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.

Ligand‐site exchange in intramolecular complexes of silicon: substituent effects

“…From the coalescence of these signals one can calculate the Δ G ≠ values . This approach has been developed in many publications . In specific cases (Table , Series IX, XII) the free energies Δ G ≠ are determined from results of NMR line‐shape analysis …”

“…Various mechanisms have been advanced to explain the ligand‐site exchange in the hypervalent compounds . A unified mechanism seems to be lacking.…”

“…For our purposes an important point is that the experiments have provided a considerable body of data on permutational isomerization . The ligand‐site exchange is a stereodynamic process; its rate ranges from 10 −1 to 10 −5 s .…”

“…Therefore, these processes are conveniently studied by using the dynamic nuclear magnetic resonance (NMR) spectroscopy (an examination of coalescence phenomena and that of the temperature dependence of the line shape). This method is widely applied to determine the free energies of activation for ligand exchange Δ G ≠ …”

Racemization Pathway for MoO2(acac)2 Favored over Ray–Dutt, Bailar, and Conte–Hippler Twists

“…An alternative is to solely use the stereogenicity of the metal center by the enantiomeric chelation of achiral ligands around the coordinating transition metal. 4 …”