Abstract
Reactive aluminum alkoxide (ASB, aluminium sec-butoxide) was chelated using β-diketone (EAA, ethyl acetoacetate) in order to gain control over rapid hydrolysis in the course of the sol-gel process. Derived chelates were analysed using several NMR spectroscopic techniques: one-dimensional 1H, 13C, 27Al NMR and two-dimensional COSY, HSQC and DOSY. The NMR analysis enabled identification of the formed chelate species, as well as determination of their quantitative relationships. Several complexation products were observed: tris-chelated monomer, Al(EAA)3, bis-chelated dimmer, Al2(OnBu)4(EAA) 2, tris-chelated dimmer, Al2(OnBu)3(EAA)3, tetra-chelated dimmer, Al2(OnBu)2(EAA)4, and monochelated trimer, Al3(OnBu)8(EAA). Of the formed oligomer compounds, this is the first evidence of Al2(OR)3L3 in any alkoxide and β-ketoester or β-diketone combination. Aluminium sec-butoxide and ethyl acetoacetate complexes Al2(OnBu)4(EAA) 2 and Al2(OnBu)2(EAA)4 were also observed for the first time. With the increase of the EAA/ASB ratio the coordination of aluminium shifts towards six, whereas above the EAA/ASB ratio of 2.5 solely six-coordinated aluminium exists.
Highlights
A LUMINA materials, especially transient forms exhibiting lower level of structural ordering are well known for their high specific surfaces
The spectra showed the presence of chelate, i.e. sec-butoxide and ethyl acetoacetate, both bound to aluminium, as well as sec-butanol released in the course of the chelation
The reactivity of aluminium sec-butoxide was controlled by chelation with ethyl acetoacetate in the course of the sol-gel process
Summary
A LUMINA materials, especially transient forms exhibiting lower level of structural ordering are well known for their high specific surfaces. The favoured coordination number for aluminium in alkoxides is 6, which is achieved through the oxygen atoms of alkoxide groups.[12] The expansion of coordination is enabled via alkoxy bridging on behalf of which oligomeric species are formed.[13] Thereby, different metal centres, ligands and alkoxide/chelator ratios will yield different oligomers.
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