Stereochemical rearrangements of dibromides of hexacoordinated germanium containing amidomethyl and lactamomethyl chelate ligands

For the first time a slow (on the NMR time scale) dynamic exchange between two diastereomers of hexacoordinated dibromogermanes containing two amidomethyl and lactamomethyl C,O-chelate ligands has been observed at room temperature in solution. The influence of temperature and solvent on the ratio of diastereomers was studied. At higher temperatures only one set of signals of the products were detected in the NMR spectra. Based on the dynamic NMR and quantum-chemical calculations (ab initio FR 3-21G), as well as on the structures determined for these compounds in the solid state, a cis-configuration of monodentate ligands was proposed for one of the diastereomers and an all-trans-configuration for the other. At low temperatures interconversion of enantiomers in the cis-diastereomer was observed. In the case of bis[(N-methylacetamido)methyl]dibromogermane the activation and thermodynamic parameters of enantiomerization (ΔG#298 12.0±0.1 kcal mol−1, ΔH# 10.7±0.3 kcal mol−1, ΔS# −4.6±2.5 cal mol−1 K−1) and diastereomeric exchange OcCtBrc⇌OtCtBrt (ΔG#298 15.0±0.1 kcal mol−1; ΔH# 0.1±0.8 kcal mol−1; ΔS#−50.0±5.8 cal mol−1 K−1; ΔG°298 −0.2±0.1 kcal mol−1; ΔH° −0.8±0.8 kcal mol−1; ΔS° −2.6±1.0 cal mol−1 K−1) were determined.