The activation parameters for the stereoisomerizations of tris(2,6-dimethyl-3,5-heptanedionato)aluminum(III) and bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)(2,6-dimethyl-3,5-heptanedionato)aluminum(III) have been determined by dynamic nuclear magnetic resonance (DNMR) spectroscopy and are reported. The reactions are found to be first order and intramolecular. Permutational analysis of the combined results of this study and those of all other DNMR studies of tris(β-diketonato) aluminum chelates reported reveal that the steric course of stereoisomerization involves the effective simultaneous interchange of the terminal groups within two of the three bidentate ligands and enantiomerization at the metal center. Consideration of the magnitudes of the activation parameters, the effective steric course, and a topological analysis leads to the following mechanistic conclusions. Chelates with alkyl or aryl substituents on the β-diketonate rings most likely stereoisomerize by a rhombic twist mechanism. Chelates with fluorocarbon substituents on the β-diketonate rings stereoisomerize by a bondrupture mechanism which proceeds via an actual square pyramidal-apical five-coordinate intermediate.
ASJC Scopus subject areas
- Colloid and Surface Chemistry