Sapphyrin dimer

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. 

Dimeric sapphyrins 5.138 and 5.139, containing chiral bridging units have also been synthesized. These were obtained by treating the activated form of the sapphyrin mono-acid 5.109 with a chiral diamine (i.e., 5.135 or 5.136 Figure 5.6.3). This afforded the corresponding sapphyrin dimers 5.138 and 5.139 in yields as high as 60% (Schemes 5.6.8 and 5.6.9). The cyclic sapphyrin dimer 5.140 was also prepared from the chiral diamine 5.137 and bis-acid sapphyrin 5.109. However, in this latter instance a stepwise synthetic approach was required. It involved reacting two equiva-... 

Figure 11, View of the self-assembled dimer formed from sapphyrin 5 in the presence of trifluoroacetic acid. This X-ray structural figure was generated information downloaded from the Cambridge Crystallographic Data Centre and corresponds to a structure originally reported in ref. 21. Atom labeling scheme carbon C nitrogen oxygen fluorine phosphorous hydrogen o. Selected hydrogen atoms...

In the specific case of the tailed sapphyrin carboxylates 5 and 6, for which evidence of self-assembly was noted in the solid state (vide supra), H NMR spectroscopic studies carried out in 4-methanol, d-chloroform, and mixtures of the two solvents showed strong line broadening, and upfield shifts of the methylene tail peaks. Such findings are, of course, fully consistent with the proposed dimerization. Further, dilution experiments performed over a concentration range of 50 to 5 mM in these solvents showed little change in the spectra, indicating that the dimeric form prevails under these conditions, even in highly polar solvents. In the case of 6, the actual dimeric stoichiometry was confirmed by vapor pressure osmometry (VPO) measurements carried out in 1,2-dichloroethane. 

Figure 13. Schematic representation showing both the dimerization of sapphyrin, 6, that occurs spontaneously in solution, and the inhibition of this dimerization process that can be effected by adding fluoride anion.

UV-vis spectroscopy was also used to probe the nature of the presumed sap-phyrin-DNA complex. Before such studies could be made, however, it proved necessary to map out the spectroscopic properties of sapphyrin itself when constrained to an aqueous medium. As the result of these latter predicative analyses, it was determined that sapphyrin can exist in three spectroscopically distinct states in aqueous media. The first spectroscopically distinct state, characterized by a Soret-like absorbance at = 450 nm, is ascribed to a monomeric form. The second, assigned to dimerized forms of sapphyrin, is readily identified by its characteristic Soret-like band at = 420 nm. The third state, identified on the basis of its signature 400-nm Soret-like absorption maximum, corresponds to the highly aggregated form of sapphyrin it is this form that is generally dominant in simple aqueous media. 

The first example of a capped sapphyrin to be reported was the face-to-face porphyrin-sapphyrin pseudo dimer 5.127. It was reported by Sessler, et al. in 1994 ... 

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