Enantiomerisation of tetrahedral homochiral [M4L6] clusters: Synchronised four Bailar twists and six atropenantiomerisation processes monitored by temperature-dependent dynamic H-1 NMR spectroscopy

Abstract

Temperature-dependent H-1 NMR studies prove homochiral, racemic [(Delta,Delta,Delta,Delta)/(Lambda,Lambda,Lambda,Lambda]-[(NH4)(4)boolean AND[Mg-4(L-1)(6)]} (1) to be kinetically stable on the NMR timescale. Due to steric reasons, rotation around the central C-C single bond in (L-1)(2-) is blocked, which prevents 1 from enantiomerisation. Most interestingly, however, the H-1 NMR spectrum of racemic 2a reveals dynamic temperature dependence. This phenomenon can be explained by simultaneous Bailar twists at the four octahedrally coordinated magnesium centres, synchronised with the sterically unhindered atropenantiomerisation processes around the C-C single bonds of the six ligands (L-2)(2-), leading to the unprecedented enantiomerisation (Delta,Delta,Delta,Delta)-2a reversible arrow (Lambda,Lambda,Lambda,Lambda)-2a. The profound nondissociative rearrangement occurs without the formation of diastereoisomers. Supplementary support for the interpretation of the temperature-dependent dynamic H-1 NMR spectra of 2 a is presented by additional studies of [(Delta,Delta,Delta,Delta)/(Lambda,Lambda,Lambda,Lambda]-{EtNH3)(4)boolean AND[Mg-4(L-2)(6)]} (2b). In 2 a and 2b, the ether methylene protons exhibit identical temperature dependence. However, with addition, the methylene protons of the ethyl ammonium groups of 2b display similar temperature dependence as the ligand ether methylene protons.