Typical Molecules with π-Acidic Surfaces

F

Figure 24: Examples of simple aromatic systems with π-acidic surfaces used for anion-πinteractions.

The main drawback in the application of anion-πinteractions is not a con-sequence of the nature of these interactions, but rather the scarce number of cases where the aromatic surface is, in fact,π-acidic and thus a small account on this seems most fitting.

1 Introduction 31

Other the aromatic molecules used by Koshi⁹⁰ (Figure 22), many aromatic systems have been used in recent years to inverstigate anion-πinteractions.

The most common ones are perfluorinated phenyl units 57. Aromatic azo-compounds like pyrazine, triazine and tetrazine have been used extensively in crystal engineering.⁹⁵Cyanuric acid derivatives have also been used.⁹⁶ These motifs are usually part of more complex molecules and not individually studied, some examples will be given later. Some of the most common ones (57 to 62) are depicted in Figure 24.

N

Figure 25: Examples of complex aromatic systems withπ-acidic surfaces used for anion-πinteractions.

More complex polyaromatic structures have attracted considerable atten-tion; some of them are depicted in Figure 25.

Compound63is a popular anion-πaromatic system that was shown to form nice and informative single crystal structures. Indeed, the structure presented in Figure 26 demonstrates, on one hand, that anion binding can happen at different positions of theπ-acidic surface and, on the other hand, it shows that charge transfer and anion-πcontacts in the solid state are feasible. In addition, a similar situation was observed in the solution phase where the classic change in color of a charge transfer complex was observed. This allowed the authors to assess the association constants which were found to be around 1·10³ -4·10³ M⁻¹ in THF, which is very high for a charge-transfer complex. The

Figure 26: Crystal structures reported by the Dunbar group between three63, four bromides and three benzenes.⁹⁷

study was completed with NMRspectroscopy and mass spectrometry evidence of the formed complex.

An original example is the case of tetrazide 64 which forms crystals with silver salts. This case is notable because silver acts as a Lewis acid and coordinates the azide moiety, thus enhancing the π-acidity of the aromatic system. This enhancement is manifested in a significantly shorter anion-π distance (ofca. 2.6 ˚A for the64:AgNO3complex; Figure 27). This approach was shown to be applicable to perchlorate as well.

The study was completed with the characterization of the complexes by ab initio computational methods at the MP2(full)/auc-cc-pVTZ // RI-MP2(full)/TZVP level of theory confirming that the silver coordination en-hances theπ-acidity of the system. In addition, the reported O-πdistances of this complex are one of the shortest distances observed for anion-π interac-tions to this date. Although this effect was not observed in solution, it clearly suggests that enhancement of theπ-acidity by a Lewis acid coordination could lead to broader applications.⁹⁸

1 Introduction 33

Downloaded by UNIVERSITE DE GENEVE on 14 March 2013 Published on 24 February 2009 on http://pubs.rsc.org | doi:10.1039/B818125J

Figure 27: Crystal structures of tetrazide64with AgNO3reported by the Domase-vitch group.⁹⁸

Naphthalenediimides (NDIs;65) and perylenediimides (PDIs;66) have been applied to anion transport with the synthesis of NDI⁹⁹and PDI¹⁰⁰anion slides.

With NDIs, compound67(Figure 28) was designed with an appropriate length (32 ˚A) to span the bilayer membrane; the ability of this molecule to promote anion transport through the lipid membrane was assessed with the HPTS assay. The observed activity was explained by a trans-membrane orientation which allows for an anion hopping mechanism from one π-acidic surface to another and thus anions were proposed to “slide” across the membrane.⁹⁹

This concept was expanded to PDIs with compound68which includes two PDIs instead of NDIs. In this case, anion transport was observed and an anion selectivity was concluded from ion exchange experiments. PDI slide 68 in addition could be used as a model photosystem in vesicles with active, photo-induced electron influx.¹⁰⁰

These two examples show the general applicability of π-acidic surfaces to the design of anion slides.

This concept was expanded later with the use of bare NDIs (69 to 72, Figure 29). These compounds were studied with several methods. Titration

N

Figure 28: NDI and PDI anion slides proposed by the Matile group.^99,100

of NDI 69 with tetrabutylammonium iodide revealed a charge transfer band that upon appropriate treatment would imply a K_D of 5.9 M. Additionally, electrospray ionization fourier-transform ion cyclotron resonance tandem mass spectrometry experiments were performed and the formation of NDI-halides could clearly been observed. This was complemented with DFT-optimized structures and MP2/6-311++G**//PBE1PBE/6-311G** calculations for the binding energies that were in the order of 140 kcal mol⁻¹ for the chloride/72 complex; moreover, the optimal position for the anion was shown to be dif-ferent to the centroid which is the case for simple aromatic rings.⁹²

Anion transport was possible with these small molecules and two main ef-fects were observed: the steric hindrance of the trimethylphenyl unit decreases the activity with respect to the phenyl substituted molecules and anion trans-port depends on theπ-acidity of the NDI. Under these conditions, compound 72 is the most active. Interestingly, with increasingπ-acidity the selectivity towards different anions would change displaying a strong nitrate selectivity for compound72; this was attributed to a favorable interaction between the planar nitrate and the flat NDI surface.

All these data would suggest that NDI73should display significant levels of selectivity and improved activities as anion transporter, but the synthesis failed after several attempts and was finally considered too reactive under normal

1 Introduction 35

Figure 29: Some NDIs used by the Matile group for the study of anion transport with anion-πinteractions.⁹²

conditions.