Sapphyrins, anion binding

In addition to phosphate and halide anion binding, carboxylate chelation by sapphyrin macrocycles has been the subject of recent investigation. To date, two crystal structures have been solved. A 2 1 complex formed between diprotonated sapphyrin 3 and trifluoroacetic acid shows that the oxyanions are chelated above and below the sapphyrin plane (Figure 10). ° Greater complexity of organi-... 

More detailed analyses of sapphyrin anion chelation in solution were made using a full range of tricks borrowed from the supramolecular field. Thus both spectroscopic techniques (e.g., NMR, UV-vis, fluorescence spectroscopy) and transport studies (carried out in a model Pressman type U-tube membrane system ) were employed. From these analyses, it became clear that sapphyrin does in fact bind various negatively charged substrates in solution, but does so both with variable affinity and oft-times remarkable selectivity. These findings/conclusions are detailed further in the paragraphs below. 

It occurred to us that the anion binding properties of sapphyrins could be employed for the self-assembly of these species. In particular, we speculated, that the zwitterionic sapphyrins 5 and 6 should self-assemble. These species contain... 

Our first generation sapphyrin-sapphyrin dimer is represented by structure 15. In this system, two covalently connected anion binding sites are contained within the same overall recognition framework. It was thus expected that system 15 would act as an effective receptor for dicarboxylate anions under conditions where the corresponding monomeric systems (e.g. 3) would not. Indeed, this proved to be the case (vide infra). 

Mass spectrometric studies were carried out as a first qualitative means of checking for dicarboxylate anion binding (see also Section 3). Here, mixtures of sapphyrin dimer 15 and several representative dicarboxylate anions, such as oxalate, 4-nitrophthalate, 5-nitroisophthalate and nitroterephthalate in methanol, were subjected to high resolution FAB mass spectrometric (HR FAB MS) analysis using FAB positive NBA matrix. In general, peaks for the putative complexes were seen, lending credence to the hypothesis that the dicarboxylate substrates in question were, in fact, being bound by 15 under the matrix desorption/gas phase conditions used to effect these mass spectrometric analyses. 

ANION BINDING BY SAPPHYRINS Jonathan L. Sessler, Andrei Andrievsky, and John W. Cenge... 

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