Stereochemical Flexibility of Bis(amidomethyl)dichlorogermanes. A Novel Dissociative Mechanism of Ligand Exchange in Neutral Hexacoordinated Bischelate Complexes

Abstract

A novel dissociative mechanism of (L,L)-ligand-site exchange in neutral hexacoordinated cis-(M←O) complexes of the type L2MX2 (M = Ge and Sn; X = Hal) is proposed. All the activation parameters of enantiomerization for (C,O)-chelate bis(amidomethyl)dichlorogermanes 4−6 have been determined by the dynamic NMR method. The fairly positive values of entropy for exchange in 4−6 (up to 17 cal·mol-1·K-1) suggest an interconversion of chelate ligands followed by a dissociative−associative mechanism. The correlation of activation barriers ΔG⧧298 in hexacoordinate L2GeCl2 species (4−6) with intramolecular Ge←O distances in the crystals of analogous pentacoordinate LGeMe2Cl complexes with the same chelate ligand was found. A two-step process for intramolecular exchange including the dissociation of the coordinate Ge←O bond and pseudorotation in a pentacoordinate intermediate is considered as a model for the enantiomerization. It is shown that the main factor controlling the stereodynamic process in dichalogermanes 1−8 is pseudorotation B⇆B* for the pentacoordinate intermediate, while in the analogous tin complexes 9−12 it is the Sn←O bond rupture A⇆B in the primary hexacoordinate state.