Comparative coordination studies of chiral supramolecular propellers of dialkyltartrate trimers with alkali ions using electrospray ionization mass spectrometry

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Eur. J. Mass Spectrom. 5, 289 - 294 (1999)

Comparative coordination studies of chiral supramolecular propellers of dialkyltartrate trimers with alkali ions using electrospray ionization mass spectrometry

Roderich Süssmuth and Günther Jung
Institut für Organische Chemie, Universität Tübingen, D-72076 Tübingen, .
F. Johann Winkler^* and Ramiro Medina
Lehrstuhl für Biologische Chemie, Technische Universität München, D-85350 Freising, .

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 M₃A⁺. In a first set of experiments using pure enantiomers of M, the chiral propeller complexes M₃A⁺ 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⁺.

Keywords: dialkyltartrates, electrospray mass spectrometry, supramolecular complexes, molecular recognition, alkali ion selectivity, chiral discrimination, comparative binding, AM1 calculations

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