with the bidentate ligand. While it could be argued that such trends are a consequence of the alterations of quantitative relationships between various oligomeric compounds in the mixture, the appearance of the pentacoordinated aluminium seems correlated with one oligomeric compound. multiple resonances, point out to the existence of inequivalent chelating sites.4] As expected, with the increase of the EAA/AI(O’Bu)3 molar ratio, the EAA resonances intensities in the chelate also increase. But their relative intensities, characteristic for EAA present in various isomers, change considerably. Some resonances occur only for the higher EAA/AI(OSBu)3 molar ratios, while other at higher ratios completely disappear. These characteristic signals change their intensity as a function of the EAA amount, which could aid the deduction of the presence of the coordinated compounds. Based on the _ intensity changes, peaks due to E3 resonance could be sorted into five groups (I — V): broad peak between 5.03 and 5.06 ppm (I), group of three peaks in the range 4.90-4.93 ppm (Il), four peaks where one is between 5.01 and 5.03 ppm and the other three in the range 4.88-4.91ppm (Ill), five peaks between 4.84 and 4.88 ppm (IV), and four peaks in the range 4.91—4.96 ppm (V) (Figure 6). multiple resonances, point out to the existence of
Table 1. Share of aluminium atoms in various coordinations for samples EAO.5—EA3.0 calculated through deconvolution of 27A NMR spectra of the tetracoordinated aluminium share and the increase of the hexacoordinated aluminium share, clearly occur on behalf of the replacement of alkoxide groups with the bidentate ligand. While it could be argued that such trends are a consequence of the alterations of quantitative relationships between various oligomeric compounds in the mixture, the appearance of the pentacoordinated aluminium seems correlated with one oligomeric compound. multiple resonances, point out to the existence of inequivalent chelating sites.4] As expected, with the increase of the EAA/AI(O’Bu)3 molar ratio, the EAA resonances intensities in the chelate also increase. But their relative intensities, characteristic for EAA present in various isomers, change considerably. Some resonances occur only for the higher EAA/AI(OSBu)3 molar ratios, while other at higher ratios completely disappear. These characteristic signals change their intensity as a function of the EAA amount, which could aid the deduction of the presence of the coordinated compounds. Based on the _ intensity changes, peaks due to E3 resonance could be sorted into five groups (I — V): broad peak between 5.03 and 5.06 ppm (I), group of three peaks in the range 4.90-4.93 ppm (Il), four peaks where one is between 5.01 and 5.03 ppm and the other three in the range 4.88-4.91ppm (Ill), five peaks between 4.84 and 4.88 ppm (IV), and four peaks in the range 4.91—4.96 ppm (V) (Figure 6). multiple resonances, point out to the existence of
Figure 6. Segment of 'H NMR E3 resonances in the spectra of samples EAO.5—EA3.0 in CDCls3. Parts of the 1H NMR spectra, where characteristic E3 ring methyne proton of EAA resonances occurs, are given in Figure 6. As discussed before, such spectra, exhibiting
Figure 6. Segment of 'H NMR E3 resonances in the spectra of samples EAO.5—EA3.0 in CDCls3. Parts of the 1H NMR spectra, where characteristic E3 ring methyne proton of EAA resonances occurs, are given in Figure 6. As discussed before, such spectra, exhibiting
DOI: 10.5562/cca3426 Croat. Chem. Acta 2019, 92(1) Table 2. Sum of area of five resonance sets for samples EAO.5S—EA3.0 calculated through the deconvolution of ‘H NMR spectra
DOI: 10.5562/cca3426 Croat. Chem. Acta 2019, 92(1) Table 2. Sum of area of five resonance sets for samples EAO.5S—EA3.0 calculated through the deconvolution of ‘H NMR spectra
Figure 7. The deconvolution of E3 1H NMR resonances for the sample EA1.5.
Figure 7. The deconvolution of E3 1H NMR resonances for the sample EA1.5.
