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Self‐Assembled Ionophores from Isoguanosine: Diffusion NMR Spectroscopy Clarifies Cation's and Anion's Influence on Supramolecular Structure
Author(s) -
EvanSalem Tamar,
Frish Limor,
van Leeuwen Fijs W. B.,
Reinhoudt David N.,
Verboom Willem,
Kaucher Mark S.,
Davis Jeffery T.,
Cohen Yoram
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200601015
Subject(s) - chemistry , tetraphenylborate , nuclear magnetic resonance spectroscopy , crystallography , alkali metal , supramolecular chemistry , salt (chemistry) , stoichiometry , inorganic chemistry , ion , stereochemistry , organic chemistry , crystal structure
Cation‐templated self‐assembly of the lipophilic isoguanosine (isoG 1 ) with different monovalent cations (M + =Li + , Na + , K + , NH 4 + , and Cs + ) was studied in solvents of different polarity by using diffusion NMR spectroscopy. Previous studies that did not use diffusion NMR techniques concluded that isoG 1 forms both pentamers (isoG 1 ) 5 ⋅ M + and decamers (isoG 1 ) 10 ⋅ M + in the presence of alkali‐metal cations. The present diffusion NMR studies demonstrate, however, that isoG 1 does not form (isoG 1 ) 5 ⋅ M + pentamers. In fact, the diffusion NMR data indicates that both doubly charged decamers of formula (isoG 1 ) 10 ⋅ 2 M + and singly charged decamers, (isoG 1 ) 10 ⋅ M + , are formed with lithium, sodium, potassium, and ammonium tetraphenylborate salts (LiB(Ph) 4 , KB(Ph) 4 , NaB(Ph) 4 and NH 4 B(Ph) 4 ), depending on the isoG 1 :salt stoichiometry of the solution. In the presence of CsB(Ph) 4 , isoG 1 affords only the singly charged decamers (isoG 1 ) 10 ⋅ Cs + . By monitoring the diffusion coefficient of the B(Ph) 4 − ion in the different mixtures of solvents, we also concluded that the anion is more strongly associated to the doubly charged decamers (isoG 1 ) 10 ⋅ 2 M + than to the singly charged decamers (isoG 1 ) 10 ⋅ M + . The (isoG 1 ) 10 ⋅ 2 M + species can, however, exist in solution without the mediation of the anion. This last conclusion was supported by the finding that the doubly charged decamers (isoG 1 ) 10 ⋅ 2 M + also prevail in 1:1 CD 3 CN:CDCl 3 , a solvent mixture in which the B(Ph) 4 − ion does not interact significantly with the self‐assembled complex. These diffusion measurements, which have provided new and improved structural information about these decameric isoG 1 assemblies, demonstrate the utility of combining diffusion NMR techniques with conventional NMR methods in seeking to characterize labile, multicomponent, supramolecular systems in solution, especially those with high symmetry.