Electronic Factors Determining the Rearrangement of Dialkylaluminum O,O‘-Chelate Compounds from Dimeric Five- to Monomeric Four-Coordinated Complexes on Dissolution. Structure Investigations of Dialkylaluminum Chelate Derivatives of α- and β-Hydroxy Carbonyl Compounds

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The synthesis and the structure characterization of dialkylaluminum O,O‘-chelate complexes derived from α- and β-hydroxy carbonyl compounds: α-tropolone (tropH), ethyl rac-lactate (elacH), and 2-acetyl-4-chlorophenol (Cl-acphH), with saturated and conjugated unsaturated bond systems, are reported. The reaction of R3Al with an equimolar amount of the hydroxy carbonyl compound leads to the formation of [R2Al(O,O‘)]n chelate complexes [where O,O‘ = Cl-acph, R = Me (1a); trop, R = Et (2b); elac, R = Me (3)]. According to the X-ray study these compounds are dimeric in the solid state. The aluminum atoms adopt a distorted trigonal bipyramidal geometry with a long internal (in 1a and 2b) or external (in 3) Al−O axial bond. In addition, complex 3 posseses a C2 symmetry, which indicates that the five-coordinated [R2Al(O,O‘)]2-type adduct is formed in a highly stereoselective manner. In solution 1a and 2b adducts rearrange to the monomeric four-coordinated chelate complexes, while 3 remains a dimeric structure found in the solid state, as confirmed by cryoscopic molecular weight determination and 1H and 27Al NMR and IR spectroscopy. The influence of the nature of the O,O‘-bifunctional ligand on the structure of dialkylaluminum chelate complexes in the solid state and in solution and the geometry of the aluminum coordination sphere are discussed on the basis of the results reported and structural data of related compounds.