As can be seen in Figure 6, four distinct signals due to compound V can be observed. It was established!” that these signals are consistent with the tris-chelated monomer Al(EAA)3, whereas four signals arise on behalf of the coexistence of different isomers in the solution. The trisubstituted Al (B-ketoesterate); compounds are mononuclear octahedral complexes with asymmetric chelating ligands.2275! They display two stereoisomers (Figure 8) due to non-equivalent oxygen atoms (close to the ethoxyde and methyl group); facial (fac) and meridional Figure 8. Schematic representation of the proposed structures of a) fac and b) mer isomers of Al(EAA)3 complex.
As can be seen in Figure 6, four distinct signals due to compound V can be observed. It was established!” that these signals are consistent with the tris-chelated monomer Al(EAA)3, whereas four signals arise on behalf of the coexistence of different isomers in the solution. The trisubstituted Al (B-ketoesterate); compounds are mononuclear octahedral complexes with asymmetric chelating ligands.2275! They display two stereoisomers (Figure 8) due to non-equivalent oxygen atoms (close to the ethoxyde and methyl group); facial (fac) and meridional Figure 8. Schematic representation of the proposed structures of a) fac and b) mer isomers of Al(EAA)3 complex.
Figure 9. Schematic representation of the proposed centrosymmetric structures of a) cis-cis, b) cis-trans and c) trans-cis isomers of Alo(OBu)2(EAA)4 complex. First prefix is omitted since the bridging butoxide groups are fixed in cis position.
Figure 9. Schematic representation of the proposed centrosymmetric structures of a) cis-cis, b) cis-trans and c) trans-cis isomers of Alo(OBu)2(EAA)4 complex. First prefix is omitted since the bridging butoxide groups are fixed in cis position.
Figure 10. Schematic representation of the proposed structure of a) one possible isomer of Al2(OBu)3(EAA)3 complex, b) or possible isomer of Al2(OBu)4(EAA)2 complex and c) Al3(OBu)s(EAA) complex.
Figure 10. Schematic representation of the proposed structure of a) one possible isomer of Al2(OBu)3(EAA)3 complex, b) or possible isomer of Al2(OBu)4(EAA)2 complex and c) Al3(OBu)s(EAA) complex.
Table 3. Share of five compounds in samples EAO.5—-EA3.0 calculated using data in Table 1 and assumed compounds compositions measured shares has been obtained. Finally, the shares of aluminium atoms in various coordination, calculated on the basis of the ‘1H NMR spectra (Table 4) were compared with the shares obtained through the deconvolution of the ?7Al NMR spectra (Table 1). Again, closely matching numbers were obtained. Therefore, the comparison of results obtained by spectra deconvolutions confirmed the assumptions on oligomer structures. matching the initial stoichiometry would be obtained. Also, the calculated butanol to butoxide share would match the share determined by *H NMR. Finally, the calculated proportions of aluminium with various coordination numbers would match those determined by ?7Al NMR. Therefore, the integral areas of the E3 resonance (Table 2) have been deconvoluted, taking into consideration the assumed oligomers in order to yield the molar fraction of the compounds. The results are presented in Table 3.
Table 3. Share of five compounds in samples EAO.5—-EA3.0 calculated using data in Table 1 and assumed compounds compositions measured shares has been obtained. Finally, the shares of aluminium atoms in various coordination, calculated on the basis of the ‘1H NMR spectra (Table 4) were compared with the shares obtained through the deconvolution of the ?7Al NMR spectra (Table 1). Again, closely matching numbers were obtained. Therefore, the comparison of results obtained by spectra deconvolutions confirmed the assumptions on oligomer structures. matching the initial stoichiometry would be obtained. Also, the calculated butanol to butoxide share would match the share determined by *H NMR. Finally, the calculated proportions of aluminium with various coordination numbers would match those determined by ?7Al NMR. Therefore, the integral areas of the E3 resonance (Table 2) have been deconvoluted, taking into consideration the assumed oligomers in order to yield the molar fraction of the compounds. The results are presented in Table 3.
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