Stereochemistry of six-coordinated silicon complexes V 1. Racemization and geometrical isomerization of tris(4-isopropyltropolonato)silicon(IV)

Abstract

The six-coordinated octahedral silicon(IV) complex [Si(ht) 3] +, where ht denotes the unsymmetrical didentate 4-isopropyltropolonate(1−) (≡ hinokitiolate(1−), C 10H 11O 2) ligand, was prepared. The complex was resolved partially by fractional crystallization of the diastereomer with d-tartratoantimonate(III), [Sb 2( d-tart) 2] 2−. The enantiomer obtained from the less soluble part was assigned to Λ on the basis of the exciton theory. The two geometrical isomers, mer (meridional) and fac (facial), were separated by high-performance ion-pair liquid chromatography. Kinetics of racemization and geometrical isomerization of mer-Λ- and fac-Λ-[Si(ht) 3] + were followed spectropolarimetrically and chromatographically respectively in methanol, 1,1,2,2-tetrachloroethane, and chloroform at 30–50°C. The first-order rate constants and activation energies for both reactions are similar to each other. The results are interpreted in terms of a common reaction pathway; they proceed by a bond-rupture mechanism through a five-coordinated intermediate.