Ditopic binding of various dinitrogen compounds to three bisporphyrin molecular tweezers with spacers of varying conformational rigidity, incorporating the planar enediyne (1), the helical stiff stilbene (2), or the semi-rigid glycoluril motif fused to the porphyrins (3), are compared. Binding constants K a = 10 4 -10 6 M´1 reveal subtle differences between these tweezers, that are discussed in terms of porphyrin dislocation modes. Exciton coupled circular dichroism (ECCD) of complexes with chiral dinitrogen guests provides experimental evidence for the conformational properties of the tweezers. The results are further supported and rationalized by conformational analysis.
Modulation of the solution conformations of N,N'-bis(benzhydryl)bispidine has been achieved by protonation. Conformers have been characterized by NMR spectroscopy using nuclear Overhauser effects and residual dipolar couplings. In contrast to the preference for the chair-chair conformation for the free base and the monoprotonated species, the diprotonated bispidine is revealed to exist as a mixture of chair-boat and boat-boat conformers. While boat-boat conformers of bispidines have previously not been detected, they are here observed to constitute up to 70% of the bispidine population.
The relative stereochemistry of acyclic diamines with several stereogenic centers has been analyzed by NMR spectroscopy in combination with conformational deconvolution. Binding to a bisporphyrin molecular clip improves the stereochemical assignment significantly. The diamines were synthesized from inexpensive sugar alcohols, and their stable hydrochlorides were quantitatively converted into free bases by treatment with ion-exchange resin.
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