A new class of macrocyclic and acyclic molecules was synthesized by the Mannich reactions of dipyrrolylmethanes to investigate anion recognition. The X-ray structures of the macrocycle and sulfate complexes are reported.
MauG is a diheme enzyme that utilizes two covalently bound c-type heme centers. We report here step-wise oxidations of a synthetic analogue of MauG in which two heme centers are bridged covalently through a flexible linker containing a pyrrole moiety. One- and two-electron oxidations produce monocation radical and dication diradical intermediates, respectively, which, being highly reactive, undergo spontaneous intramolecular rearrangement involving the pyrrole bridge itself to form indolizinium-fused chlorin-porphyrin and spiro-porphyrinato heterodimers. Unlike in MauG, where the two oxidizing equivalents produce the bis-Fe redox state, the synthetic analogue of the same, however, stabilizes two ferric hemes, each coupled with a porphyrin π-cation radical. The present study highlights the possible role played by the bridge in the electronic communication.
A new class of tripyrrolylmethane-based triazacryptand with bridgehead carbons and acyclic molecules were synthesized by the Mannich reaction of tripyrrolylmethane with primary or secondary amine hydrochloride and formaldehyde, respectively. The structure of the triazacryptand was determined by X-ray diffraction (XRD) method. The anion binding properties of both the bicyclic and acyclic receptors were studied by (1)H NMR titration method. The binding studies showed that both receptors exhibit very high affinity and bind strongly with the F(-) ion in DMSO-d(6). However, the binding constant of azacryptand with F(-) is much higher than that of the acyclic receptor. This is attributed to the preorganization of the azacryptand having a specific cavity size, and the strength and the number of hydrogen bonds formed by the F(-) ion. This is supported by the crystal structures of F(-), Cl(-), and Br(-) ion complexes of the bicyclic receptor and by DFT calculations. The X-ray structures showed that the azacryptand receptor forms an inclusion complex with only the F(-) ion; other anions bind in the clefts of the macrobicycle, thus supporting a size-selective anion binding behavior. The high affinity and the selectivity of the macrobicycle as a neutral receptor of the F(-) ion in the presence of other competitive anions in DMSO-d(6) were confirmed by (1)H NMR spectroscopy. Furthermore, the F(-)-ion-mediated hydrogen-deuterium exchanges were monitored by (19)F NMR spectroscopy, showing multiplets based on the formation of all possible deuterium-exchanged fluoride complexes in solution.
The [2 + 2] Schiff base condensation reactions between the newly synthesized dialdehyde, N,N-di(α-formylpyrrolyl-α-methyl)-N-methylamine), and ethylenediamine or p-phenylenediamine dihydrochloride readily afforded the 30- and 34-membered large size macrocycles in very high yields. Subsequent reduction reactions of these macrocycles with NaBH(4) gave the corresponding saturated macrocyclic hexaamines in good yields. The analogous reaction of the new dialdehyde with a triamine molecule afforded the [3 + 2] Schiff base macrobicycle in high yield, which was then reduced by reaction with NaBH(4) to give the saturated macrobicycle. All these compounds were characterized by spectroscopic methods. The anion binding properties of the saturated macrocycles having the ethylene and the phenylene linkers in CDCl(3) were studied by NMR titration methods. Although they have similar pyrrolic and amine NH groups their binding properties are different and interesting, owing to the conformational flexibility or rigidness rendered by the ethylene or phenylene groups, respectively. The macrocycle having the ethylene linkers binds anions in a 1:1 fashion, while the other receptor having the phenylene linkers prefers to bind anions in a sequential 1:2 fashion and has a multiple equilibria between a 1:1 and a 1:2 complexes, as shown by their binding constants, curve fittings by EQNMR, and Job plots. The X-ray structures of the 1:2 methanol, the aqua and the benzoate anion complexes of the macrocycles show two cavities in which the guests are bound, correlating with the high affinity found for the formation of stable 1:2 complexes in solution. The X-ray structure showed that the macrobicycle Schiff base adopts an eclipsed paddle-wheel shaped conformation and exhibits an out-out configuration at the bridgehead nitrogen atoms.
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