Comparative Coordination Studies of Chiral Supramolecular Propellers of Dialkyltartrate Trimers with Alkali Ions Using Electrospray Ionization Mass Spectrometry

Abstract

Electrospray ionization mass spectrometry with a quadrupole analyzer has been used to compare the assembly of all stereoisomers of dialkyltartrates M with alkali ions A⁺ to form supramolecular trimer complexes M₃·A⁺. In a first set of experiments using pure enantiomers of M, the chiral propeller complexes M₃·A⁺ showed the highest stability with K⁺ as the central ion. For the ligand M = dimethyltartrate, the relative ionophore selectivities decreased in the order of K⁺(100), Na⁺(78), Rb⁺(65), Cs⁺(28), Li⁺(6). For both the pseudo crown ether complex M₃·K⁺ and the corresponding 18-crown-6·K⁺ complex, the relative ion abundances were the same when a 15-fold excess of the formal trimer M₃, in comparison with the 18-crown-6 ligand, was present in the experiment. In a second set of experiments, we investigated the chirality effect on M₃·A⁺-propeller formation by using enantiomer-labeled racemic mixtures of (S)- and (R)-dialkyltartrates and comparing the formation of the diastereomeric clusters SSS·A⁺, SSR·A⁺, SRR·A⁺ and RRR·A⁺. The chiral discrimination ratio (CDR) in favor of the homochiral propellers SSS·A⁺ and RRR·A⁺ compared with the heterochiral clusters SSR·A⁺ and SRR·A⁺ shows unusually high values of about 18 for K⁺ and Rb⁺ with dimethyltartrate (DMT) ligands. The CDR values are not as high for complexes with other alkali ions and/or the bulkier diisopropyltartrate (DIPT) ligands. The maximum value of CDR-18 falls between comparable maximum values of CDR = 5 from chemical ionization studies and CDR = 45 from studies using Fourier transform ion cyclotron resonance mass spectrometry. Semiempirical AM1 calculations have been applied to predict plausible structures for the homochiral propeller complexes (DMT)³·K⁺.