ELECTRONIC FACTORS DETERMINING THE REARRANGEMENT OF DIALKYLALUMINUM O,O'-CHELATE COMPOUNDS FROM DIMERIC 5-COORDINATED TO MONOMERIC 4-COORDINATED COMPLEXES ON DISSOLUTION - STRUCTURE INVESTIGATIONS OF DIALKYLALUMINUM CHELATE DERIVATIVES OF ALPHA-HYDROXY AND BETA-HYDROXY CARBONYL-COMPOUNDS

The synthesis and the structure characterization of dialkylaluminum O, O'-chelate complexes derived from alpha- and beta-hydroxy carbonyl com pounds: alpha-tropolone (tropH), ethyl rac-lactate (elacH), and 2-acet yl-4-chlorophenol (Cl-acphH), with saturated and conjugated unsaturate d bond systems, are reported. The reaction of R3Al with an equimolar a mount 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 co mpounds are dimeric in the solid state. The aluminum atoms adopt a dis torted trigonal bipyramidal geometry with a long internal (in 1a and 2 b) or external (in 3) Al-O axial bond. In addition, complex 3 posseses a C-2 symmetry, which indicates that the five-coordinated [R2Al(O,O') ](2)-type adduct is formed in a highly stereoselective manner. In solu tion 1a and 2b adducts rearrange to the monomeric four-coordinated che late complexes, while 3 remains a dimeric structure found in the solid state, as confirmed by cryoscopic molecular weight determination and H-1 and Al-27 NMR and IR spectroscopy. The influence of the nature of the O,O'-bifunctional ligand on the structure of dialkylaluminum chela te complexes in the solid state and in solution and the geometry of th e aluminum coordination sphere are discussed on the basis of the resul ts reported and structural data of related compounds.