Nitrogen, protonated anion binding

A range of cyclophanes (compounds containing a bridged aromatic ring - for a full discussion of cyclophane chemistry see Section 6.5) with protonated nitrogen functionalities have been used with great success as anion binding hosts. In particular, the diphenylmethane moiety is commonly used... 

The central, pyridine-like nitrogens of porphyrins are notoriously non-basic. As a result, the porphyrins remain neutral in charge at or near physiological pH. By contrast, many of the expanded porphyrins contain internal cores that are both readily protonated and intrinsically large. As a result, they act as anion binding agents. This ability, of course, is without parallel in the chemistry of the porphyrins. Thus, it, perhaps more than any other aspect of expanded porphyrin chemistry, has served to animate this still-emerging field. 

Fig. 9.10 Schematic depicting left) metal-centered active site structure in proximity to a surface nitrogen species, center) protonation of the surface nitrogen species with the active site having a high turnover frequency, and right) anion binding to protonated surface nitrogen species dramatically reducing active site turnover frequency (reprinted with permission from [74], copyright 2011 American Chemical Society)...

Ditopic binding of the perchlorate anions is observed in both L38 and L39 in hexa- and tetraprotonated forms, respectively, resulting into ditopic complexes in which the two perchlorate ions are coordinated by the hydrogenbonding interactions within their protonated forms. The mode of ditopic encapsulation of perchlorate ions can be described as face binding. In L39, all four protonated sites are utilized in binding anions however, in L38, two protonated nitrogen atoms remain unbonded. X-ray crystal structure analysis of perchlorate complex of L38 indicates the involvement of total six hydrogen bonds (N- O = 2.808-2.960 A) linked with the two perchlorate anions inside, where each anion binds to both of the central NH2 and also one secondary NH2 linked with aromatic unit (Scheme 16). 

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